Re: [tip:x86/mm] x86, mm: NX protection for kernel data

[Siarhei Liakh]

On Sat, Mar 13, 2010 at 8:12 AM, matthieu castet
<castet.matthieu@xxxxxxx> wrote:
> Hi,
>
>> > looking for c17ebdb8 in system.map points to a location in pgd_lock:
>> > ============================================
>> > $grep c17ebd System.map
>> > c17ebd68 d bios_check_work
>> > c17ebda8 d highmem_pages
>> > c17ebdac D pgd_lock
>> > c17ebdc8 D pgd_list
>> > c17ebdd0 D show_unhandled_signals
>> > c17ebdd4 d cpa_lock
>> > c17ebdf0 d memtype_lock
>> > ============================================
>> >
>> > I've looked at the lock debugging and could not find any place that
>> > would look like an attempt to execute data. This would lead me to
>> > think that calling set_memory_nx from kernel_init somehow confuses the
>> > lock debugging subsystem, or set_memory_nx does not change page
>> > attributes in a safe manner (for example when a lock is stored inside
>> > the page whose attributes are being changed).
>>
>> I've done some extra debugging and it really does look like the crash
>> happens when we are setting NX on a large page which has pgd_lock
>> inside it.
>>
>> Here is a trace of printk's that I added to troubleshoot this issue:
>> =========================
>> [    3.072003] try_preserve_large_page - enter
>> [    3.073185] try_preserve_large_page - address: 0xc1600000
>> [    3.074513] try_preserve_large_page - 2M page
>> [    3.075606] try_preserve_large_page - about to call static_protections
>> [    3.076000] try_preserve_large_page - back from static_protections
>> [    3.076000] try_preserve_large_page - past loop
>> [    3.076000] try_preserve_large_page - new_prot != old_prot
>> [    3.076000] try_preserve_large_page - the address is aligned and
>> the number of pages covers the full range
>> [    3.076000] try_preserve_large_page - about to call __set_pmd_pte
>> [    3.076000] __set_pmd_pte - enter
>> [    3.076000] __set_pmd_pte - address: 0xc1600000
>> [    3.076000] __set_pmd_pte - about to call
>> set_pte_atomic(*0xc18c0058(low=0x16001e3, high=0x0), (low=0x16001e1,
>> high=0x80000000))
>> [lock-up here]
>> =========================
>>
[...]
> 0xc1600000 2MB page is in 0xc1600000-0xc1800000 range.  pgd_lock
> (0xc17ebdac) seems to be in that range.

That's what I was thinking...

> You change attribute from (low=0x16001e3, high=0x0) to (low=0x16001e1,
> high=0x80000000). IE you set
> NX bit (bit 63), but you also clear R/W bit (bit 2). So the page become read
> only, but you are using a lock
> inside this page that need RW access. So you got a page fault.

Yes, that would do it.

> Now I don't know what should be done.
> Is that normal we set the page RO ?

No, this page should not be RO, as it contains kernel's RW data.
The interesting part is that the call that initiates the change is
set_memory_nx(), so it should not be clearing RW bit... The
interesting part is that the kernel does not crash with lock debugging
disabled.

Thanks for your help.
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