| From bippy-7c5fe7eed585 Mon Sep 17 00:00:00 2001 |
| From: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| To: <linux-cve-announce@vger.kernel.org> |
| Reply-to: <cve@kernel.org>, <linux-kernel@vger.kernel.org> |
| Subject: CVE-2025-21897: sched_ext: Fix pick_task_scx() picking non-queued tasks when it's called without balance() |
| |
| Description |
| =========== |
| |
| In the Linux kernel, the following vulnerability has been resolved: |
| |
| sched_ext: Fix pick_task_scx() picking non-queued tasks when it's called without balance() |
| |
| a6250aa251ea ("sched_ext: Handle cases where pick_task_scx() is called |
| without preceding balance_scx()") added a workaround to handle the cases |
| where pick_task_scx() is called without prececing balance_scx() which is due |
| to a fair class bug where pick_taks_fair() may return NULL after a true |
| return from balance_fair(). |
| |
| The workaround detects when pick_task_scx() is called without preceding |
| balance_scx() and emulates SCX_RQ_BAL_KEEP and triggers kicking to avoid |
| stalling. Unfortunately, the workaround code was testing whether @prev was |
| on SCX to decide whether to keep the task running. This is incorrect as the |
| task may be on SCX but no longer runnable. |
| |
| This could lead to a non-runnable task to be returned from pick_task_scx() |
| which cause interesting confusions and failures. e.g. A common failure mode |
| is the task ending up with (!on_rq && on_cpu) state which can cause |
| potential wakers to busy loop, which can easily lead to deadlocks. |
| |
| Fix it by testing whether @prev has SCX_TASK_QUEUED set. This makes |
| @prev_on_scx only used in one place. Open code the usage and improve the |
| comment while at it. |
| |
| The Linux kernel CVE team has assigned CVE-2025-21897 to this issue. |
| |
| |
| Affected and fixed versions |
| =========================== |
| |
| Issue introduced in 6.12 with commit a6250aa251eacaf3ebfcfe152a96a727fd483ecd and fixed in 6.12.18 with commit 5324c459f90d16b0c43a78b494c598915d782b7a |
| Issue introduced in 6.12 with commit a6250aa251eacaf3ebfcfe152a96a727fd483ecd and fixed in 6.13.6 with commit de60a31cb0bcacfaf9487546eac5e70e0a9c66d7 |
| Issue introduced in 6.12 with commit a6250aa251eacaf3ebfcfe152a96a727fd483ecd and fixed in 6.14 with commit 8fef0a3b17bb258130a4fcbcb5addf94b25e9ec5 |
| |
| Please see https://www.kernel.org for a full list of currently supported |
| kernel versions by the kernel community. |
| |
| Unaffected versions might change over time as fixes are backported to |
| older supported kernel versions. The official CVE entry at |
| https://cve.org/CVERecord/?id=CVE-2025-21897 |
| will be updated if fixes are backported, please check that for the most |
| up to date information about this issue. |
| |
| |
| Affected files |
| ============== |
| |
| The file(s) affected by this issue are: |
| kernel/sched/ext.c |
| |
| |
| Mitigation |
| ========== |
| |
| The Linux kernel CVE team recommends that you update to the latest |
| stable kernel version for this, and many other bugfixes. Individual |
| changes are never tested alone, but rather are part of a larger kernel |
| release. Cherry-picking individual commits is not recommended or |
| supported by the Linux kernel community at all. If however, updating to |
| the latest release is impossible, the individual changes to resolve this |
| issue can be found at these commits: |
| https://git.kernel.org/stable/c/5324c459f90d16b0c43a78b494c598915d782b7a |
| https://git.kernel.org/stable/c/de60a31cb0bcacfaf9487546eac5e70e0a9c66d7 |
| https://git.kernel.org/stable/c/8fef0a3b17bb258130a4fcbcb5addf94b25e9ec5 |
