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

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: [email protected] # 6.13+
Tested-by: Ivan Shapovalov <[email protected]>
Link: https://lore.kernel.org/linux-btrfs/[email protected]/
Reviewed-by: Johannes Thumshirn <[email protected]>
Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Filipe Manana <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: fdmanana

fs/btrfs/extent_map.c

@@ -1222,8 +1222,7 @@ static long btrfs_scan_root(struct btrfs_root *root, struct btrfs_em_shrink_ctx
 		fs_info->em_shrinker_last_ino = btrfs_ino(inode);
 		btrfs_add_delayed_iput(inode);
 
-		if (ctx->scanned >= ctx->nr_to_scan ||
-		    btrfs_fs_closing(inode->root->fs_info))
+		if (ctx->scanned >= ctx->nr_to_scan || btrfs_fs_closing(fs_info))
 			break;
 
 		cond_resched();