Re: WARNING in get_pat_info

[Jens Axboe]

On 10/15/24 2:04 PM, Dave Hansen wrote:
> On 10/15/24 11:55, Marius Fleischer wrote:
>> Hope you are doing well!
>>
>> Quick update from our side: The reproducer from the previous email
>> still triggers a WARNING on v5.15 (commit hash
>> 3a5928702e7120f83f703fd566082bfb59f1a57e). Happy to also test on
>> other kernel versions if that helps.
>>
>> Please let us know if there is any other helpful information we can provide.
> 
> I don't know for sure, but I suspect that io_uring is triggering this.
> The reproducer is:
> 
> 	syz_io_uring_setup(0x6f7e, &(0x7f0000000080), 0x0, 0x0)
> 	syz_clone(0x24080, 0x0, 0x0, 0x0, 0x0, 0x0) (fail_nth: 40)
> 
> and the stack trace shows:
> 
>  untrack_pfn+0xdc/0x240 arch/x86/mm/pat/memtype.c:1122
>  ...
>  __mmput+0x122/0x4b0 kernel/fork.c:1126
>  ...
>  __do_sys_clone+0xc8/0x110 kernel/fork.c:2721
> 
> So whatever is happening is going on with a VM_PFNMAP VMA.  Those aren't
> super common except when you're mmap()'ing something from a device
> driver.  I would randomly guess that io_uring_setup() is setting up a
> VM_PFNMAP VMA and untrack_pfn() is getting called when that VMA is
> getting torn down.
> 
> The other goofiness is that the copy_mm() path is ending up in
> exit_mmap().  I think the only way to end up doing that is in the
> failure path of dup_mm().
> 
> So I *think* what happens is that a io_uring VMA gets created in
> dup_mmap(), but never gets any pages faulted in.  Some later setup fails
> and the new mm needs to be torn down.  *Something* about the io_uring
> VMA screws up the untrack_pfn() code.
> 
> I'm hoping that this rings a bell with the io_uring folks and this is a
> bug they've found and fixed in mainline that just got missed backporting
> to stable.

Doesn't ring a bell, and haven't had anyone report that before. The
older io_uring code does indeed use remap_pfn_range(), this is gone in
newer kernels. But in any case, I can't seem to trigger this on
5.15-stable, I might be missing something in my .config...

-- 
Jens Axboe