GHSA-MQH3-5X68-9V64
Vulnerability from github – Published: 2025-03-27 18:31 – Updated: 2026-01-22 21:33In 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.
{
"affected": [],
"aliases": [
"CVE-2023-53024"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2025-03-27T17:15:51Z",
"severity": "HIGH"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nbpf: Fix pointer-leak due to insufficient speculative store bypass mitigation\n\nTo mitigate Spectre v4, 2039f26f3aca (\"bpf: Fix leakage due to\ninsufficient speculative store bypass mitigation\") inserts lfence\ninstructions after 1) initializing a stack slot and 2) spilling a\npointer to the stack.\n\nHowever, this does not cover cases where a stack slot is first\ninitialized with a pointer (subject to sanitization) but then\noverwritten with a scalar (not subject to sanitization because\nthe slot was already initialized). In this case, the second write\nmay be subject to speculative store bypass (SSB) creating a\nspeculative pointer-as-scalar type confusion. This allows the\nprogram to subsequently leak the numerical pointer value using,\nfor example, a branch-based cache side channel.\n\nTo fix this, also sanitize scalars if they write a stack slot\nthat previously contained a pointer. Assuming that pointer-spills\nare only generated by LLVM on register-pressure, the performance\nimpact on most real-world BPF programs should be small.\n\nThe following unprivileged BPF bytecode drafts a minimal exploit\nand the mitigation:\n\n [...]\n // r6 = 0 or 1 (skalar, unknown user input)\n // r7 = accessible ptr for side channel\n // r10 = frame pointer (fp), to be leaked\n //\n r9 = r10 # fp alias to encourage ssb\n *(u64 *)(r9 - 8) = r10 // fp[-8] = ptr, to be leaked\n // lfence added here because of pointer spill to stack.\n //\n // Ommitted: Dummy bpf_ringbuf_output() here to train alias predictor\n // for no r9-r10 dependency.\n //\n *(u64 *)(r10 - 8) = r6 // fp[-8] = scalar, overwrites ptr\n // 2039f26f3aca: no lfence added because stack slot was not STACK_INVALID,\n // store may be subject to SSB\n //\n // fix: also add an lfence when the slot contained a ptr\n //\n r8 = *(u64 *)(r9 - 8)\n // r8 = architecturally a scalar, speculatively a ptr\n //\n // leak ptr using branch-based cache side channel:\n r8 \u0026= 1 // choose bit to leak\n if r8 == 0 goto SLOW // no mispredict\n // architecturally dead code if input r6 is 0,\n // only executes speculatively iff ptr bit is 1\n r8 = *(u64 *)(r7 + 0) # encode bit in cache (0: slow, 1: fast)\nSLOW:\n [...]\n\nAfter running this, the program can time the access to *(r7 + 0) to\ndetermine whether the chosen pointer bit was 0 or 1. Repeat this 64\ntimes to recover the whole address on amd64.\n\nIn summary, sanitization can only be skipped if one scalar is\noverwritten with another scalar. Scalar-confusion due to speculative\nstore bypass can not lead to invalid accesses because the pointer\nbounds deducted during verification are enforced using branchless\nlogic. See 979d63d50c0c (\"bpf: prevent out of bounds speculation on\npointer arithmetic\") for details.\n\nDo not make the mitigation depend on !env-\u003eallow_{uninit_stack,ptr_leaks}\nbecause speculative leaks are likely unexpected if these were enabled.\nFor example, leaking the address to a protected log file may be acceptable\nwhile disabling the mitigation might unintentionally leak the address\ninto the cached-state of a map that is accessible to unprivileged\nprocesses.",
"id": "GHSA-mqh3-5x68-9v64",
"modified": "2026-01-22T21:33:42Z",
"published": "2025-03-27T18:31:28Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2023-53024"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/01bdcc73dbe7be3ad4d4ee9a59b71e42f461a528"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/81b3374944d201872cfcf82730a7860f8e7c31dd"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/aae109414a57ab4164218f36e2e4a17f027fcaaa"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/b0c89ef025562161242a7c19b213bd6b272e93df"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/da75dec7c6617bddad418159ffebcb133f008262"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/e4f4db47794c9f474b184ee1418f42e6a07412b6"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H",
"type": "CVSS_V3"
}
]
}
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