Re: [PATCH] mm/hugetlb: Fix uffd wr-protection for CoW optimization path
From: David Hildenbrand
Date: Fri Mar 24 2023 - 04:52:17 EST
On 23.03.23 23:11, Peter Xu wrote:
On Thu, Mar 23, 2023 at 08:33:07PM +0500, Muhammad Usama Anjum wrote:
Hi Peter,
Sorry for late reply.
On 3/22/23 12:50 AM, Peter Xu wrote:
On Tue, Mar 21, 2023 at 08:36:35PM +0100, David Hildenbrand wrote:
On 21.03.23 20:18, Peter Xu wrote:
This patch fixes an issue that a hugetlb uffd-wr-protected mapping can be
writable even with uffd-wp bit set. It only happens with all these
conditions met: (1) hugetlb memory (2) private mapping (3) original mapping
was missing, then (4) being wr-protected (IOW, pte marker installed). Then
write to the page to trigger.
Userfaultfd-wp trap for hugetlb was implemented in hugetlb_fault() before
even reaching hugetlb_wp() to avoid taking more locks that userfault won't
need. However there's one CoW optimization path for missing hugetlb page
that can trigger hugetlb_wp() inside hugetlb_no_page(), that can bypass the
userfaultfd-wp traps.
A few ways to resolve this:
(1) Skip the CoW optimization for hugetlb private mapping, considering
that private mappings for hugetlb should be very rare, so it may not
really be helpful to major workloads. The worst case is we only skip the
optimization if userfaultfd_wp(vma)==true, because uffd-wp needs another
fault anyway.
(2) Move the userfaultfd-wp handling for hugetlb from hugetlb_fault()
into hugetlb_wp(). The major cons is there're a bunch of locks taken
when calling hugetlb_wp(), and that will make the changeset unnecessarily
complicated due to the lock operations.
(3) Carry over uffd-wp bit in hugetlb_wp(), so it'll need to fault again
for uffd-wp privately mapped pages.
This patch chose option (3) which contains the minimum changeset (simplest
for backport) and also make sure hugetlb_wp() itself will start to be
always safe with uffd-wp ptes even if called elsewhere in the future.
This patch will be needed for v5.19+ hence copy stable.
Reported-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
Cc: linux-stable <stable@xxxxxxxxxxxxxxx>
Fixes: 166f3ecc0daf ("mm/hugetlb: hook page faults for uffd write protection")
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
mm/hugetlb.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 8bfd07f4c143..22337b191eae 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5478,7 +5478,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
struct folio *pagecache_folio, spinlock_t *ptl)
{
const bool unshare = flags & FAULT_FLAG_UNSHARE;
- pte_t pte;
+ pte_t pte, newpte;
struct hstate *h = hstate_vma(vma);
struct page *old_page;
struct folio *new_folio;
@@ -5622,8 +5622,10 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
mmu_notifier_invalidate_range(mm, range.start, range.end);
page_remove_rmap(old_page, vma, true);
hugepage_add_new_anon_rmap(new_folio, vma, haddr);
- set_huge_pte_at(mm, haddr, ptep,
- make_huge_pte(vma, &new_folio->page, !unshare));
+ newpte = make_huge_pte(vma, &new_folio->page, !unshare);
+ if (huge_pte_uffd_wp(pte))
+ newpte = huge_pte_mkuffd_wp(newpte);
+ set_huge_pte_at(mm, haddr, ptep, newpte);
folio_set_hugetlb_migratable(new_folio);
/* Make the old page be freed below */
new_folio = page_folio(old_page);
Looks correct to me. Do we have a reproducer?
I used a reproducer for the async mode I wrote (patch 2 attached, need to
change to VM_PRIVATE):
https://lore.kernel.org/all/ZBNr4nohj%2FTw4Zhw@x1n/
I don't think kernel kselftest can trigger it because we don't do strict
checks yet with uffd-wp bits. I've already started looking into cleanup
the test cases and I do plan to add new tests to cover this.
Meanwhile, let's also wait for an ack from Muhammad. Even though the async
mode is not part of the code base, it'll be a good test for verifying every
single uffd-wp bit being set or cleared as expected.
I've tested by applying this patch. But the bug is still there. Just like
Peter has mentioned, we are using our in progress patches related to
pagemap_scan ioctl and userfaultd wp async patches to reproduce it.
To reproduce please build kernel and run pagemap_ioctl test in mm in
hugetlb_mem_reproducer branch:
https://gitlab.collabora.com/usama.anjum/linux-mainline/-/tree/hugetlb_mem_reproducer
In case you have any question on how to reproduce, please let me know. I'll
try to provide a cleaner alternative.
Hmm, I think my current fix is incomplete if not wrong. The root cause
should still be valid, however I overlooked another path:
if (page_mapcount(old_page) == 1 && PageAnon(old_page)) {
if (!PageAnonExclusive(old_page))
page_move_anon_rmap(old_page, vma);
if (likely(!unshare))
set_huge_ptep_writable(vma, haddr, ptep);
delayacct_wpcopy_end();
return 0;
}
We should bail out early in this path, and it'll be even easier we always
bail out hugetlb_wp() as long as uffd-wp is detected because userfault
should always be handled before any decision to CoW.
v2 attached.. Please give it another shot.
Hmmm, I think you must only do that for !unshare (FAULT_FLAG_WRITE).
Otherwise you'll never be able to resolve an unsharing request on a r/o
mapped hugetlb page that has the uffd-wp set?
Or am I missing something?
--
Thanks,
David / dhildenb