ghsa-w3rc-7c7w-5vq5
Vulnerability from github
In the Linux kernel, the following vulnerability has been resolved:
bpf: Defer work in bpf_timer_cancel_and_free
Currently, the same case as previous patch (two timer callbacks trying to cancel each other) can be invoked through bpf_map_update_elem as well, or more precisely, freeing map elements containing timers. Since this relies on hrtimer_cancel as well, it is prone to the same deadlock situation as the previous patch.
It would be sufficient to use hrtimer_try_to_cancel to fix this problem, as the timer cannot be enqueued after async_cancel_and_free. Once async_cancel_and_free has been done, the timer must be reinitialized before it can be armed again. The callback running in parallel trying to arm the timer will fail, and freeing bpf_hrtimer without waiting is sufficient (given kfree_rcu), and bpf_timer_cb will return HRTIMER_NORESTART, preventing the timer from being rearmed again.
However, there exists a UAF scenario where the callback arms the timer before entering this function, such that if cancellation fails (due to timer callback invoking this routine, or the target timer callback running concurrently). In such a case, if the timer expiration is significantly far in the future, the RCU grace period expiration happening before it will free the bpf_hrtimer state and along with it the struct hrtimer, that is enqueued.
Hence, it is clear cancellation needs to occur after async_cancel_and_free, and yet it cannot be done inline due to deadlock issues. We thus modify bpf_timer_cancel_and_free to defer work to the global workqueue, adding a work_struct alongside rcu_head (both used at different points of time, so can share space).
Update existing code comments to reflect the new state of affairs.
{
"affected": [],
"aliases": [
"CVE-2024-41045"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2024-07-29T15:15:12Z",
"severity": null
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nbpf: Defer work in bpf_timer_cancel_and_free\n\nCurrently, the same case as previous patch (two timer callbacks trying\nto cancel each other) can be invoked through bpf_map_update_elem as\nwell, or more precisely, freeing map elements containing timers. Since\nthis relies on hrtimer_cancel as well, it is prone to the same deadlock\nsituation as the previous patch.\n\nIt would be sufficient to use hrtimer_try_to_cancel to fix this problem,\nas the timer cannot be enqueued after async_cancel_and_free. Once\nasync_cancel_and_free has been done, the timer must be reinitialized\nbefore it can be armed again. The callback running in parallel trying to\narm the timer will fail, and freeing bpf_hrtimer without waiting is\nsufficient (given kfree_rcu), and bpf_timer_cb will return\nHRTIMER_NORESTART, preventing the timer from being rearmed again.\n\nHowever, there exists a UAF scenario where the callback arms the timer\nbefore entering this function, such that if cancellation fails (due to\ntimer callback invoking this routine, or the target timer callback\nrunning concurrently). In such a case, if the timer expiration is\nsignificantly far in the future, the RCU grace period expiration\nhappening before it will free the bpf_hrtimer state and along with it\nthe struct hrtimer, that is enqueued.\n\nHence, it is clear cancellation needs to occur after\nasync_cancel_and_free, and yet it cannot be done inline due to deadlock\nissues. We thus modify bpf_timer_cancel_and_free to defer work to the\nglobal workqueue, adding a work_struct alongside rcu_head (both used at\n_different_ points of time, so can share space).\n\nUpdate existing code comments to reflect the new state of affairs.",
"id": "GHSA-w3rc-7c7w-5vq5",
"modified": "2024-07-29T15:30:42Z",
"published": "2024-07-29T15:30:42Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2024-41045"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/7aa5a19279c3639ae8b758b63f05d0c616a39fa1"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/a6fcd19d7eac1335eb76bc16b6a66b7f574d1d69"
}
],
"schema_version": "1.4.0",
"severity": []
}
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