CVE-2023-53024
bpf: Fix pointer-leak due to insufficient speculative store bypass mitigation
We use these services and cookies to improve your user experience. You may opt out if you wish, however, this may limit some features on this site.
Please see our statement on Data Privacy.
bpf: Fix pointer-leak due to insufficient speculative store bypass mitigation
In the Linux kernel, the following vulnerability has been resolved: bpf: Fix pointer-leak due to insufficient speculative store bypass mitigation To mitigate Spectre v4, 2039f26f3aca ("bpf: Fix leakage due to insufficient speculative store bypass mitigation") inserts lfence instructions after 1) initializing a stack slot and 2) spilling a pointer to the stack. However, this does not cover cases where a stack slot is first initialized with a pointer (subject to sanitization) but then overwritten with a scalar (not subject to sanitization because the slot was already initialized). In this case, the second write may be subject to speculative store bypass (SSB) creating a speculative pointer-as-scalar type confusion. This allows the program to subsequently leak the numerical pointer value using, for example, a branch-based cache side channel. To fix this, also sanitize scalars if they write a stack slot that previously contained a pointer. Assuming that pointer-spills are only generated by LLVM on register-pressure, the performance impact on most real-world BPF programs should be small. The following unprivileged BPF bytecode drafts a minimal exploit and the mitigation: [...] // r6 = 0 or 1 (skalar, unknown user input) // r7 = accessible ptr for side channel // r10 = frame pointer (fp), to be leaked // r9 = r10 # fp alias to encourage ssb *(u64 *)(r9 - 8) = r10 // fp[-8] = ptr, to be leaked // lfence added here because of pointer spill to stack. // // Ommitted: Dummy bpf_ringbuf_output() here to train alias predictor // for no r9-r10 dependency. // *(u64 *)(r10 - 8) = r6 // fp[-8] = scalar, overwrites ptr // 2039f26f3aca: no lfence added because stack slot was not STACK_INVALID, // store may be subject to SSB // // fix: also add an lfence when the slot contained a ptr // r8 = *(u64 *)(r9 - 8) // r8 = architecturally a scalar, speculatively a ptr // // leak ptr using branch-based cache side channel: r8 &= 1 // choose bit to leak if r8 == 0 goto SLOW // no mispredict // architecturally dead code if input r6 is 0, // only executes speculatively iff ptr bit is 1 r8 = *(u64 *)(r7 + 0) # encode bit in cache (0: slow, 1: fast) SLOW: [...] After running this, the program can time the access to *(r7 + 0) to determine whether the chosen pointer bit was 0 or 1. Repeat this 64 times to recover the whole address on amd64. In summary, sanitization can only be skipped if one scalar is overwritten with another scalar. Scalar-confusion due to speculative store bypass can not lead to invalid accesses because the pointer bounds deducted during verification are enforced using branchless logic. See 979d63d50c0c ("bpf: prevent out of bounds speculation on pointer arithmetic") for details. Do not make the mitigation depend on !env->allow_{uninit_stack,ptr_leaks} because speculative leaks are likely unexpected if these were enabled. For example, leaking the address to a protected log file may be acceptable while disabling the mitigation might unintentionally leak the address into the cached-state of a map that is accessible to unprivileged processes.
Reserved 2025-03-27 | Published 2025-03-27 | Updated 2025-05-04 | Assigner Linuxgit.kernel.org/...c/aae109414a57ab4164218f36e2e4a17f027fcaaa
git.kernel.org/...c/81b3374944d201872cfcf82730a7860f8e7c31dd
git.kernel.org/...c/da75dec7c6617bddad418159ffebcb133f008262
git.kernel.org/...c/01bdcc73dbe7be3ad4d4ee9a59b71e42f461a528
git.kernel.org/...c/b0c89ef025562161242a7c19b213bd6b272e93df
git.kernel.org/...c/e4f4db47794c9f474b184ee1418f42e6a07412b6
Support options
