bpf: Keep fault protection when merging pointer types

When the same BPF_LDX instruction is reached through paths that yield
different pointer types, save_aux_ptr_type() merges them into a single
type which is later used by bpf_convert_ctx_accesses() to decide whether
the load has to be rewritten into a BPF_PROBE_MEM one.

Before f2362a57aeff ("bpf: allow void* cast using bpf_rdonly_cast()")
the merge only accepted two PTR_TO_BTF_ID pointers and unconditionally
fell back to PTR_TO_BTF_ID | PTR_UNTRUSTED, so the merged type was always
one that gets the BPF_PROBE_MEM rewrite. However, the mentioned commit
widened the merge to also cover a PTR_TO_MEM base and replaced the
fallback by a union of the PTR_UNTRUSTED and MEM_RDONLY flags.

A union of flags though cannot express the property the later rewrite
is built upon, some examples:

  - PTR_TO_MEM merged with PTR_TO_BTF_ID | PTR_UNTRUSTED gets
    PTR_TO_MEM | PTR_UNTRUSTED but only the MEM_RDONLY variant is valid
  - PTR_TO_MEM merged with a plain PTR_TO_BTF_ID gets PTR_TO_MEM
    dropping the rewrite the latter type would have gotten
  - PTR_TO_MEM | MEM_RDONLY merged with a plain PTR_TO_BTF_ID gets
    PTR_TO_MEM | MEM_RDONLY which is not rewritten either since only
    its PTR_UNTRUSTED variant is

In all three cases a program can take the unsafe path at runtime with a
NULL or otherwise bad pointer and panic the kernel on the faulting load:

  BUG: kernel NULL pointer dereference, address: 0000000000000038
  RIP: 0010:bpf_prog_77531a87032eeaf1_mixed_mem_btf_id_type+0x4b/0x65
  Call Trace:
   <TASK>
   bpf_test_run+0x20b/0x460
   bpf_prog_test_run_skb+0x650/0xbe0
   __sys_bpf+0xb96/0x3140
   __x64_sys_bpf+0x2c/0x40
   do_syscall_64+0xba/0x590
  Kernel panic - not syncing: Fatal exception in interrupt

Note that the last two shapes have to be fixed right here, otherwise
the merged type retains nothing which marks the load as fault prone,
thus no rule in bpf_convert_ctx_accesses() can recover it. Fix it by
normalizing the merged type instead.

Reuse it in is_load_acq_unsafe() to avoid open coding, and trim the
overly verbose comment which is more of an implementation detail of
bpf_convert_ctx_accesses() anyway.

Fixes: f2362a57aeff ("bpf: allow void* cast using bpf_rdonly_cast()")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20260814215301.709827-1-daniel@iogearbox.net

0 files changed, 0 insertions, 0 deletions