nilfs2: fix use-after-free bug of nilfs_root in nilfs_evict_inode()

During unmount process of nilfs2, nothing holds nilfs_root structure after
nilfs2 detaches its writer in nilfs_detach_log_writer().  However, since
nilfs_evict_inode() uses nilfs_root for some cleanup operations, it may
cause use-after-free read if inodes are left in "garbage_list" and
released by nilfs_dispose_list() at the end of nilfs_detach_log_writer().

Fix this issue by modifying nilfs_evict_inode() to only clear inode
without additional metadata changes that use nilfs_root if the file system
is degraded to read-only or the writer is detached.

Link: https://lkml.kernel.org/r/20230509152956.8313-1-konishi.ryusuke@gmail.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by: syzbot+78d4495558999f55d1da@syzkaller.appspotmail.com
Closes: https://lkml.kernel.org/r/00000000000099e5ac05fb1c3b85@google.com
Tested-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/nilfs2/inode.c b/fs/nilfs2/inode.c
index 1310d2d5feb38778188d894fa0bc189c4a7f30e3..a8ce522ac747931575104447288cdbe2ed317d0f 100644 (file)
--- a/fs/nilfs2/inode.c
+++ b/fs/nilfs2/inode.c
@@ -917,6 +917,7 @@ void nilfs_evict_inode(struct inode *inode)
        struct nilfs_transaction_info ti;
        struct super_block *sb = inode->i_sb;
        struct nilfs_inode_info *ii = NILFS_I(inode);
+       struct the_nilfs *nilfs;
        int ret;
 
        if (inode->i_nlink || !ii->i_root || unlikely(is_bad_inode(inode))) {
@@ -929,6 +930,23 @@ void nilfs_evict_inode(struct inode *inode)
 
        truncate_inode_pages_final(&inode->i_data);
 
+       nilfs = sb->s_fs_info;
+       if (unlikely(sb_rdonly(sb) || !nilfs->ns_writer)) {
+               /*
+                * If this inode is about to be disposed after the file system
+                * has been degraded to read-only due to file system corruption
+                * or after the writer has been detached, do not make any
+                * changes that cause writes, just clear it.
+                * Do this check after read-locking ns_segctor_sem by
+                * nilfs_transaction_begin() in order to avoid a race with
+                * the writer detach operation.
+                */
+               clear_inode(inode);
+               nilfs_clear_inode(inode);
+               nilfs_transaction_abort(sb);
+               return;
+       }
+
        /* TODO: some of the following operations may fail.  */
        nilfs_truncate_bmap(ii, 0);
        nilfs_mark_inode_dirty(inode);