linux-stable.git/kernel/bpf/core.c, branch v4.14.331

bpf: Adjust insufficient default bpf_jit_limit

2023-04-05T09:14:15+00:00

[ Upstream commit 10ec8ca8ec1a2f04c4ed90897225231c58c124a7 ]

We've seen recent AWS EKS (Kubernetes) user reports like the following:

  After upgrading EKS nodes from v20230203 to v20230217 on our 1.24 EKS
  clusters after a few days a number of the nodes have containers stuck
  in ContainerCreating state or liveness/readiness probes reporting the
  following error:

    Readiness probe errored: rpc error: code = Unknown desc = failed to
    exec in container: failed to start exec "4a11039f730203ffc003b7[...]":
    OCI runtime exec failed: exec failed: unable to start container process:
    unable to init seccomp: error loading seccomp filter into kernel:
    error loading seccomp filter: errno 524: unknown

  However, we had not been seeing this issue on previous AMIs and it only
  started to occur on v20230217 (following the upgrade from kernel 5.4 to
  5.10) with no other changes to the underlying cluster or workloads.

  We tried the suggestions from that issue (sysctl net.core.bpf_jit_limit=452534528)
  which helped to immediately allow containers to be created and probes to
  execute but after approximately a day the issue returned and the value
  returned by cat /proc/vmallocinfo | grep bpf_jit | awk '{s+=$2} END {print s}'
  was steadily increasing.

I tested bpf tree to observe bpf_jit_charge_modmem, bpf_jit_uncharge_modmem
their sizes passed in as well as bpf_jit_current under tcpdump BPF filter,
seccomp BPF and native (e)BPF programs, and the behavior all looks sane
and expected, that is nothing "leaking" from an upstream perspective.

The bpf_jit_limit knob was originally added in order to avoid a situation
where unprivileged applications loading BPF programs (e.g. seccomp BPF
policies) consuming all the module memory space via BPF JIT such that loading
of kernel modules would be prevented. The default limit was defined back in
2018 and while good enough back then, we are generally seeing far more BPF
consumers today.

Adjust the limit for the BPF JIT pool from originally 1/4 to now 1/2 of the
module memory space to better reflect today's needs and avoid more users
running into potentially hard to debug issues.

Fixes: fdadd04931c2 ("bpf: fix bpf_jit_limit knob for PAGE_SIZE >= 64K")
Reported-by: Stephen Haynes 
Reported-by: Lefteris Alexakis 
Signed-off-by: Daniel Borkmann 
Link: https://github.com/awslabs/amazon-eks-ami/issues/1179
Link: https://github.com/awslabs/amazon-eks-ami/issues/1219
Reviewed-by: Kuniyuki Iwashima 
Link: https://lore.kernel.org/r/20230320143725.8394-1-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov 
Signed-off-by: Sasha Levin

bpf: fix subprog verifier bypass by div/mod by 0 exception

2023-03-11T15:26:32+00:00

Commit f6b1b3bf0d5f681631a293cfe1ca934b81716f1e upstream.

One of the ugly leftovers from the early eBPF days is that div/mod
operations based on registers have a hard-coded src_reg == 0 test
in the interpreter as well as in JIT code generators that would
return from the BPF program with exit code 0. This was basically
adopted from cBPF interpreter for historical reasons.

There are multiple reasons why this is very suboptimal and prone
to bugs. To name one: the return code mapping for such abnormal
program exit of 0 does not always match with a suitable program
type's exit code mapping. For example, '0' in tc means action 'ok'
where the packet gets passed further up the stack, which is just
undesirable for such cases (e.g. when implementing policy) and
also does not match with other program types.

While trying to work out an exception handling scheme, I also
noticed that programs crafted like the following will currently
pass the verifier:

