bpf: fix bpf_d_path() helper prototype #6443
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Upstream branch: 30f0920 |
Commit 37cce22 ("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 37cce22, 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: 37cce22 ("bpf: verifier: Refactor helper access type tracking") Co-developed-by: Zesen Liu <[email protected]> Signed-off-by: Zesen Liu <[email protected]> Co-developed-by: Peili Gao <[email protected]> Signed-off-by: Peili Gao <[email protected]> Co-developed-by: Haoran Ni <[email protected]> Signed-off-by: Haoran Ni <[email protected]> Signed-off-by: Shuran Liu <[email protected]> Reviewed-by: Matt Bobrowski <[email protected]>
Add a simple LSM BPF program and a corresponding test_progs test case to exercise bpf_d_path() and ensure that prefix comparisons on the returned path keep working. The LSM program hooks bprm_check_security, calls bpf_d_path() on the binary being executed, and compares the returned path against the "/tmp/" prefix. The result is recorded in an array map. The user space test runs /tmp/bpf_d_path_test (copied from /bin/true) and checks that the BPF program records a successful prefix match. Without the preceding fix to bpf_d_path()'s helper prototype, the test can fail due to the verifier incorrectly assuming that the buffer contents are unchanged across the helper call and misoptimizing the program. With the fix applied, the test passes. Co-developed-by: Zesen Liu <[email protected]> Signed-off-by: Zesen Liu <[email protected]> Co-developed-by: Peili Gao <[email protected]> Signed-off-by: Peili Gao <[email protected]> Co-developed-by: Haoran Ni <[email protected]> Signed-off-by: Haoran Ni <[email protected]> Signed-off-by: Shuran Liu <[email protected]>
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Upstream branch: 30f0920 |
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Pull request for series with
subject: bpf: fix bpf_d_path() helper prototype
version: 1
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1029325