diff options
| author | Thomas Gleixner <tglx@kernel.org> | 2026-02-10 17:20:51 +0100 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2026-02-11 12:59:56 -0800 |
| commit | 1e83ccd5921a610ef409a7d4e56db27822b4ea39 (patch) | |
| tree | ad4ec05303b8afcf7653827874599894a09b2ab9 /kernel | |
| parent | 939faf71cf7ca9ab3d1bd2912ac0e203d4d7156a (diff) | |
Shinichiro reported a KASAN UAF, which is actually an out of bounds access
in the MMCID management code.
CPU0 CPU1
T1 runs in userspace
T0: fork(T4) -> Switch to per CPU CID mode
fixup() set MM_CID_TRANSIT on T1/CPU1
T4 exit()
T3 exit()
T2 exit()
T1 exit() switch to per task mode
---> Out of bounds access.
As T1 has not scheduled after T0 set the TRANSIT bit, it exits with the
TRANSIT bit set. sched_mm_cid_remove_user() clears the TRANSIT bit in
the task and drops the CID, but it does not touch the per CPU storage.
That's functionally correct because a CID is only owned by the CPU when
the ONCPU bit is set, which is mutually exclusive with the TRANSIT flag.
Now sched_mm_cid_exit() assumes that the CID is CPU owned because the
prior mode was per CPU. It invokes mm_drop_cid_on_cpu() which clears the
not set ONCPU bit and then invokes clear_bit() with an insanely large
bit number because TRANSIT is set (bit 29).
Prevent that by actually validating that the CID is CPU owned in
mm_drop_cid_on_cpu().
Fixes: 007d84287c74 ("sched/mmcid: Drop per CPU CID immediately when switching to per task mode")
Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Cc: stable@vger.kernel.org
Closes: https://lore.kernel.org/aYsZrixn9b6s_2zL@shinmob
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'kernel')
| -rw-r--r-- | kernel/sched/core.c | 7 | ||||
| -rw-r--r-- | kernel/sched/sched.h | 6 |
2 files changed, 7 insertions, 6 deletions
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 7c8b769c0d0d..759777694c78 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -10788,10 +10788,9 @@ void sched_mm_cid_exit(struct task_struct *t) return; /* * Mode change. The task has the CID unset - * already. The CPU CID is still valid and - * does not have MM_CID_TRANSIT set as the - * mode change has just taken effect under - * mm::mm_cid::lock. Drop it. + * already and dealt with an eventually set + * TRANSIT bit. If the CID is owned by the CPU + * then drop it. */ mm_drop_cid_on_cpu(mm, this_cpu_ptr(mm->mm_cid.pcpu)); } diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index e51bfa3586fa..b82fb70a9d54 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -3813,8 +3813,10 @@ static __always_inline void mm_unset_cid_on_task(struct task_struct *t) static __always_inline void mm_drop_cid_on_cpu(struct mm_struct *mm, struct mm_cid_pcpu *pcp) { /* Clear the ONCPU bit, but do not set UNSET in the per CPU storage */ - pcp->cid = cpu_cid_to_cid(pcp->cid); - mm_drop_cid(mm, pcp->cid); + if (cid_on_cpu(pcp->cid)) { + pcp->cid = cpu_cid_to_cid(pcp->cid); + mm_drop_cid(mm, pcp->cid); + } } static inline unsigned int __mm_get_cid(struct mm_struct *mm, unsigned int max_cids) |