  0: (bf) r6 = r1
  1: (85) call pc+8
  caller:
   R6=ctx(id=0,off=0,imm=0) R10=fp0,call_-1
  callee:
   frame1: R1=ctx(id=0,off=0,imm=0) R10=fp0,call_1
  10: (b4) (u32) r2 = (u32) 0
  11: (b4) (u32) r3 = (u32) 1
  12: (3c) (u32) r3 /= (u32) r2
  13: (61) r0 = *(u32 *)(r1 +76)
  14: (95) exit
  returning from callee:
   frame1: R0_w=pkt(id=0,off=0,r=0,imm=0)
           R1=ctx(id=0,off=0,imm=0) R2_w=inv0
           R3_w=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff))
           R10=fp0,call_1
  to caller at 2:
   R0_w=pkt(id=0,off=0,r=0,imm=0) R6=ctx(id=0,off=0,imm=0)
   R10=fp0,call_-1

  from 14 to 2: R0=pkt(id=0,off=0,r=0,imm=0)
                R6=ctx(id=0,off=0,imm=0) R10=fp0,call_-1
  2: (bf) r1 = r6
  3: (61) r1 = *(u32 *)(r1 +80)
  4: (bf) r2 = r0
  5: (07) r2 += 8
  6: (2d) if r2 > r1 goto pc+1
   R0=pkt(id=0,off=0,r=8,imm=0) R1=pkt_end(id=0,off=0,imm=0)
   R2=pkt(id=0,off=8,r=8,imm=0) R6=ctx(id=0,off=0,imm=0)
   R10=fp0,call_-1
  7: (71) r0 = *(u8 *)(r0 +0)
  8: (b7) r0 = 1
  9: (95) exit

  from 6 to 8: safe
  processed 16 insns (limit 131072), stack depth 0+0

Basically what happens is that in the subprog we make use of a
div/mod by 0 exception and in the 'normal' subprog's exit path
we just return skb->data back to the main prog. This has the
implication that the verifier thinks we always get a pkt pointer
in R0 while we still have the implicit 'return 0' from the div
as an alternative unconditional return path earlier. Thus, R0
then contains 0, meaning back in the parent prog we get the
address range of [0x0, skb->data_end] as read and writeable.
Similar can be crafted with other pointer register types.

Since i) BPF_ABS/IND is not allowed in programs that contain
BPF to BPF calls (and generally it's also disadvised to use in
native eBPF context), ii) unknown opcodes don't return zero
anymore, iii) we don't return an exception code in dead branches,
the only last missing case affected and to fix is the div/mod
handling.

What we would really need is some infrastructure to propagate
exceptions all the way to the original prog unwinding the
current stack and returning that code to the caller of the
BPF program. In user space such exception handling for similar
runtimes is typically implemented with setjmp(3) and longjmp(3)
as one possibility which is not available in the kernel,
though (kgdb used to implement it in kernel long time ago). I
implemented a PoC exception handling mechanism into the BPF
interpreter with porting setjmp()/longjmp() into x86_64 and
adding a new internal BPF_ABRT opcode that can use a program
specific exception code for all exception cases we have (e.g.
div/mod by 0, unknown opcodes, etc). While this seems to work
in the constrained BPF environment (meaning, here, we don't
need to deal with state e.g. from memory allocations that we
would need to undo before going into exception state), it still
has various drawbacks: i) we would need to implement the
setjmp()/longjmp() for every arch supported in the kernel and
for x86_64, arm64, sparc64 JITs currently supporting calls,
ii) it has unconditional additional cost on main program
entry to store CPU register state in initial setjmp() call,
and we would need some way to pass the jmp_buf down into
___bpf_prog_run() for main prog and all subprogs, but also
storing on stack is not really nice (other option would be
per-cpu storage for this, but it also has the drawback that
we need to disable preemption for every BPF program types).
All in all this approach would add a lot of complexity.

Another poor-man's solution would be to have some sort of
additional shared register or scratch buffer to hold state
for exceptions, and test that after every call return to
chain returns and pass R0 all the way down to BPF prog caller.
This is also problematic in various ways: i) an additional
register doesn't map well into JITs, and some other scratch
space could only be on per-cpu storage, which, again has the
side-effect that this only works when we disable preemption,
or somewhere in the input context which is not available
everywhere either, and ii) this adds significant runtime
overhead by putting conditionals after each and every call,
as well as implementation complexity.

Yet another option is to teach verifier that div/mod can
return an integer, which however is also complex to implement
as verifier would need to walk such fake 'mov r0,; exit;'
sequeuence and there would still be no guarantee for having
propagation of this further down to the BPF caller as proper
exception code. For parent prog, it is also is not distinguishable
from a normal return of a constant scalar value.

