md/raid5: Fix UAF on IO across the reshape position

If make_stripe_request() returns STRIPE_WAIT_RESHAPE,
raid5_make_request() will free the cloned bio. But raid5_make_request()
can call make_stripe_request() multiple times, writing to the various
stripes. If that bio got added to the toread or towrite lists of a
stripe disk in an earlier call to make_stripe_request(), then it's not
safe to just free the bio if a later part of it is found to cross the
reshape position. Doing so can lead to a UAF error, when bio_endio()
is called on the bio for the earlier stripes.

Instead, raid5_make_request() needs to wait until all parts of the bio
have called bio_endio(). To do this, bios that cross the reshape
position while the reshape can't make progress are flagged as needing to
wait for all parts to complete. When raid5_make_request() has a bio that
failed make_stripe_request() with STRIPE_WAIT_RESHAPE, it sets
bi->bi_private to a completion struct and waits for completion after
ending the bio.  When the bio_endio() is called for the last time on a
clone bio with bi->bi_private set, it wakes up the waiter. This
guarantees that raid5_make_request() doesn't return until the cloned bio
needing a retry for io across the reshape boundary is safely cleaned up.

There is a simple reproducer available at [1]. Compile the kernel with
KASAN for more useful reporting when the error is triggered (this is not
necessary to see the bug).

[1] https://gist.github.com/bmarzins/e48598824305cf2171289e47d7241fa5

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Reviewed-by: Xiao Ni <xni@redhat.com>
Link: https://lore.kernel.org/r/20260408043548.1695157-1-bmarzins@redhat.com
Signed-off-by: Yu Kuai <yukuai@fnnas.com>

3 files changed, 14 insertions, 25 deletions

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 5fb5ae8368ba..8656f2eff40d 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -9341,9 +9341,11 @@ static void md_bitmap_end(struct mddev *mddev, struct md_io_clone *md_io_clone)
 
 static void md_end_clone_io(struct bio *bio)
 {
-	struct md_io_clone *md_io_clone = bio->bi_private;
+	struct md_io_clone *md_io_clone = container_of(bio, struct md_io_clone,
+						       bio_clone);
 	struct bio *orig_bio = md_io_clone->orig_bio;
 	struct mddev *mddev = md_io_clone->mddev;
+	struct completion *reshape_completion = bio->bi_private;
 
 	if (bio_data_dir(orig_bio) == WRITE && md_bitmap_enabled(mddev, false))
 		md_bitmap_end(mddev, md_io_clone);
@@ -9355,7 +9357,10 @@ static void md_end_clone_io(struct bio *bio)
 		bio_end_io_acct(orig_bio, md_io_clone->start_time);
 
 	bio_put(bio);
-	bio_endio(orig_bio);
+	if (unlikely(reshape_completion))
+		complete(reshape_completion);
+	else
+		bio_endio(orig_bio);
 	percpu_ref_put(&mddev->active_io);
 }
 
@@ -9380,7 +9385,7 @@ static void md_clone_bio(struct mddev *mddev, struct bio **bio)
 	}
 
 	clone->bi_end_io = md_end_clone_io;
-	clone->bi_private = md_io_clone;
+	clone->bi_private = NULL;
 	*bio = clone;
 }
 
@@ -9391,26 +9396,6 @@ void md_account_bio(struct mddev *mddev, struct bio **bio)
 }
 EXPORT_SYMBOL_GPL(md_account_bio);
 
-void md_free_cloned_bio(struct bio *bio)
-{
-	struct md_io_clone *md_io_clone = bio->bi_private;
-	struct bio *orig_bio = md_io_clone->orig_bio;
-	struct mddev *mddev = md_io_clone->mddev;
-
-	if (bio_data_dir(orig_bio) == WRITE && md_bitmap_enabled(mddev, false))
-		md_bitmap_end(mddev, md_io_clone);
-
-	if (bio->bi_status && !orig_bio->bi_status)
-		orig_bio->bi_status = bio->bi_status;
-
-	if (md_io_clone->start_time)
-		bio_end_io_acct(orig_bio, md_io_clone->start_time);
-
-	bio_put(bio);
-	percpu_ref_put(&mddev->active_io);
-}
-EXPORT_SYMBOL_GPL(md_free_cloned_bio);
-
 /* md_allow_write(mddev)
  * Calling this ensures that the array is marked 'active' so that writes
  * may proceed without blocking.  It is important to call this before
diff --git a/drivers/md/md.h b/drivers/md/md.h
index d6f5482e2479..12d86aa7e6f9 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -920,7 +920,6 @@ extern void md_finish_reshape(struct mddev *mddev);
 void md_submit_discard_bio(struct mddev *mddev, struct md_rdev *rdev,
 			struct bio *bio, sector_t start, sector_t size);
 void md_account_bio(struct mddev *mddev, struct bio **bio);
-void md_free_cloned_bio(struct bio *bio);
 
 extern bool __must_check md_flush_request(struct mddev *mddev, struct bio *bio);
 void md_write_metadata(struct mddev *mddev, struct md_rdev *rdev,
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 6e79829c5acb..0d76e82f4506 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -6217,7 +6217,12 @@ static bool raid5_make_request(struct mddev *mddev, struct bio * bi)
 
 	mempool_free(ctx, conf->ctx_pool);
 	if (res == STRIPE_WAIT_RESHAPE) {
-		md_free_cloned_bio(bi);
+		DECLARE_COMPLETION_ONSTACK(done);
+		WRITE_ONCE(bi->bi_private, &done);
+
+		bio_endio(bi);
+
+		wait_for_completion(&done);
 		return false;
 	}