Re: [PATCH] ext4: fix bug in extents parsing when number of entries in header is zero

[Baokun Li]

在 2022/8/5 22:00, Luís Henriques 写道:
> When walking through an inode extents, the ext4_ext_binsearch_idx() function
> assumes that the extent header has been previously validated.  However,
> there are no checks that verify that the number of entries (eh->eh_entries)
> is non-zero.  And this will lead to problems because the EXT_FIRST_INDEX()
> and EXT_LAST_INDEX() will return garbage and result in this:
>
> [  135.245946] ------------[ cut here ]------------
> [  135.247579] kernel BUG at fs/ext4/extents.c:2258!
> [  135.249045] invalid opcode: 0000 [#1] PREEMPT SMP
> [  135.250320] CPU: 2 PID: 238 Comm: tmp118 Not tainted 5.19.0-rc8+ #4
> [  135.252067] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.15.0-0-g2dd4b9b-rebuilt.opensuse.org 04/01/2014
> [  135.255065] RIP: 0010:ext4_ext_map_blocks+0xc20/0xcb0
> [  135.256475] Code:
> [  135.261433] RSP: 0018:ffffc900005939f8 EFLAGS: 00010246
> [  135.262847] RAX: 0000000000000024 RBX: ffffc90000593b70 RCX: 0000000000000023
> [  135.264765] RDX: ffff8880038e5f10 RSI: 0000000000000003 RDI: ffff8880046e922c
> [  135.266670] RBP: ffff8880046e9348 R08: 0000000000000001 R09: ffff888002ca580c
> [  135.268576] R10: 0000000000002602 R11: 0000000000000000 R12: 0000000000000024
> [  135.270477] R13: 0000000000000000 R14: 0000000000000024 R15: 0000000000000000
> [  135.272394] FS:  00007fdabdc56740(0000) GS:ffff88807dd00000(0000) knlGS:0000000000000000
> [  135.274510] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  135.276075] CR2: 00007ffc26bd4f00 CR3: 0000000006261004 CR4: 0000000000170ea0
> [  135.277952] Call Trace:
> [  135.278635]  <TASK>
> [  135.279247]  ? preempt_count_add+0x6d/0xa0
> [  135.280358]  ? percpu_counter_add_batch+0x55/0xb0
> [  135.281612]  ? _raw_read_unlock+0x18/0x30
> [  135.282704]  ext4_map_blocks+0x294/0x5a0
> [  135.283745]  ? xa_load+0x6f/0xa0
> [  135.284562]  ext4_mpage_readpages+0x3d6/0x770
> [  135.285646]  read_pages+0x67/0x1d0
> [  135.286492]  ? folio_add_lru+0x51/0x80
> [  135.287441]  page_cache_ra_unbounded+0x124/0x170
> [  135.288510]  filemap_get_pages+0x23d/0x5a0
> [  135.289457]  ? path_openat+0xa72/0xdd0
> [  135.290332]  filemap_read+0xbf/0x300
> [  135.291158]  ? _raw_spin_lock_irqsave+0x17/0x40
> [  135.292192]  new_sync_read+0x103/0x170
> [  135.293014]  vfs_read+0x15d/0x180
> [  135.293745]  ksys_read+0xa1/0xe0
> [  135.294461]  do_syscall_64+0x3c/0x80
> [  135.295284]  entry_SYSCALL_64_after_hwframe+0x46/0xb0
>
> Unfortunately, __ext4_ext_check() only verifies that eh->eh_entries doesn't
> exceed eh->eh_max.  And since an empty leaf seems to be a valid value in
> same cases, adding this extra check there isn't an option.
>
> This patch simply adds the check directly in ext4_ext_binsearch_idx() and
> propagates this error so that the kernel doesn't hit this BUG_ON() in
> ext4_ext_determine_hole().
>
> Link:https://bugzilla.kernel.org/show_bug.cgi?id=215941
> Signed-off-by: Luís Henriques<lhenriques@xxxxxxx>
> ---
>   fs/ext4/extents.c | 13 ++++++++++---
>   1 file changed, 10 insertions(+), 3 deletions(-)
>
> Hi!
>
> This bug is easily reproducible using the filesystem image provided --
> it's just a matter of mounting it and run:
>
>      $ cat /mnt/foo/bar/xattr

Hi Luís,
yeah, that's a good catch!
> Anyway, I hope my analysis of the bug is correct -- the root cause seems
> to be an extent header with an invalid value for in eh_entries, which will
> later cause the BUG_ON().
>
> Cheers,
> --
> Luís
But there's a little bit of a deviation in your understanding of the 
problem,
so the patch doesn't look good.
The issue is caused by the contradiction between eh_entries and eh_depth.
Therefore, we need to check the contradiction instead of adding a 
judgment to ext4_ext_binsearch_idx.
So the right fix is to add a check to __ext4_ext_check like:

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index c148bb97b527..2dfd35f727cb 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -460,6 +460,10 @@ static int __ext4_ext_check(const char *function, 
unsigned int line,
                error_msg = "invalid eh_entries";
                goto corrupted;
        }
+       if (unlikely((eh->eh_entries == 0) && (depth > 0))) {
+               error_msg = "contradictory eh_entries and eh_depth";
+               goto corrupted;
+       }
        if (!ext4_valid_extent_entries(inode, eh, lblk, &pblk, depth)) {
                error_msg = "invalid extent entries";
                goto corrupted;

In this way, we can fix this issue and check for header exceptions 
before calling ext4_ext_binsearch_idx.

Thanks!
--
With Best Regards,
Baokun Li