The approach taken here is a completely different one with
little complexity and no additional overhead involved in
that we make use of the fact that a div/mod by 0 is undefined
behavior. Instead of bailing out, we adapt the same behavior
as on some major archs like ARMv8 [0] into eBPF as well:
X div 0 results in 0, and X mod 0 results in X. aarch64 and
aarch32 ISA do not generate any traps or otherwise aborts
of program execution for unsigned divides. I verified this
also with a test program compiled by gcc and clang, and the
behavior matches with the spec. Going forward we adapt the
eBPF verifier to emit such rewrites once div/mod by register
was seen. cBPF is not touched and will keep existing 'return 0'
semantics. Given the options, it seems the most suitable from
all of them, also since major archs have similar schemes in
place. Given this is all in the realm of undefined behavior,
we still have the option to adapt if deemed necessary and
this way we would also have the option of more flexibility
from LLVM code generation side (which is then fully visible
to verifier). Thus, this patch i) fixes the panic seen in
above program and ii) doesn't bypass the verifier observations.

  [0] ARM Architecture Reference Manual, ARMv8 [ARM DDI 0487B.b]
      http://infocenter.arm.com/help/topic/com.arm.doc.ddi0487b.b/DDI0487B_b_armv8_arm.pdf
      1) aarch64 instruction set: section C3.4.7 and C6.2.279 (UDIV)
         "A division by zero results in a zero being written to
          the destination register, without any indication that
          the division by zero occurred."
      2) aarch32 instruction set: section F1.4.8 and F5.1.263 (UDIV)
         "For the SDIV and UDIV instructions, division by zero
          always returns a zero result."

Fixes: f4d7e40a5b71 ("bpf: introduce function calls (verification)")
Signed-off-by: Daniel Borkmann 
Acked-by: Alexei Starovoitov 
Signed-off-by: Alexei Starovoitov 
Signed-off-by: Thadeu Lima de Souza Cascardo 
Signed-off-by: Edward Liaw 
Signed-off-by: Greg Kroah-Hartman



bpf: Do not use ax register in interpreter on div/mod
2023-03-11T15:26:32+00:00

Commit c348d806ed1d3075af52345344243824d72c4945 upstream.

Partially undo old commit 144cd91c4c2b ("bpf: move tmp variable into ax
register in interpreter"). The reason we need this here is because ax
register will be used for holding temporary state for div/mod instruction
which otherwise interpreter would corrupt. This will cause a small +8 byte
stack increase for interpreter, but with the gain that we can use it from
verifier rewrites as scratch register.

Signed-off-by: Daniel Borkmann 
Reviewed-by: John Fastabend 
[cascardo: This partial revert is needed in order to support using AX for
the following two commits, as there is no JMP32 on 4.19.y]
Signed-off-by: Thadeu Lima de Souza Cascardo 
[edliaw: Removed redeclaration of tmp introduced by patch differences
between 4.14 and 4.19]
Signed-off-by: Edward Liaw 
Signed-off-by: Greg Kroah-Hartman 




bpf: Make sure mac_header was set before using it
2022-07-29T15:06:51+00:00

commit 0326195f523a549e0a9d7fd44c70b26fd7265090 upstream.

Classic BPF has a way to load bytes starting from the mac header.

Some skbs do not have a mac header, and skb_mac_header()
in this case is returning a pointer that 65535 bytes after
skb->head.

Existing range check in bpf_internal_load_pointer_neg_helper()
was properly kicking and no illegal access was happening.

New sanity check in skb_mac_header() is firing, so we need
to avoid it.

WARNING: CPU: 1 PID: 28990 at include/linux/skbuff.h:2785 skb_mac_header include/linux/skbuff.h:2785 [inline]
WARNING: CPU: 1 PID: 28990 at include/linux/skbuff.h:2785 bpf_internal_load_pointer_neg_helper+0x1b1/0x1c0 kernel/bpf/core.c:74
Modules linked in:
CPU: 1 PID: 28990 Comm: syz-executor.0 Not tainted 5.19.0-rc4-syzkaller-00865-g4874fb9484be #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/29/2022
RIP: 0010:skb_mac_header include/linux/skbuff.h:2785 [inline]
RIP: 0010:bpf_internal_load_pointer_neg_helper+0x1b1/0x1c0 kernel/bpf/core.c:74
Code: ff ff 45 31 f6 e9 5a ff ff ff e8 aa 27 40 00 e9 3b ff ff ff e8 90 27 40 00 e9 df fe ff ff e8 86 27 40 00 eb 9e e8 2f 2c f3 ff <0f> 0b eb b1 e8 96 27 40 00 e9 79 fe ff ff 90 41 57 41 56 41 55 41
RSP: 0018:ffffc9000309f668 EFLAGS: 00010216
RAX: 0000000000000118 RBX: ffffffffffeff00c RCX: ffffc9000e417000
RDX: 0000000000040000 RSI: ffffffff81873f21 RDI: 0000000000000003
RBP: ffff8880842878c0 R08: 0000000000000003 R09: 000000000000ffff
R10: 000000000000ffff R11: 0000000000000001 R12: 0000000000000004
R13: ffff88803ac56c00 R14: 000000000000ffff R15: dffffc0000000000
FS: 00007f5c88a16700(0000) GS:ffff8880b9b00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fdaa9f6c058 CR3: 000000003a82c000 CR4: 00000000003506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:

