Re: [PATCH v2] ovl: keep err zero after successful ovl_cache_get()

Hi Amir,

On 15.05.26 19:39, Amir Goldstein wrote:
> On Fri, May 15, 2026 at 1:16 PM Nirmoy Das <[email protected]> wrote:
>> Hi Amir,
>>
>> On 14.05.26 22:19, Amir Goldstein wrote:
>>> On Thu, May 14, 2026 at 5:26 PM Amir Goldstein <[email protected]> wrote:
>>>> On Thu, May 14, 2026 at 4:43 PM Nirmoy Das <[email protected]> wrote:
>>>>> ovl_iterate_merged() stores PTR_ERR(cache) in err before checking
>>>>> IS_ERR(cache). On success err holds the truncated cache pointer and
>>>>> can be returned as a bogus non-zero error.
>>>>>
>>>>> The syzbot reproducer reaches this through overlay-on-overlay readdir:
>>>>>
>>>>>     getdents64
>>>>>       iterate_dir(outer overlay file)
>>>>>         ovl_iterate_merged()
>>>>>           ovl_cache_get()
>>>>>             ovl_dir_read_merged()
>>>>>               ovl_dir_read()
>>>>>                 iterate_dir(inner overlay file)
>>>>>                   ovl_iterate_merged()
>>>>>
>>>>> Only compute PTR_ERR(cache) on the error path.
>>>>>
>>>>> Fixes: d25e4b739f83 ("ovl: refactor ovl_iterate() and port to cred guard")
>>>>> Reported-by: [email protected]
>>>>> Closes: https://syzkaller.appspot.com/bug?extid=a16fb0cce329a320661c
>>>>> Cc: [email protected]
>>>>> Signed-off-by: Nirmoy Das <[email protected]>
>>>>> ---
>>>>> v2:
>>>>>    - Drop the now-redundant 'int err = 0' initializer and the trailing
>>>>>      'return err' in ovl_iterate_merged(); err is only used inside the
>>>>>      loop's update-check, so the function can just return 0 on success.
>>>>>      (Amir Goldstein)
>>>>>    - Link to v1:
>>>>>      https://lore.kernel.org/all/[email protected]/
>>>>>
>>>> I queue this up and will work on fortifying patches.
>>> Nirmoy,
>>>
>>> I pushed fortify patches to ovl-fixes on my github [1].
>>>
>>> Can you verify that the assertions trigger if you revert your fix
>>> and run the reproducer?
>>>
>>> I imagine they would trigger much more frequently than the KASAN
>>> warnings do.
>>
>> Yes, the assertion triggers with your ovl-fixes branch after reverting
>> my fix.
>>
>> 9541f25af774 Revert "ovl: keep err zero after successful ovl_cache_get()"
>> 1c067d912e47 ovl: add assertions in dir cache code
>> 98e3a2d258e9 ovl: fix race between copy-up and open of a directory
>> 4f80bb375112 ovl: keep err zero after successful ovl_cache_get()
>> 18de6460b6bd ovl: opt-in for fortified ERR_PTR()
>> 690bd87e1fef err_ptr.h: introduce ERR_PTR_SAFE()
>> 7fd2df204f34 Linux 7.1-rc2
>>
>> Running the syz reproducer with panic_on_warn=1 triggered:
>>
>> [   55.404636] ------------[ cut here ]------------
>> [   55.404646] WARNING: fs/overlayfs/readdir.c:511 at
>> ovl_iterate+0x4c0/0x5bc, CPU#2: syz-ovl-iterate/14575
>> [   55.406875] CPU: 2 UID: 0 PID: 14575 Comm: syz-ovl-iterate Not
>> tainted 7.1.0-rc2-g9541f25af774 #1 PREEMPT
>> [   55.408328] pc : ovl_iterate+0x4c0/0x5bc
>> [   55.408632] lr : ovl_iterate+0x4b4/0x5bc
>> [   55.413504] x2 : 0000000000000000 x1 : 0000000000000000 x0 :
>> ffffffffc152db40
>> [   55.414036] Call trace:
>> [   55.414209]  ovl_iterate+0x4c0/0x5bc (P)
>> [   55.414503]  wrap_directory_iterator+0x60/0x90
>> [   55.414809]  shared_ovl_iterate+0x18/0x24
>> [   55.415125]  iterate_dir+0x10c/0x3a4
>> [   55.415365]  __arm64_sys_getdents64+0xe0/0x1e4
>> [   55.417312] Kernel panic - not syncing: kernel: panic_on_warn set ...
>>
> Thanks for testing!
>
> Did it trigger faster than the KASAN warning?
> I'd imagine that it would?


I lost my setup. I think it was quicker but I don't conclusive data. I 
will update you next week.


Regards,

Nirmoy

>
> Thanks,
> Amir.