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 <libaokun1@xxxxxxxxxx> > --- > 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 >
