Re: 6.10/bisected/regression - commit 8430557fc584 cause warning at mm/page_table_check.c:198 __page_table_check_ptes_set+0x306

David Hildenbrand
2024-05-22T20:26:07Z

On 22.05.24 18:10, Peter Xu wrote:
> On Wed, May 22, 2024 at 05:34:21PM +0200, David Hildenbrand wrote:
>> On 22.05.24 17:18, Peter Xu wrote:
>>> On Wed, May 22, 2024 at 09:48:51AM +0200, David Hildenbrand wrote:
>>>> On 22.05.24 00:36, Peter Xu wrote:
>>>>> On Wed, May 22, 2024 at 03:21:04AM +0500, Mikhail Gavrilov wrote:
>>>>>> On Wed, May 22, 2024 at 2:37 AM Peter Xu <peterx@redhat.com> wrote:
>>>>>>> Hmm I still cannot reproduce. Weird.
>>>>>>>
>>>>>>> Would it be possible for you to identify which line in debug_vm_pgtable.c
>>>>>>> triggered that issue?
>>>>>>>
>>>>>>> I think it should be some set_pte_at() but I'm not sure, as there aren't a
>>>>>>> lot and all of them look benign so far. It could be that I missed
>>>>>>> something important.
>>>>>>
>>>>>> I hope it's helps:
>>>>>
>>>>> Thanks for offering this, it's just that it doesn't look coherent with what
>>>>> was reported for some reason.
>>>>>
>>>>>>
>>>>>>> sh /usr/src/kernels/(uname -r)/scripts/faddr2line /lib/debug/lib/modules/(uname -r)/vmlinux debug_vm_pgtable+0x1c04
>>>>>> debug_vm_pgtable+0x1c04/0x3360:
>>>>>> native_ptep_get_and_clear at arch/x86/include/asm/pgtable_64.h:94
>>>>>> (inlined by) ptep_get_and_clear at arch/x86/include/asm/pgtable.h:1262
>>>>>> (inlined by) ptep_clear at include/linux/pgtable.h:509
>>>>>
>>>>> This is a pte_clear(), and pte_clear() shouldn't even do the set() checks,
>>>>> and shouldn't stumble over what I added.
>>>>>
>>>>> IOW, it doesn't match with the real stack dump previously:
>>>>>
>>>>> [ 5.581003] ? __page_table_check_ptes_set+0x306/0x3c0
>>>>> [ 5.581274] ? __pfx___page_table_check_ptes_set+0x10/0x10
>>>>> [ 5.581544] ? __pfx_check_pgprot+0x10/0x10
>>>>> [ 5.581806] set_ptes.constprop.0+0x66/0xd0
>>>>> [ 5.582072] ? __pfx_set_ptes.constprop.0+0x10/0x10
>>>>> [ 5.582333] ? __pfx_pte_val+0x10/0x10
>>>>> [ 5.582595] debug_vm_pgtable+0x1c04/0x3360
>>>>>
>>>>
>>>> Staring at pte_clear_tests():
>>>>
>>>> #ifndef CONFIG_RISCV
>>>> pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
>>>> #endif
>>>> set_pte_at(args->mm, args->vaddr, args->ptep, pte);
>>>>
>>>> So we set random PTE bits, probably setting the present, uffd and write bit
>>>> at the same time. That doesn't make too much sense when we want to perform
>>>> that such combinations cannot exist.
>>>
>>> Here the issue is I don't think it should set W bit anyway, as we init
>>> page_prot to be RWX but !shared:
>>>
>>> args->page_prot = vm_get_page_prot(VM_ACCESS_FLAGS);
>>>
>>> On x86_64 (Mikhail's system) it should have W bit cleared afaict, meanwhile
>>> the RANDOM_ORVALUE won't touch bit W due to S390_SKIP_MASK (which contains
>>> bit W / bit 1, which is another "accident"..). Then even if with that it
>>> should not trigger.. I think that's also why I cannot reproduce this
>>> problem locally.
>>
>> Why oh why are skip mask applied independently of the architecture.
>>
>> While _PAGE_RW should indeed be masked out by RANDOM_ORVALUE.
>>
>> But with shadow stacks we consider a PTE writable (see
>> pte_write()->pte_shstk()) if
>> (1) X86_FEATURE_SHSTK is enabled
>> (2) _PAGE_RW is clear
>> (3) _PAGE_DIRTY is set
>>
>> _PAGE_DIRTY is bit 6.
>>
>> Likely your CPU does not support shadow stacks.
>
> Good point. My host has it, but I tested in the VM which doesn't. I
> suppose we can wait and double check whether Mikhail should see the issue
> went away with that patch provided.
>
> In this case, instead of keep fiddling with random bits to apply and
> further work on top of per-arch random bits, I'd hope we can simply drop
> that random mechanism as I don't think it'll be pxx_none() now. I attached
> a patch I plan to post. Does it look reasonable?

I doubt that randomness ever helped in finding a BUG. Clearing is just
too simple ... but I might just be wrong :)

I'd vote for removing that, this will likely not be the last issue we
run into once we add more sanity checks during set_pte_at().

--
Cheers,

David / dhildenb