GHSA-8J25-5VWV-HM2F
Vulnerability from github – Published: 2024-02-27 12:31 – Updated: 2024-04-10 15:30In the Linux kernel, the following vulnerability has been resolved:
locking/qrwlock: Fix ordering in queued_write_lock_slowpath()
While this code is executed with the wait_lock held, a reader can acquire the lock without holding wait_lock. The writer side loops checking the value with the atomic_cond_read_acquire(), but only truly acquires the lock when the compare-and-exchange is completed successfully which isn’t ordered. This exposes the window between the acquire and the cmpxchg to an A-B-A problem which allows reads following the lock acquisition to observe values speculatively before the write lock is truly acquired.
We've seen a problem in epoll where the reader does a xchg while holding the read lock, but the writer can see a value change out from under it.
Writer | Reader
ep_scan_ready_list() | |- write_lock_irq() | |- queued_write_lock_slowpath() | |- atomic_cond_read_acquire() | | read_lock_irqsave(&ep->lock, flags); --> (observes value before unlock) | chain_epi_lockless() | | epi->next = xchg(&ep->ovflist, epi); | | read_unlock_irqrestore(&ep->lock, flags); | | | atomic_cmpxchg_relaxed() | |-- READ_ONCE(ep->ovflist); |
A core can order the read of the ovflist ahead of the atomic_cmpxchg_relaxed(). Switching the cmpxchg to use acquire semantics addresses this issue at which point the atomic_cond_read can be switched to use relaxed semantics.
[peterz: use try_cmpxchg()]
{
"affected": [],
"aliases": [
"CVE-2021-46921"
],
"database_specific": {
"cwe_ids": [
"CWE-668"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2024-02-27T10:15:06Z",
"severity": "MODERATE"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nlocking/qrwlock: Fix ordering in queued_write_lock_slowpath()\n\nWhile this code is executed with the wait_lock held, a reader can\nacquire the lock without holding wait_lock. The writer side loops\nchecking the value with the atomic_cond_read_acquire(), but only truly\nacquires the lock when the compare-and-exchange is completed\nsuccessfully which isn\u2019t ordered. This exposes the window between the\nacquire and the cmpxchg to an A-B-A problem which allows reads\nfollowing the lock acquisition to observe values speculatively before\nthe write lock is truly acquired.\n\nWe\u0027ve seen a problem in epoll where the reader does a xchg while\nholding the read lock, but the writer can see a value change out from\nunder it.\n\n Writer | Reader\n --------------------------------------------------------------------------------\n ep_scan_ready_list() |\n |- write_lock_irq() |\n |- queued_write_lock_slowpath() |\n\t|- atomic_cond_read_acquire() |\n\t\t\t\t | read_lock_irqsave(\u0026ep-\u003elock, flags);\n --\u003e (observes value before unlock) | chain_epi_lockless()\n | | epi-\u003enext = xchg(\u0026ep-\u003eovflist, epi);\n | | read_unlock_irqrestore(\u0026ep-\u003elock, flags);\n | |\n | atomic_cmpxchg_relaxed() |\n |-- READ_ONCE(ep-\u003eovflist); |\n\nA core can order the read of the ovflist ahead of the\natomic_cmpxchg_relaxed(). Switching the cmpxchg to use acquire\nsemantics addresses this issue at which point the atomic_cond_read can\nbe switched to use relaxed semantics.\n\n[peterz: use try_cmpxchg()]",
"id": "GHSA-8j25-5vwv-hm2f",
"modified": "2024-04-10T15:30:32Z",
"published": "2024-02-27T12:31:09Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2021-46921"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/5902f9453a313be8fe78cbd7e7ca9dba9319fc6e"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/82808cc026811fbc3ecf0c0b267a12a339eead56"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/82fa9ced35d88581cffa4a1c856fc41fca96d80a"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/d558fcdb17139728347bccc60a16af3e639649d2"
}
],
"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:N",
"type": "CVSS_V3"
}
]
}
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