lkml.org 
[lkml]   [2022]   [Apr]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH v3 15/16] mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page
From
On 3/29/22 18:04, David Hildenbrand wrote:
> Whenever GUP currently ends up taking a R/O pin on an anonymous page that
> might be shared -- mapped R/O and !PageAnonExclusive() -- any write fault
> on the page table entry will end up replacing the mapped anonymous page
> due to COW, resulting in the GUP pin no longer being consistent with the
> page actually mapped into the page table.
>
> The possible ways to deal with this situation are:
> (1) Ignore and pin -- what we do right now.
> (2) Fail to pin -- which would be rather surprising to callers and
> could break user space.
> (3) Trigger unsharing and pin the now exclusive page -- reliable R/O
> pins.
>
> Let's implement 3) because it provides the clearest semantics and
> allows for checking in unpin_user_pages() and friends for possible BUGs:
> when trying to unpin a page that's no longer exclusive, clearly
> something went very wrong and might result in memory corruptions that
> might be hard to debug. So we better have a nice way to spot such
> issues.
>
> This change implies that whenever user space *wrote* to a private
> mapping (IOW, we have an anonymous page mapped), that GUP pins will
> always remain consistent: reliable R/O GUP pins of anonymous pages.
>
> As a side note, this commit fixes the COW security issue for hugetlb with
> FOLL_PIN as documented in:
> https://lore.kernel.org/r/3ae33b08-d9ef-f846-56fb-645e3b9b4c66@redhat.com
> The vmsplice reproducer still applies, because vmsplice uses FOLL_GET
> instead of FOLL_PIN.
>
> Note that follow_huge_pmd() doesn't apply because we cannot end up in
> there with FOLL_PIN.
>
> This commit is heavily based on prototype patches by Andrea.
>
> Co-developed-by: Andrea Arcangeli <aarcange@redhat.com>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

\
 
 \ /
  Last update: 2022-04-19 17:57    [W:0.252 / U:0.124 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site