____bpf_skb_load_helper_32 net/core/filter.c:276 [inline]
bpf_skb_load_helper_32+0x191/0x220 net/core/filter.c:264

Fixes: f9aefd6b2aa3 ("net: warn if mac header was not set")
Reported-by: syzbot 
Signed-off-by: Eric Dumazet 
Signed-off-by: Daniel Borkmann 
Link: https://lore.kernel.org/bpf/20220707123900.945305-1-edumazet@google.com
Signed-off-by: Greg Kroah-Hartman 




bpf: fix truncated jump targets on heavy expansions
2022-02-08T17:16:27+00:00

commit 050fad7c4534c13c8eb1d9c2ba66012e014773cb upstream.

Recently during testing, I ran into the following panic:

  [  207.892422] Internal error: Accessing user space memory outside uaccess.h routines: 96000004 [#1] SMP
  [  207.901637] Modules linked in: binfmt_misc [...]
  [  207.966530] CPU: 45 PID: 2256 Comm: test_verifier Tainted: G        W         4.17.0-rc3+ #7
  [  207.974956] Hardware name: FOXCONN R2-1221R-A4/C2U4N_MB, BIOS G31FB18A 03/31/2017
  [  207.982428] pstate: 60400005 (nZCv daif +PAN -UAO)
  [  207.987214] pc : bpf_skb_load_helper_8_no_cache+0x34/0xc0
  [  207.992603] lr : 0xffff000000bdb754
  [  207.996080] sp : ffff000013703ca0
  [  207.999384] x29: ffff000013703ca0 x28: 0000000000000001
  [  208.004688] x27: 0000000000000001 x26: 0000000000000000
  [  208.009992] x25: ffff000013703ce0 x24: ffff800fb4afcb00
  [  208.015295] x23: ffff00007d2f5038 x22: ffff00007d2f5000
  [  208.020599] x21: fffffffffeff2a6f x20: 000000000000000a
  [  208.025903] x19: ffff000009578000 x18: 0000000000000a03
  [  208.031206] x17: 0000000000000000 x16: 0000000000000000
  [  208.036510] x15: 0000ffff9de83000 x14: 0000000000000000
  [  208.041813] x13: 0000000000000000 x12: 0000000000000000
  [  208.047116] x11: 0000000000000001 x10: ffff0000089e7f18
  [  208.052419] x9 : fffffffffeff2a6f x8 : 0000000000000000
  [  208.057723] x7 : 000000000000000a x6 : 00280c6160000000
  [  208.063026] x5 : 0000000000000018 x4 : 0000000000007db6
  [  208.068329] x3 : 000000000008647a x2 : 19868179b1484500
  [  208.073632] x1 : 0000000000000000 x0 : ffff000009578c08
  [  208.078938] Process test_verifier (pid: 2256, stack limit = 0x0000000049ca7974)
  [  208.086235] Call trace:
  [  208.088672]  bpf_skb_load_helper_8_no_cache+0x34/0xc0
  [  208.093713]  0xffff000000bdb754
  [  208.096845]  bpf_test_run+0x78/0xf8
  [  208.100324]  bpf_prog_test_run_skb+0x148/0x230
  [  208.104758]  sys_bpf+0x314/0x1198
  [  208.108064]  el0_svc_naked+0x30/0x34
  [  208.111632] Code: 91302260 f9400001 f9001fa1 d2800001 (29500680)
  [  208.117717] ---[ end trace 263cb8a59b5bf29f ]---

The program itself which caused this had a long jump over the whole
instruction sequence where all of the inner instructions required
heavy expansions into multiple BPF instructions. Additionally, I also
had BPF hardening enabled which requires once more rewrites of all
constant values in order to blind them. Each time we rewrite insns,
bpf_adj_branches() would need to potentially adjust branch targets
which cross the patchlet boundary to accommodate for the additional
delta. Eventually that lead to the case where the target offset could
not fit into insn->off's upper 0x7fff limit anymore where then offset
wraps around becoming negative (in s16 universe), or vice versa
depending on the jump direction.

