Re: [PATCH v28 09/32] x86/mm: Introduce _PAGE_COW

Yu, Yu-cheng
2021-08-17T18:24:44Z

On 8/16/2021 3:43 AM, Borislav Petkov wrote:
> On Thu, Jul 22, 2021 at 01:51:56PM -0700, Yu-cheng Yu wrote:
>> @@ -153,13 +178,23 @@ static inline int pud_young(pud_t pud)
>>
>> static inline int pte_write(pte_t pte)
>> {
>> - return pte_flags(pte) & _PAGE_RW;
>> + /*
>> + * Shadow stack pages are always writable - but not by normal
>> + * instructions, and only by shadow stack operations. Therefore,
>> + * the W=0,D=1 test with pte_shstk().
>> + */
>> + return (pte_flags(pte) & _PAGE_RW) || pte_shstk(pte);
>
> Well, this is weird: if some kernel code queries a shstk page and this
> here function says it is writable but then goes and tries to write into
> it and that write fails, then it'll confuse the user.
>
> IOW, from where I'm standing, that should be:
>
> return (pte_flags(pte) & _PAGE_RW) && !pte_shstk(pte);
>
> as in, a writable page is one which has _PAGE_RW and it is *not* a
> shadow stack page because latter is special and not really writable.
> > Hmmm?
>

Indeed, this can be looked at in a few ways. We can visualize
pte_write() as 'CPU can write to it with MOV' or 'CPU can write to it
with any opcodes'. Depending on whatever pte_write() is, copy-on-write
code can be adjusted accordingly.

Yu-cheng