[PATCH 6.19 43/49] sched/mmcid: Dont assume CID is CPU owned on mode switch

Greg Kroah-Hartman
2026-02-13T13:51:21Z

From: Thomas Gleixner <tglx@kernel.org>

commit 1e83ccd5921a610ef409a7d4e56db27822b4ea39 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/sched/core.c | 7 +++----
kernel/sched/sched.h | 6 ++++--
2 files changed, 7 insertions(+), 6 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10729,10 +10729,9 @@ void sched_mm_cid_exit(struct task_struc
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));
}
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -3758,8 +3758,10 @@ static __always_inline void mm_unset_cid
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)