Merge branch 'bpf-verifier-fix-ptr_to_flow_keys-constant-offset-oob'

Nuoqi Gui says:

====================
bpf, verifier: fix PTR_TO_FLOW_KEYS constant-offset OOB

A constant offset added to a PTR_TO_FLOW_KEYS register lands in
reg->var_off, but check_flow_keys_access() bounds-checks only insn->off
and never folds reg->var_off.value.  A BPF_PROG_TYPE_FLOW_DISSECTOR
program can therefore do "flow_keys += 0x1000; *(flow_keys + 0)" and have
it accepted, then read/write kernel stack past struct bpf_flow_keys at
runtime.  Patch 1 folds reg->var_off.value into the offset (and rejects
non-constant offsets), mirroring check_ctx_access(); patch 2 adds verifier
selftests.

This is a regression introduced in the 7.1 development cycle by commit
022ac0750883 ("bpf: use reg->var_off instead of reg->off for pointers"),
which moved the constant offset from reg->off (folded generically before
022ac0750883) into reg->var_off without updating the flow_keys path.  No
released kernel is affected: v7.0.x rejects the program above, and the bug
reproduces only on v7.1-rc1..rc5, so no stable backport is needed.

It was first reported privately to security@kernel.org; per their guidance
it is handled in the open as a normal regression fix.  Found by manual
verifier audit and confirmed dynamically in a disposable QEMU/KVM guest:
the load above is accepted, a runtime read leaked a kernel-stack pointer
0x1000 past bpf_flow_keys, and a runtime write of a marker faulted the
guest in net_rx_action.

An alternative -- forbidding pointer arithmetic on PTR_TO_FLOW_KEYS
outright by dropping "if (known) break;" in adjust_ptr_min_max_vals() --
was rejected because v7.0.x accepted (and correctly bounds-checked)
constant arithmetic on the keys pointer; restoring the fold preserves that
behaviour while closing the divergence.

Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
---
v2 -> v3:
 - Pass existing reg/argno context into check_flow_keys_access(), avoiding
   a stale regno reference in check_mem_access().
 - Add a variable-offset selftest using bpf_get_prandom_u32().

v1 -> v2:
 - Target bpf-next instead of bpf (per reviewer feedback).
 - Base-commit updated to bpf-next/master.

v2: https://lore.kernel.org/bpf/20260604180730.2518088-1-gnq25@mails.tsinghua.edu.cn/
v1: https://lore.kernel.org/bpf/20260604150755.2487555-1-gnq25@mails.tsinghua.edu.cn/
====================

Link: https://patch.msgid.link/20260606-c3-01-v3-v3-0-97c51f592f15@mails.tsinghua.edu.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 15 insertions, 3 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 935595138aa0..68ddd465584c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4728,9 +4728,21 @@ static int check_ctx_access(struct bpf_verifier_env *env, int insn_idx, struct b
 	return err;
 }
 
-static int check_flow_keys_access(struct bpf_verifier_env *env, int off,
-				  int size)
+static int check_flow_keys_access(struct bpf_verifier_env *env,
+				  struct bpf_reg_state *reg, argno_t argno,
+				  int off, int size)
 {
+	/* Only a constant offset is allowed here; fold it into off. */
+	if (!tnum_is_const(reg->var_off)) {
+		char tn_buf[48];
+
+		tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
+		verbose(env, "%s invalid variable offset to flow keys: off=%d, var_off=%s\n",
+			reg_arg_name(env, argno), off, tn_buf);
+		return -EACCES;
+	}
+	off += reg->var_off.value;
+
 	if (size < 0 || off < 0 ||
 	    (u64)off + size > sizeof(struct bpf_flow_keys)) {
 		verbose(env, "invalid access to flow keys off=%d size=%d\n",
@@ -6239,7 +6251,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, struct b
 			return -EACCES;
 		}
 
-		err = check_flow_keys_access(env, off, size);
+		err = check_flow_keys_access(env, reg, argno, off, size);
 		if (!err && t == BPF_READ && value_regno >= 0)
 			mark_reg_unknown(env, regs, value_regno);
 	} else if (type_is_sk_pointer(reg->type)) {