btrfs: fix use-after-free on inode when scanning root during em shrinking

[ Upstream commit 59f3703 ]

At btrfs_scan_root() we are accessing the inode's root (and fs_info) in a
call to btrfs_fs_closing() after we have scheduled the inode for a delayed
iput, and that can result in a use-after-free on the inode in case the
cleaner kthread does the iput before we dereference the inode in the call
to btrfs_fs_closing().

Fix this by using the fs_info stored already in a local variable instead
of doing inode->root->fs_info.

Fixes: 1020443 ("btrfs: make the extent map shrinker run asynchronously as a work queue job")
CC: stable@vger.kernel.org # 6.13+
Tested-by: Ivan Shapovalov <intelfx@intelfx.name>
Link: https://lore.kernel.org/linux-btrfs/0414d690ac5680d0d77dfc930606cdc36e42e12f.camel@intelfx.name/
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: Sasha Levin

fs/btrfs/extent_map.c

@@ -1250,6 +1250,7 @@ static struct btrfs_inode *find_first_inode_to_shrink(struct btrfs_root *root,
 
 static long btrfs_scan_root(struct btrfs_root *root, struct btrfs_em_shrink_ctx *ctx)
 {
+	struct btrfs_fs_info *fs_info = root->fs_info;
 	struct btrfs_inode *inode;
 	long nr_dropped = 0;
 	u64 min_ino = ctx->last_ino + 1;
@@ -1264,7 +1265,7 @@ static long btrfs_scan_root(struct btrfs_root *root, struct btrfs_em_shrink_ctx
 		iput(&inode->vfs_inode);
 
 		if (ctx->scanned >= ctx->nr_to_scan ||
-		    btrfs_fs_closing(inode->root->fs_info))
+		    btrfs_fs_closing(fs_info))
 			break;
 
 		cond_resched();