On Fri, Dec 16, 2022 at 11:41:16AM +0800, Jun Nie wrote: > This patch[1] is needed on linux-5.15.y because the panic[2] is also found on > linux-5.15.y when debugging bug[3]. Back ported patch[4] is confirmed to fix > the bug on linux-5.15.y in the latest test of page[3]. Maybe back port on more > branches is needed per patch comments. This is not a proper fix for the syzkaller report being reported here: https://syzkaller.appspot.com/bug?id=3613786cb88c93aa1c6a279b1df6a7b201347d08 It's true that the reproducer will no longer trigger, but that's just because the reproducer is just exiting early because it is passing in a mount option which is no longer being accepted. In fact, that mount option is completely unneeded and it's a failing of syzkaller that it doesn't adequately minimize the reproducer by trying to remove various random mount options that are not actually needed. For example, running the reproducer will trigger warnings like this: EXT4-fs: Ignoring removed nobh option If we modify the kernel to simply ignore nouser_xattr, then the reproducer will still trigger. So this is not the right patch to backport. It's important that people who are trying to fix syzkaller bugs understand what is fundamentally going on, instead of using blunt force patches that simple paper over the issue. Please remember that syzkaller is supposed to help us improve the kernel, and it's not just about trying to reduce the count of open syzkaller reports for its own sake. (This is really much more of a quality of implementation issue, since this is not something that would really ever trigger in real life, nor is it really a security issue --- despite some people thinking that all syzkaller reports are actually security issues, and we must run around like chickens with their heads cut off and until they are all fixed.) The real root cause of the problem is that the file system is getting mounted with these mount options: nouser_xattr,acl,debug_want_extra_isize=0x0000000000000080,lazytime,nobh,quota Of which nouser_attr, acl, nobh, and quota are completely pointless. It's also **super** unfortunate that the reproducer isn't written in C, but this horrible psuedo-assimply language: memcpy( (void*)0x20000000, "\x6e\x6f\x75\x73\x65\x72\x5f\x78\x61\x74\x74\x72\x2c\x61\x63\x6c\x2c\x64" "\x65\x62\x75\x67\x5f\x77\x61\x6e\x74\x5f\x65\x78\x74\x72\x61\x5f\x69\x73" "\x69\x7a\x65\x3d\x30\x78\x30\x30\x30\x30\x30\x30\x30\x30\x30\x30\x30\x30" "\x30\x30\x38\x30\x2c\x6c\x61\x7a\x79\x74\x69\x6d\x65\x2c\x6e\x6f\x62\x68" "\x2c\x71\x75\x6f\x74\x61\x2c\x00\x3d\x93\x09\x61\x36\x5d\x73\x58\x9c", 89); ... syz_mount_image(0x20000440, 0x20000480, 0x1e, 0x20000000, 2, 0x427, ^^^^^^^^^^ 0x200004c0); (And again, this is stuff that I've complained to the syzkaller team for years and years and years as being fundamentally developer hostile and disrespects the time of upstream maintainers. ARGH!!!!) Anyway..... So now let's look at the stack trace: ext4_xattr_block_set+0x8f8/0x3820 fs/ext4/xattr.c:1971 ext4_xattr_move_to_block fs/ext4/xattr.c:2603 [inline] ext4_xattr_make_inode_space fs/ext4/xattr.c:2672 [inline] ext4_expand_extra_isize_ea+0x1591/0x1f30 fs/ext4/xattr.c:2764 __ext4_expand_extra_isize+0x29e/0x3d0 fs/ext4/inode.c:5826 ext4_try_to_expand_extra_isize fs/ext4/inode.c:5869 [inline] __ext4_mark_inode_dirty+0x4bf/0x7a0 fs/ext4/inode.c:5947 ext4_dirty_inode+0xbc/0x100 fs/ext4/inode.c:5979 __mark_inode_dirty+0x1f9/0x9d0 fs/fs-writeback.c:2431 mark_inode_dirty_sync include/linux/fs.h:2429 [inline] iput+0x155/0x7d0 fs/inode.c:1686 dentry_unlink_inode+0x349/0x430 fs/dcache.c:376 __dentry_kill+0x3e2/0x5d0 fs/dcache.c:582 shrink_dentry_list+0x379/0x4d0 fs/dcache.c:1176 shrink_dcache_parent+0xcd/0x350 do_one_tree fs/dcache.c:1657 [inline] shrink_dcache_for_umount+0x7c/0x1a0 fs/dcache.c:1674 generic_shutdown_super+0x69/0x2d0 fs/super.c:447 kill_block_super+0x80/0xe0 fs/super.c:1395 Because lazytime is enabled, after running the reproducer under strace, what happens is that inode #12 gets touched so its access time is modified, but because lazytime is enabled, we don't actually update the on-disk until we actually unmount the superblock. That's why generic_shutdown_super() is in the stack trace. At that point, when we shrink the dentry cache, when we eject the inode from memory, iput() needs to update the on-disk inode with the updated atime. So far, so good. But then we call ext4_dirty_inode(), and then that interacts with the "debug_want_extra_isize-=128" mount option. So at this point, we try to expand inode's extra isize space, and in order to do that we have to move some extended attributes. Unfortunately, how ext4 currently does this is a bit stupid, and it reads the contents of the ea_inode into memory, deletes the ea_inode and then creates a new ea_inode. That works, but it's horribly inefficient, and **that's*** what we should actually fix. Unfortunately, because we try to create a new ea_inode, when ext4_xattr_block_set() calls the static function (which gets inlined) ext4_xattr_inode_create(), and at that point, the call to ext4_new_inode trips over the fact that the file system is being unmoutned, and sb->s_root has already been set to NULL. So this is what actually goes *boom*: ea_inode = ext4_new_inode(handle, inode->i_sb->s_root->d_inode, ^^^^^^^^^^^^^^^^^^^ NULL ptr, oops! S_IFREG | 0600, NULL, inode->i_ino + 1, owner, EXT4_EA_INODE_FL); We can prove this is the issue by using the following debugging patch, which prevents the reproducer from triggering after prining the "fs being unmouinted" message: --- a/fs/ext4/xattr.c +++ b/fs/ext4/xattr.c @@ -2697,6 +2697,13 @@ int ext4_expand_extra_isize_ea(struct inode *inode, int new_extra_isize, int s_min_extra_isize = le16_to_cpu(sbi->s_es->s_min_extra_isize); int isize_diff; /* How much do we need to grow i_extra_isize */ + pr_err("ext4_expand_extra_isize_ea ino %lu new_extra_isize %d curr %d\n", + inode->i_ino, new_extra_isize, EXT4_I(inode)->i_extra_isize); + if (inode->i_sb->s_root == NULL) { + pr_err("ext4_expand_extra_isize_ea: fs being unmounted\n"); + return -EINVAL; + } + retry: isize_diff = new_extra_isize - EXT4_I(inode)->i_extra_isize; if (EXT4_I(inode)->i_extra_isize >= new_extra_isize) Fixing this the clean and proper way, which is by making ext4_xattr_move_to_block() more intelligent/efficient, is left as an exercise to the reader. Cheers, - Ted P.S. Note that this fix is actually needed for the current upstream kernel; the reproducer will trigger in 6.1, although we need to either modify the reproducer to drop the completely pointless nouser_xattr mount option (which is a bit painful since the !@?! mount options is obfuscated by virtue of being in hex for no particular good reason) or by hacking the kernel to ignore that mount options, via a patch like this: --- a/fs/ext4/super.c +++ b/fs/ext4/super.c @@ -1658,6 +1658,7 @@ static const struct fs_parameter_spec ext4_param_specs[] = { fsparam_flag ("oldalloc", Opt_removed), fsparam_flag ("orlov", Opt_removed), fsparam_flag ("user_xattr", Opt_user_xattr), + fsparam_flag ("nouser_xattr", Opt_removed), fsparam_flag ("acl", Opt_acl), fsparam_flag ("norecovery", Opt_noload), fsparam_flag ("noload", Opt_noload),
