Re: [RFC] ext4: don't remove already removed extent

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

On 9/20/23 5:41 AM, Eric Whitney wrote:
> * Muhammad Usama Anjum <usama.anjum@collabora.com>:
>> Syzbot has hit the following bug on current and all older kernels:
>> BUG: KASAN: out-of-bounds in ext4_ext_rm_leaf fs/ext4/extents.c:2736 [inline]
>> BUG: KASAN: out-of-bounds in ext4_ext_remove_space+0x2482/0x4d90 fs/ext4/extents.c:2958
>> Read of size 18446744073709551508 at addr ffff888073aea078 by task syz-executor420/6443
>>
>> On investigation, I've found that eh->eh_entries is zero, ex is
>> referring to last entry and EXT_LAST_EXTENT(eh) is referring to first.
>> Hence EXT_LAST_EXTENT(eh) - ex becomes negative and causes the wrong
>> buffer read.
>>
>> element: FFFF8882F8F0D06C       <----- ex
>> element: FFFF8882F8F0D060
>> element: FFFF8882F8F0D054
>> element: FFFF8882F8F0D048
>> element: FFFF8882F8F0D03C
>> element: FFFF8882F8F0D030
>> element: FFFF8882F8F0D024
>> element: FFFF8882F8F0D018
>> element: FFFF8882F8F0D00C	<------  EXT_FIRST_EXTENT(eh)
>> header:  FFFF8882F8F0D000	<------  EXT_LAST_EXTENT(eh) and eh
>>
>> Cc: stable@vger.kernel.org
>> Reported-by: syzbot+6e5f2db05775244c73b7@syzkaller.appspotmail.com
>> Closes: https://groups.google.com/g/syzkaller-bugs/c/G6zS-LKgDW0/m/63MgF6V7BAAJ
>> Fixes: d583fb87a3ff ("ext4: punch out extents")
>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
>> ---
>> This patch is only fixing the local issue. There may be bigger bug. Why
>> is ex set to last entry if the eh->eh_entries is 0. If any ext4
>> developer want to look at the bug, please don't hesitate.
>> ---
>>  fs/ext4/extents.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
>> index e4115d338f101..7b7779b4cb87f 100644
>> --- a/fs/ext4/extents.c
>> +++ b/fs/ext4/extents.c
>> @@ -2726,7 +2726,7 @@ ext4_ext_rm_leaf(handle_t *handle, struct inode *inode,
>>  		 * If the extent was completely released,
>>  		 * we need to remove it from the leaf
>>  		 */
>> -		if (num == 0) {
>> +		if (num == 0 && eh->eh_entries) {
>>  			if (end != EXT_MAX_BLOCKS - 1) {
>>  				/*
>>  				 * For hole punching, we need to scoot all the
>> -- 
>> 2.40.1
>>
> 
> Hi:
> 
> First, thanks for taking the time to look at this.
Thank you for replying and giving me pointers that I need to start looking
at problem from first warning until the bug which can be difficult until I
debug the problem smartly and learn at least the basics of ext4.

> 
> I'm suspicious that syzbot may be fuzzing an extent header or other extent
> tree components.  As you noticed, eh_entries and ex appear to be inconsistent.
> Also, note the long series of corrupted file system reports in the console log
> occurring before the KASAN bug - ext4 had been detecting and rejecting bad
> data up to that point.  The file system on the disk image provided by sysbot
> indicates that metadata checksumming was enabled (and it fscks cleanly).
> That should have caught a corrupted extent header or inode, but perhaps
> there's a problem.
> 
> The console log indicates that the problem occurred on inode #16.  Does the
> information you've provided above come from testing you did on inode #16
> (looks like the name was /bin/base64)?
I couldn't analyze the problem in broad spectrum. There must be some bigger
thing wrong here.

> 
> By any chance, have you found a simpler reproducer than what syzbot provides?
Not yet, this gets reproduced after a while. I'll try to come up with
better reproducer if I can.

> 
> Thanks,
> Eric
> 
> 

-- 
BR,
Muhammad Usama Anjum