Re: [PATCH 2/2] ext4: Avoid cycles in directory h-tree

["Theodore Ts'o" <tytso@mit.edu>]

On Thu, Apr 28, 2022 at 08:31:38PM +0200, Jan Kara wrote:
> A maliciously corrupted filesystem can contain cycles in the h-tree
> stored inside a directory. That can easily lead to the kernel corrupting
> tree nodes that were already verified under its hands while doing a node
> split and consequently accessing unallocated memory. Fix the problem by
> verifying traversed block numbers are unique.
> 
> CC: stable@vger.kernel.org
> Signed-off-by: Jan Kara <jack@suse.cz>

When I tried applying this patch, I got this crash:

ext4/052 23s ... 	[19:28:41][    2.683407] run fstests ext4/052 at 2022-05-17 19:28:41
[    5.433672] Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: dx_probe+0x629/0x630
[    5.434449] CPU: 0 PID: 2468 Comm: dirstress Not tainted 5.18.0-rc5-xfstests-00019-g204e6b4d4cc1 #610
[    5.435012] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
[    5.435501] Call Trace:
[    5.435659]  <TASK>
[    5.435791]  dump_stack_lvl+0x34/0x44
[    5.436027]  panic+0x107/0x28f
[    5.436221]  ? dx_probe+0x629/0x630
[    5.436430]  __stack_chk_fail+0x10/0x10
[    5.436663]  dx_probe+0x629/0x630
[    5.436869]  ext4_dx_add_entry+0x54/0x700
[    5.437176]  ext4_add_entry+0x38d/0x4e0
[    5.437421]  ext4_add_nondir+0x2b/0xc0
[    5.437647]  ext4_symlink+0x1c5/0x390
[    5.437869]  vfs_symlink+0x184/0x220
[    5.438095]  do_symlinkat+0x7a/0x110
[    5.438313]  __x64_sys_symlink+0x38/0x40
[    5.438548]  do_syscall_64+0x38/0x90
[    5.438762]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[    5.439070] RIP: 0033:0x7f8137109a97
[    5.439285] Code: f0 ff ff 73 01 c3 48 8b 0d f6 d3 0c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 b8 58 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d c9 d3 0c 00 f7 d8 64 89 01 48
[    5.440374] RSP: 002b:00007ffc514a2428 EFLAGS: 00000246 ORIG_RAX: 0000000000000058
[    5.440820] RAX: ffffffffffffffda RBX: 0000000000035d4a RCX: 00007f8137109a97
[    5.441278] RDX: 0000000000000000 RSI: 00007ffc514a2450 RDI: 00007ffc514a2450
[    5.441734] RBP: 0000000000000003 R08: 0000000000000000 R09: 0000000000000013
[    5.442153] R10: 00007ffc514a21c2 R11: 0000000000000246 R12: 0000000000061a80
[    5.442571] R13: 0000000000000000 R14: 00007ffc514a2450 R15: 00007ffc514a2c50
[    5.442989]  </TASK>
[    5.443289] Kernel Offset: disabled
[    5.443517] Rebooting in 5 seconds..

Applying just the first patch series (plus the patch hunk from this
commit needed so that the first patch compiles) does not result in a
crash, so the problem is clearly with this change.

Looking more closely at ext4/052 which tests the large_dir feature,
and then looking at the patch, I suspect the fix which is needed
is:

> +	ext4_lblk_t blocks[EXT4_HTREE_LEVEL];

needs to be

	ext4_lblk_t blocks[EXT4_HTREE_LEVEL + 1];

Otherwise on large htree which is 3 levels deep, this

> +		blocks[++level] = block;

is going to end up smashing the stack.

Jan, do you agree?

							- Ted