Re: [PATCH v2] ext4: fix bug in extents parsing when eh_entries == 0 and eh_depth > 0

[Baokun Li <libaokun1@huawei.com>]

Hi Luís,

On 8/12/2022 6:53 PM, Luís Henriques wrote:
> 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 when depth is > 0.  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
>
> This patch simply adds an extra check in __ext4_ext_check(), verifying that
> eh_entries is not 0 when eh_depth is > 0.
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=215941
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=216283
> Cc: Baokun Li <libaokun1@huawei.com>
> Signed-off-by: Luís Henriques <lhenriques@suse.de>
> ---
>   fs/ext4/extents.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
> Hi!
>
> Baokun's feedback showed me that I had a partial understanding of the
> problem.  Thus, I'm sending v2 which pretty much uses Baokun's suggestion
> and simplifies the solution.  I've also added the link to the 2nd bugzilla
> to the commit text.
>
> Cheers,
> --
> Luís
>
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 53cfe2c681c4..a5457ac1999c 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -460,6 +460,11 @@ static int __ext4_ext_check(const char *function, unsigned int line,
>   		error_msg = "invalid eh_entries";
>   		goto corrupted;
>   	}
> +	if (unlikely((le16_to_cpu(eh->eh_entries) == 0) &&
> +		     (le16_to_cpu(eh->eh_depth > 0)))) {

The parentheses are misplaced, and le16_to_cpu is not needed here.

> +		error_msg = "eh_entries is 0 but eh_depth is > 0";
> +		goto corrupted;
> +	}
>   	if (!ext4_valid_extent_entries(inode, eh, lblk, &pblk, depth)) {
>   		error_msg = "invalid extent entries";
>   		goto corrupted;
> .

-- 
With Best Regards,
Baokun Li