nilfs2: add missing check for inode numbers on directory entries

commit bb76c6c upstream.

Syzbot reported that mounting and unmounting a specific pattern of
corrupted nilfs2 filesystem images causes a use-after-free of metadata
file inodes, which triggers a kernel bug in lru_add_fn().

As Jan Kara pointed out, this is because the link count of a metadata file
gets corrupted to 0, and nilfs_evict_inode(), which is called from iput(),
tries to delete that inode (ifile inode in this case).

The inconsistency occurs because directories containing the inode numbers
of these metadata files that should not be visible in the namespace are
read without checking.

Fix this issue by treating the inode numbers of these internal files as
errors in the sanity check helper when reading directory folios/pages.

Also thanks to Hillf Danton and Matthew Wilcox for their initial mm-layer
analysis.

Link: https://lkml.kernel.org/r/20240623051135.4180-3-konishi.ryusuke@gmail.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by: syzbot+d79afb004be235636ee8@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d79afb004be235636ee8
Reported-by: Jan Kara <jack@suse.cz>
Closes: https://lkml.kernel.org/r/20240617075758.wewhukbrjod5fp5o@quack3
Tested-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: konis

fs/nilfs2/dir.c

@@ -143,6 +143,9 @@ static bool nilfs_check_page(struct page *page)
 			goto Enamelen;
 		if (((offs + rec_len - 1) ^ offs) & ~(chunk_size-1))
 			goto Espan;
+		if (unlikely(p->inode &&
+			     NILFS_PRIVATE_INODE(le64_to_cpu(p->inode))))
+			goto Einumber;
 	}
 	if (offs != limit)
 		goto Eend;
@@ -168,6 +171,9 @@ static bool nilfs_check_page(struct page *page)
 	goto bad_entry;
 Espan:
 	error = "directory entry across blocks";
+	goto bad_entry;
+Einumber:
+	error = "disallowed inode number";
 bad_entry:
 	nilfs_error(sb,
 		    "bad entry in directory #%lu: %s - offset=%lu, inode=%lu, rec_len=%d, name_len=%d",

fs/nilfs2/nilfs.h

@@ -121,6 +121,11 @@ enum {
 	((ino) >= NILFS_FIRST_INO(sb) ||				\
 	 ((ino) < NILFS_USER_INO && (NILFS_SYS_INO_BITS & BIT(ino))))
 
+#define NILFS_PRIVATE_INODE(ino) ({					\
+	ino_t __ino = (ino);						\
+	((__ino) < NILFS_USER_INO && (__ino) != NILFS_ROOT_INO &&	\
+	 (__ino) != NILFS_SKETCH_INO); })
+
 /**
  * struct nilfs_transaction_info: context information for synchronization
  * @ti_magic: Magic number