On 28.04.23 17:56, Peter Xu wrote:
On Fri, Apr 28, 2023 at 05:34:35PM +0200, David Hildenbrand wrote:
On 28.04.23 17:33, Lorenzo Stoakes wrote:
On Fri, Apr 28, 2023 at 05:23:29PM +0200, David Hildenbrand wrote:
Security is the primary case where we have historically closed uAPI
items.
As this patch
1) Does not tackle GUP-fast
2) Does not take care of !FOLL_LONGTERM
I am not convinced by the security argument in regard to this patch.
If we want to sells this as a security thing, we have to block it
*completely* and then CC stable.
Regarding GUP-fast, to fix the issue there as well, I guess we could do
something similar as I did in gup_must_unshare():
If we're in GUP-fast (no VMA), and want to pin a !anon page writable,
fallback to ordinary GUP. IOW, if we don't know, better be safe.
How do we determine it's non-anon in the first place? The check is on the
VMA. We could do it by following page tables down to folio and checking
folio->mapping for PAGE_MAPPING_ANON I suppose?
PageAnon(page) can be called from GUP-fast after grabbing a reference. See
gup_must_unshare().
Hmm.. Is it a good idea at all to sacrifise all "!anon" fast-gups for this?
People will silently got degrade even on legal pins on shmem/hugetlb, I
think, which seems to be still a very major use case.
Right. Optimizing for hugetlb should be easy. Shmem is problematic.
I once raised to John that PageAnonExclusive is essentially a "anon page
is pinnable" flag. Too bad we don't have spare page flags ;)
--
Thanks,
David / dhildenb
