On 28.04.23 18:09, Kirill A . Shutemov wrote:
On Fri, Apr 28, 2023 at 05:43:52PM +0200, David Hildenbrand wrote:
On 28.04.23 17:34, 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().
IIRC, PageHuge() can also be called from GUP-fast and could special-case
hugetlb eventually, as it's table while we hold a (temporary) reference.
Shmem might be not so easy ...
page->mapping->a_ops should be enough to whitelist whatever fs you want.
The issue is how to stabilize that from GUP-fast, such that we can
safely dereference the mapping. Any idea?
At least for anon page I know that page->mapping only gets cleared when
freeing the page, and we don't dereference the mapping but only check a
single flag stored alongside the mapping. Therefore, PageAnon() is fine
in GUP-fast context.
--
Thanks,
David / dhildenb
