[PATCH v5 02/20] fs: Fix possible UAF in mark_buffer_write_io_error()

When filesystem is freeing inode it calls mmb_invalidate() which removes
bhs from inode's metadata bh tracking and clears b_mmb for them. However
if the inode is getting deleted, we don't bother with calling mmb_sync()
before and thus these buffers can be under IO and we can be racing with
IO completion handler calling mark_buffer_write_io_error(). This race
can lead to mark_buffer_write_io_error() either hitting NULL pointer
reference or trying to operate on already freed inode. Protect the
mapping handling with RCU to make sure mmb and inode aren't freed before
we are done with them.

Reported-by: Sashiko <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
---
 fs/buffer.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 9af5f061a1f8..daaa6614a6d6 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1123,12 +1123,18 @@ EXPORT_SYMBOL(mark_buffer_dirty);
 
 void mark_buffer_write_io_error(struct buffer_head *bh)
 {
+	struct mapping_metadata_bhs *mmb;
+
 	set_buffer_write_io_error(bh);
 	/* FIXME: do we need to set this in both places? */
 	if (bh->b_folio && bh->b_folio->mapping)
 		mapping_set_error(bh->b_folio->mapping, -EIO);
-	if (bh->b_mmb)
-		mapping_set_error(bh->b_mmb->mapping, -EIO);
+	/* Protect us from mmb & inode getting freed while we work on it */
+	rcu_read_lock();
+	mmb = READ_ONCE(bh->b_mmb);
+	if (mmb)
+		mapping_set_error(mmb->mapping, -EIO);
+	rcu_read_unlock();
 }
 EXPORT_SYMBOL(mark_buffer_write_io_error);
 
-- 
2.51.0