Therefore it becomes necessary to detect and reject any such occasions
in a generic way for native eBPF and cBPF to eBPF migrations. For
the latter we can simply check bounds in the bpf_convert_filter()'s
BPF_EMIT_JMP helper macro and bail out once we surpass limits. The
bpf_patch_insn_single() for native eBPF (and cBPF to eBPF in case
of subsequent hardening) is a bit more complex in that we need to
detect such truncations before hitting the bpf_prog_realloc(). Thus
the latter is split into an extra pass to probe problematic offsets
on the original program in order to fail early. With that in place
and carefully tested I no longer hit the panic and the rewrites are
rejected properly. The above example panic I've seen on bpf-next,
though the issue itself is generic in that a guard against this issue
in bpf seems more appropriate in this case.

Signed-off-by: Daniel Borkmann 
Acked-by: Martin KaFai Lau 
Signed-off-by: Alexei Starovoitov 
[ab: Dropped BPF_PSEUDO_CALL hardening, introoduced in 4.16]
Signed-off-by: Alessio Balsini 
Acked-by: Thadeu Lima de Souza Cascardo 
Signed-off-by: Greg Kroah-Hartman 




bpf: Prevent increasing bpf_jit_limit above max
2021-11-26T10:40:22+00:00

[ Upstream commit fadb7ff1a6c2c565af56b4aacdd086b067eed440 ]

Restrict bpf_jit_limit to the maximum supported by the arch's JIT.

Signed-off-by: Lorenz Bauer 
Signed-off-by: Alexei Starovoitov 
Link: https://lore.kernel.org/bpf/20211014142554.53120-4-lmb@cloudflare.com
Signed-off-by: Sasha Levin 




bpf: fix bpf_jit_limit knob for PAGE_SIZE >= 64K
2019-08-25T08:50:23+00:00

[ Upstream commit fdadd04931c2d7cd294dc5b2b342863f94be53a3 ]

Michael and Sandipan report:

  Commit ede95a63b5 introduced a bpf_jit_limit tuneable to limit BPF
  JIT allocations. At compile time it defaults to PAGE_SIZE * 40000,
  and is adjusted again at init time if MODULES_VADDR is defined.

  For ppc64 kernels, MODULES_VADDR isn't defined, so we're stuck with
  the compile-time default at boot-time, which is 0x9c400000 when
  using 64K page size. This overflows the signed 32-bit bpf_jit_limit
  value:

  root@ubuntu:/tmp# cat /proc/sys/net/core/bpf_jit_limit
  -1673527296

  and can cause various unexpected failures throughout the network
  stack. In one case `strace dhclient eth0` reported:

  setsockopt(5, SOL_SOCKET, SO_ATTACH_FILTER, {len=11, filter=0x105dd27f8},
             16) = -1 ENOTSUPP (Unknown error 524)

  and similar failures can be seen with tools like tcpdump. This doesn't
  always reproduce however, and I'm not sure why. The more consistent
  failure I've seen is an Ubuntu 18.04 KVM guest booted on a POWER9
  host would time out on systemd/netplan configuring a virtio-net NIC
  with no noticeable errors in the logs.

Given this and also given that in near future some architectures like
arm64 will have a custom area for BPF JIT image allocations we should
get rid of the BPF_JIT_LIMIT_DEFAULT fallback / default entirely. For
4.21, we have an overridable bpf_jit_alloc_exec(), bpf_jit_free_exec()
so therefore add another overridable bpf_jit_alloc_exec_limit() helper
function which returns the possible size of the memory area for deriving
the default heuristic in bpf_jit_charge_init().

