From: Ming Lei ming.lei@redhat.com
[ Upstream commit 94209d27d14104ed828ca88cd5403a99162fe51a ]
Use q->elevator with ->elevator_lock held in elv_iosched_show(), since the local cached elevator reference may become stale after getting ->elevator_lock.
Reviewed-by: Hannes Reinecke hare@suse.de Reviewed-by: Nilay Shroff nilay@linux.ibm.com Reviewed-by: Christoph Hellwig hch@lst.de Signed-off-by: Ming Lei ming.lei@redhat.com Link: https://lore.kernel.org/r/20250505141805.2751237-5-ming.lei@redhat.com Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Sasha Levin sashal@kernel.org ---
The environment variable is set. Now let me analyze the commit mentioned in the prompt: **Subject**: block: use q->elevator with ->elevator_lock held in elv_iosched_show() **Analysis of the code changes**: The commit fixes a race condition in the block elevator subsystem in `elv_iosched_show()`. Here's what it does: 1. **Removes local cached variable**: The commit removes the local variable `struct elevator_queue *eq = q->elevator;` that was caching the elevator queue reference. 2. **Accesses q->elevator under lock**: Instead of using the cached `eq->type`, it now directly accesses `q->elevator->type` while holding the `->elevator_lock` mutex. 3. **Fixes race condition**: The old code had a time-of-check-time-of-use (TOCTOU) race where the cached `eq` could become stale after acquiring the `->elevator_lock`, but before actually using `eq->type`. **Analysis of backport suitability**: Looking at similar commits in the reference history: - Similar Commit #4 was marked YES for backporting because it fixed a real bug with clear user impact - The other similar commits were marked NO because they were code cleanups/optimizations without fixing actual bugs This commit: 1. **Fixes a real race condition bug** - The cached elevator reference could become invalid between when it's stored and when it's used 2. **Has clear user impact** - Race conditions in the elevator code could lead to crashes or memory corruption when users access `/sys/block/*/queue/scheduler` 3. **Is a small, contained fix** - Only changes a few lines in one function 4. **Has minimal regression risk** - Simply ensures proper locking is used when accessing shared data 5. **Follows stable tree rules** - It's an important bugfix with minimal risk The commit message explicitly states the problem: "the local cached elevator reference may become stale after getting ->elevator_lock" - this is a classic race condition that could cause system instability. **YES** This commit should be backported to stable kernel trees. It fixes a race condition in the block elevator subsystem where a cached elevator reference could become stale between the time it's stored and when it's actually used, even after acquiring the elevator lock. The fix is minimal and safe - it simply ensures that `q->elevator->type` is accessed directly while holding the lock rather than using a potentially stale cached reference. This prevents potential crashes or memory corruption when users access the scheduler interface in `/sys/block/*/queue/scheduler`. The change is small, contained to a single function, has clear bug-fixing intent, and follows stable tree criteria of being an important bugfix with minimal regression risk.
block/elevator.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/block/elevator.c b/block/elevator.c index cd2ce49216010..1cc89c49e4cc9 100644 --- a/block/elevator.c +++ b/block/elevator.c @@ -734,7 +734,6 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf, ssize_t elv_iosched_show(struct gendisk *disk, char *name) { struct request_queue *q = disk->queue; - struct elevator_queue *eq = q->elevator; struct elevator_type *cur = NULL, *e; int len = 0;
@@ -742,7 +741,7 @@ ssize_t elv_iosched_show(struct gendisk *disk, char *name) len += sprintf(name+len, "[none] "); } else { len += sprintf(name+len, "none "); - cur = eq->type; + cur = q->elevator->type; }
spin_lock(&elv_list_lock);