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SubjectRe: [PATCH v2] mm/hugetlb: fix a addressing exception caused by huge_pte_offset()
On Mon, Mar 23, 2020 at 10:27:48AM -0700, Mike Kravetz wrote:

> > pgd = pgd_offset(mm, addr);
> > - if (!pgd_present(*pgd))
> > + if (!pgd_present(READ_ONCE(*pgd)))
> > return NULL;
> > p4d = p4d_offset(pgd, addr);
> > - if (!p4d_present(*p4d))
> > + if (!p4d_present(READ_ONCE(*p4d)))
> > return NULL;
> >
> > pud = pud_offset(p4d, addr);
>
> One would argue that pgd and p4d can not change from present to !present
> during the execution of this code. To me, that seems like the issue which
> would cause an issue. Of course, I could be missing something.

This I am not sure of, I think it must be true under the read side of
the mmap_sem, but probably not guarenteed under RCU..

In any case, it doesn't matter, the fact that *p4d can change at all
is problematic. Unwinding the above inlines we get:

p4d = p4d_offset(pgd, addr)
if (!p4d_present(*p4d))
return NULL;
pud = (pud_t *)p4d_page_vaddr(*p4d) + pud_index(address);

According to our memory model the compiler/CPU is free to execute this
as:

p4d = p4d_offset(pgd, addr)
p4d_for_vaddr = *p4d;
if (!p4d_present(*p4d))
return NULL;
pud = (pud_t *)p4d_page_vaddr(p4d_for_vaddr) + pud_index(address);

In the case where p4 goes from !present -> present (ie
handle_mm_fault()):

p4d_for_vaddr == p4d_none, and p4d_present(*p4d) == true, meaning the
p4d_page_vaddr() will crash.

Basically the problem here is not just missing READ_ONCE, but that the
p4d is read multiple times at all. It should be written like gup_fast
does, to guarantee a single CPU read of the unstable data:

p4d = READ_ONCE(*p4d_offset(pgdp, addr));
if (!p4d_present(p4))
return NULL;
pud = pud_offset(&p4d, addr);

At least this is what I've been able to figure out :\

> > Also, the remark about pmd_offset() seems accurate. The
> > get_user_fast_pages() pattern seems like the correct one to emulate:
> >
> > pud = READ_ONCE(*pudp);
> > if (pud_none(pud))
> > ..
> > if (!pud_'is a pmd pointer')
> > ..
> > pmdp = pmd_offset(&pud, address);
> > pmd = READ_ONCE(*pmd);
> > [...]
> >
> > Passing &pud in avoids another de-reference of the pudp. Honestly all
> > these APIs that take in page table pointers and internally
> > de-reference them seem very hard to use correctly when the page table
> > access isn't fully locked against write.

And the same protocol for the PUD, etc.

> > It looks like at least the p4d read from the pgd is also unlocked here
> > as handle_mm_fault() writes to it??
>
> Yes, there is no locking required to call huge_pte_offset().

None? Not RCU or read mmap_sem?

Jason

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