Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default

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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




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