Messages in this thread | | | Date | Fri, 28 Jul 2023 11:08:26 +0200 | From | David Hildenbrand <> | Subject | Re: [PATCH v1 2/4] mm/gup: Make follow_page() succeed again on PROT_NONE PTEs/PMDs |
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On 28.07.23 04:30, John Hubbard wrote: > On 7/27/23 14:28, David Hildenbrand wrote: >> We accidentally enforced PROT_NONE PTE/PMD permission checks for >> follow_page() like we do for get_user_pages() and friends. That was >> undesired, because follow_page() is usually only used to lookup a currently >> mapped page, not to actually access it. Further, follow_page() does not >> actually trigger fault handling, but instead simply fails. > > I see that follow_page() is also completely undocumented. And that > reduces us to deducing how it should be used...these things that > change follow_page()'s behavior maybe should have a go at documenting > it too, perhaps.
I can certainly be motivated to do that. :)
> >> >> Let's restore that behavior by conditionally setting FOLL_FORCE if >> FOLL_WRITE is not set. This way, for example KSM and migration code will >> no longer fail on PROT_NONE mapped PTEs/PMDS. >> >> Handling this internally doesn't require us to add any new FOLL_FORCE >> usage outside of GUP code. >> >> While at it, refuse to accept FOLL_FORCE: we don't even perform VMA >> permission checks like in check_vma_flags(), so especially >> FOLL_FORCE|FOLL_WRITE would be dodgy. >> >> This issue was identified by code inspection. We'll add some >> documentation regarding FOLL_FORCE next. >> >> Reported-by: Peter Xu <peterx@redhat.com> >> Fixes: 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()") >> Cc: <stable@vger.kernel.org> >> Signed-off-by: David Hildenbrand <david@redhat.com> >> --- >> mm/gup.c | 10 +++++++++- >> 1 file changed, 9 insertions(+), 1 deletion(-) >> >> diff --git a/mm/gup.c b/mm/gup.c >> index 2493ffa10f4b..da9a5cc096ac 100644 >> --- a/mm/gup.c >> +++ b/mm/gup.c >> @@ -841,9 +841,17 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address, >> if (vma_is_secretmem(vma)) >> return NULL; >> >> - if (WARN_ON_ONCE(foll_flags & FOLL_PIN)) >> + if (WARN_ON_ONCE(foll_flags & (FOLL_PIN | FOLL_FORCE))) >> return NULL; > > This is not a super happy situation: follow_page() is now prohibited > (see above: we should document that interface) from passing in > FOLL_FORCE...
I guess you saw my patch #4.
If you take a look at the existing callers (that are fortunately very limited), you'll see that nobody cares.
Most of the FOLL flags don't make any sense for follow_page(), and limiting further (ab)use is at least to me very appealing.
> >> >> + /* >> + * Traditionally, follow_page() succeeded on PROT_NONE-mapped pages >> + * but failed follow_page(FOLL_WRITE) on R/O-mapped pages. Let's >> + * keep these semantics by setting FOLL_FORCE if FOLL_WRITE is not set. >> + */ >> + if (!(foll_flags & FOLL_WRITE)) >> + foll_flags |= FOLL_FORCE; >> + > > ...but then we set it anyway, for special cases. It's awkward because > FOLL_FORCE is not an "internal to gup" flag (yet?). > > I don't yet have suggestions, other than: > > 1) Yes, the FOLL_NUMA made things bad. > > 2) And they are still very confusing, especially the new use of > FOLL_FORCE. > > ...I'll try to let this soak in and maybe recommend something > in a more productive way. :)
What I can offer that might be very appealing is the following:
Get rid of the flags parameter for follow_page() *completely*. Yes, then we can even rename FOLL_ to something reasonable in the context where it is nowadays used ;)
Internally, we'll then set
FOLL_GET | FOLL_DUMP | FOLL_FORCE
and document exactly what this functions does. Any user that needs something different should just look into using get_user_pages() instead.
I can prototype that on top of this work easily.
-- Cheers,
David / dhildenb
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