Like bpf_jit_alloc_exec() and bpf_jit_free_exec(), the new
bpf_jit_alloc_exec_limit() assumes that module_alloc() is the default
JIT memory provider, and therefore in case archs implement their custom
module_alloc() we use MODULES_{END,_VADDR} for limits and otherwise for
vmalloc_exec() cases like on ppc64 we use VMALLOC_{END,_START}.

Additionally, for archs supporting large page sizes, we should change
the sysctl to be handled as long to not run into sysctl restrictions
in future.

Fixes: ede95a63b5e8 ("bpf: add bpf_jit_limit knob to restrict unpriv allocations")
Reported-by: Sandipan Das 
Reported-by: Michael Roth 
Signed-off-by: Daniel Borkmann 
Tested-by: Michael Roth 
Signed-off-by: Alexei Starovoitov 
Signed-off-by: Sasha Levin 




bpf: add bpf_jit_limit knob to restrict unpriv allocations
2019-08-25T08:50:03+00:00

commit ede95a63b5e84ddeea6b0c473b36ab8bfd8c6ce3 upstream.

Rick reported that the BPF JIT could potentially fill the entire module
space with BPF programs from unprivileged users which would prevent later
attempts to load normal kernel modules or privileged BPF programs, for
example. If JIT was enabled but unsuccessful to generate the image, then
before commit 290af86629b2 ("bpf: introduce BPF_JIT_ALWAYS_ON config")
we would always fall back to the BPF interpreter. Nowadays in the case
where the CONFIG_BPF_JIT_ALWAYS_ON could be set, then the load will abort
with a failure since the BPF interpreter was compiled out.

Add a global limit and enforce it for unprivileged users such that in case
of BPF interpreter compiled out we fail once the limit has been reached
or we fall back to BPF interpreter earlier w/o using module mem if latter
was compiled in. In a next step, fair share among unprivileged users can
be resolved in particular for the case where we would fail hard once limit
is reached.

Fixes: 290af86629b2 ("bpf: introduce BPF_JIT_ALWAYS_ON config")
Fixes: 0a14842f5a3c ("net: filter: Just In Time compiler for x86-64")
Co-Developed-by: Rick Edgecombe 
Signed-off-by: Daniel Borkmann 
Acked-by: Alexei Starovoitov 
Cc: Eric Dumazet 
Cc: Jann Horn 
Cc: Kees Cook 
Cc: LKML 
Signed-off-by: Alexei Starovoitov 
Signed-off-by: Ben Hutchings 
Signed-off-by: Greg Kroah-Hartman 




bpf: get rid of pure_initcall dependency to enable jits
2019-08-25T08:50:02+00:00

commit fa9dd599b4dae841924b022768354cfde9affecb upstream.

Having a pure_initcall() callback just to permanently enable BPF
JITs under CONFIG_BPF_JIT_ALWAYS_ON is unnecessary and could leave
a small race window in future where JIT is still disabled on boot.
Since we know about the setting at compilation time anyway, just
initialize it properly there. Also consolidate all the individual
bpf_jit_enable variables into a single one and move them under one
location. Moreover, don't allow for setting unspecified garbage
values on them.

Signed-off-by: Daniel Borkmann 
Acked-by: Alexei Starovoitov 
Signed-off-by: Alexei Starovoitov 
[bwh: Backported to 4.14 as dependency of commit 2e4a30983b0f
 "bpf: restrict access to core bpf sysctls":
 - Adjust context]
Signed-off-by: Ben Hutchings 
Signed-off-by: Greg Kroah-Hartman 




bpf: enable access to ax register also from verifier rewrite
2019-04-20T07:15:08+00:00

commit 9b73bfdd08e73231d6a90ae6db4b46b3fbf56c30 upstream.

Right now we are using BPF ax register in JIT for constant blinding as
well as in interpreter as temporary variable. Verifier will not be able
to use it simply because its use will get overridden from the former in
bpf_jit_blind_insn(). However, it can be made to work in that blinding
will be skipped if there is prior use in either source or destination
register on the instruction. Taking constraints of ax into account, the
verifier is then open to use it in rewrites under some constraints. Note,
ax register already has mappings in every eBPF JIT.


Signed-off-by: Daniel Borkmann 
Acked-by: Alexei Starovoitov 
Signed-off-by: Alexei Starovoitov 
[backported to 4.14 sblbir]
Signed-off-by: Balbir Singh 
Signed-off-by: Greg Kroah-Hartman