[PATCH] ext4: fix use-after-free in ext4 delayed I/O completion

[Longxing Li <coregee2000@gmail.com>]

ext4_add_complete_io() queues the inode's i_rsv_conversion_work without
holding an extra reference to the inode. If the inode is unlinked and
evicted before the delayed work runs, the inode slab object gets freed.
When ext4_do_flush_completed_IO() later accesses io_end->inode, it is
accessing a freed/reallocated object -- a use-after-free detected by KASAN.

Take igrab(inode) when first queueing the work (only when list is empty to
avoid duplicate references), and drop the reference in
ext4_do_flush_completed_IO() after processing all io_ends.
A per-inode bit (i_rsv_need_iput) tracks whether the extra reference
was taken.

This is a defense-in-depth fix complementary to the upstream fix
c678bdc99875 ("ext4: fix inode use after free in ext4_end_io_rsv_work()")
which adds consistency checks in ext4_io_end_defer_completion(). Both fixes
address the same root race but from different angles.

Fixes: ce51afb8cc5e ("ext4: abort journal on data writeback failure if in data_err=abort mode")
Reported-by: Longxing Li <coregee2000@gmail.com>
Signed-off-by: Longxing Li <coregee2000@gmail.com>
---
 fs/ext4/ext4.h    |  2 ++
 fs/ext4/page-io.c | 15 +++++++++++++--
 fs/ext4/super.c   |  1 +
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 56112f201cac..239e6c0a3d3c 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1179,6 +1179,8 @@ struct ext4_inode_info {
 
 	/* Lock protecting lists below */
 	spinlock_t i_completed_io_lock;
+	/* Track if ext4_add_complete_io() took an extra inode reference. */
+	unsigned short i_rsv_need_iput:1;
 	/*
 	 * Completed IOs that need unwritten extents handling and have
 	 * transaction reserved
diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
index 39abfeec5f36..294c0bcbbb7f 100644
--- a/fs/ext4/page-io.c
+++ b/fs/ext4/page-io.c
@@ -262,8 +262,12 @@ static void ext4_add_complete_io(ext4_io_end_t *io_end)
 
 	spin_lock_irqsave(&ei->i_completed_io_lock, flags);
 	wq = sbi->rsv_conversion_wq;
-	if (list_empty(&ei->i_rsv_conversion_list))
-		queue_work(wq, &ei->i_rsv_conversion_work);
+	if (list_empty(&ei->i_rsv_conversion_list)) {
+		if (igrab(io_end->inode)) {
+			ei->i_rsv_need_iput = 1;
+			queue_work(wq, &ei->i_rsv_conversion_work);
+		}
+	}
 	list_add_tail(&io_end->list, &ei->i_rsv_conversion_list);
 	spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
 }
@@ -291,6 +295,13 @@ static int ext4_do_flush_completed_IO(struct inode *inode,
 		if (unlikely(!ret && err))
 			ret = err;
 	}
+
+	/* Release inode reference from ext4_add_complete_io. */
+	if (ei->i_rsv_need_iput) {
+		ei->i_rsv_need_iput = 0;
+		iput(&ei->vfs_inode);
+	}
+
 	return ret;
 }
 
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 87205660c5d0..10a5d863d9b9 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1418,6 +1418,7 @@ static struct inode *ext4_alloc_inode(struct super_block *sb)
 	ei->jinode = NULL;
 	INIT_LIST_HEAD(&ei->i_rsv_conversion_list);
 	spin_lock_init(&ei->i_completed_io_lock);
+	ei->i_rsv_need_iput = 0;
 	ei->i_sync_tid = 0;
 	ei->i_datasync_tid = 0;
 	INIT_WORK(&ei->i_rsv_conversion_work, ext4_end_io_rsv_work);
-- 
2.34.1