On Fri, Aug 05, 2022 at 08:39:47PM +0800, Baokun Li wrote:
> I caught a null-ptr-deref bug as follows:
> ==================================================================
> KASAN: null-ptr-deref in range [0x0000000000000068-0x000000000000006f]
> CPU: 1 PID: 1589 Comm: umount Not tainted 5.10.0-02219-dirty #339
> RIP: 0010:ext4_write_info+0x53/0x1b0
> [...]
> Call Trace:
> dquot_writeback_dquots+0x341/0x9a0
> ext4_sync_fs+0x19e/0x800
> __sync_filesystem+0x83/0x100
> sync_filesystem+0x89/0xf0
> generic_shutdown_super+0x79/0x3e0
> kill_block_super+0xa1/0x110
> deactivate_locked_super+0xac/0x130
> deactivate_super+0xb6/0xd0
> cleanup_mnt+0x289/0x400
> __cleanup_mnt+0x16/0x20
> task_work_run+0x11c/0x1c0
> exit_to_user_mode_prepare+0x203/0x210
> syscall_exit_to_user_mode+0x5b/0x3a0
> do_syscall_64+0x59/0x70
> entry_SYSCALL_64_after_hwframe+0x44/0xa9
> ==================================================================
>
> Above issue may happen as follows:
> -------------------------------------
> exit_to_user_mode_prepare
> task_work_run
> __cleanup_mnt
> cleanup_mnt
> deactivate_super
> deactivate_locked_super
> kill_block_super
> generic_shutdown_super
> shrink_dcache_for_umount
> dentry = sb->s_root
> sb->s_root = NULL <--- Here set NULL
> sync_filesystem
> __sync_filesystem
> sb->s_op->sync_fs > ext4_sync_fs
> dquot_writeback_dquots
> sb->dq_op->write_info > ext4_write_info
> ext4_journal_start(d_inode(sb->s_root), EXT4_HT_QUOTA, 2)
> d_inode(sb->s_root)
> s_root->d_inode <--- Null pointer dereference
>
> To solve this problem, we use ext4_journal_start_sb directly
> to avoid s_root being used.
Are we syncing the file system after the superblock shutdown and getting
away with it? This does not look good. Do you have a reproducer?
Thanks!
-Lukas
>
> Signed-off-by: Baokun Li <[email protected]>
> ---
> fs/ext4/super.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 9a66abcca1a8..0ce4565422f6 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -6653,7 +6653,7 @@ static int ext4_write_info(struct super_block *sb, int type)
> handle_t *handle;
>
> /* Data block + inode block */
> - handle = ext4_journal_start(d_inode(sb->s_root), EXT4_HT_QUOTA, 2);
> + handle = ext4_journal_start_sb(sb, EXT4_HT_QUOTA, 2);
> if (IS_ERR(handle))
> return PTR_ERR(handle);
> ret = dquot_commit_info(sb, type);
> --
> 2.31.1
>