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bpf: Fix verifier assumptions of bpf_d_path's output buffer

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[ Upstream commit ac44dcc788 ]

Commit 37cce22dbd ("bpf: verifier: Refactor helper access type
tracking") started distinguishing read vs write accesses performed by
helpers.

The second argument of bpf_d_path() is a pointer to a buffer that the
helper fills with the resulting path. However, its prototype currently
uses ARG_PTR_TO_MEM without MEM_WRITE.

Before 37cce22dbd, helper accesses were conservatively treated as
potential writes, so this mismatch did not cause issues. Since that
commit, the verifier may incorrectly assume that the buffer contents
are unchanged across the helper call and base its optimizations on this
wrong assumption. This can lead to misbehaviour in BPF programs that
read back the buffer, such as prefix comparisons on the returned path.

Fix this by marking the second argument of bpf_d_path() as
ARG_PTR_TO_MEM | MEM_WRITE so that the verifier correctly models the
write to the caller-provided buffer.

Fixes: 37cce22dbd ("bpf: verifier: Refactor helper access type tracking")
Co-developed-by: Zesen Liu <ftyg@live.com>
Signed-off-by: Zesen Liu <ftyg@live.com>
Co-developed-by: Peili Gao <gplhust955@gmail.com>
Signed-off-by: Peili Gao <gplhust955@gmail.com>
Co-developed-by: Haoran Ni <haoran.ni.cs@gmail.com>
Signed-off-by: Haoran Ni <haoran.ni.cs@gmail.com>
Signed-off-by: Shuran Liu <electronlsr@gmail.com>
Reviewed-by: Matt Bobrowski <mattbobrowski@google.com>
Link: https://lore.kernel.org/r/20251206141210.3148-2-electronlsr@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Shuran Liu 2025-12-06 22:12:09 +08:00 • committed by Greg Kroah-Hartman
commit d0e177e176
1 changed files with 1 additions and 1 deletions
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