CVE-2023-53024

Description

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.

Gravité et risque d'exploitation

CVE-2023-53024 obtient un score CVSS de 0.0/10, soit une gravité non évaluée. Selon le modèle EPSS, la probabilité qu'elle soit exploitée dans les 30 prochains jours est de 0.20 %, ce qui la place devant 9.5 % des CVE référencées.

Que faire ?

• Vérifiez si Linux (versions <=6.1.*, 5.4.231, <01bdcc73dbe7be3ad4d4ee9a59b71e42f461a528, <5.10.166, 0, <aae109414a57ab4164218f36e2e4a17f027fcaaa, <4.19.272, 4.19.272, 5.13.8, <5.14, 0e9280654aa482088ee6ef3deadef331f5ac5fb0, <5.4.231, <=5.10.*, <da75dec7c6617bddad418159ffebcb133f008262, 5.14, 5.10.166, <81b3374944d201872cfcf82730a7860f8e7c31dd, 6.1.9, 2039f26f3aca5b0e419b98f65dd36481337b86ee, <b0c89ef025562161242a7c19b213bd6b272e93df, <=5.4.*, <=4.19.*, <e4f4db47794c9f474b184ee1418f42e6a07412b6, 4.19.207, 5.4.146, f5893af2704eb763eb982f01d573f5b19f06b623, 5.10.56, <=*, 0b27bdf02c400684225ee5ee99970bcbf5082282, <=5.15.*, 872968502114d68c21419cf7eb5ab97717e7b803, 5.15.91, 6.2) est présent sur votre parc.
• Appliquez le correctif ou la version corrigée indiqués dans les références de l'éditeur.
• À défaut de correctif, limitez l'exposition (filtrage réseau, désactivation du composant).
• Automatisez la détection : les agents CVE Heimdall (gratuits, open source) corrèlent l'inventaire de vos serveurs avec la base CVE.

Références

• https://git.kernel.org/stable/c/aae109414a57ab4164218f36e2e4a17f027fcaaa
• https://git.kernel.org/stable/c/81b3374944d201872cfcf82730a7860f8e7c31dd
• https://git.kernel.org/stable/c/da75dec7c6617bddad418159ffebcb133f008262
• https://git.kernel.org/stable/c/01bdcc73dbe7be3ad4d4ee9a59b71e42f461a528
• https://git.kernel.org/stable/c/b0c89ef025562161242a7c19b213bd6b272e93df
• https://git.kernel.org/stable/c/e4f4db47794c9f474b184ee1418f42e6a07412b6