Patch "mm/hugetlb: fix uffd wr-protection for CoW optimization path" has been added to the 6.1-stable tree

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This is a note to let you know that I've just added the patch titled

    mm/hugetlb: fix uffd wr-protection for CoW optimization path

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     mm-hugetlb-fix-uffd-wr-protection-for-cow-optimization-path.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.


>From 60d5b473d61be61ac315e544fcd6a8234a79500e Mon Sep 17 00:00:00 2001
From: Peter Xu <peterx@xxxxxxxxxx>
Date: Tue, 21 Mar 2023 15:18:40 -0400
Subject: mm/hugetlb: fix uffd wr-protection for CoW optimization path

From: Peter Xu <peterx@xxxxxxxxxx>

commit 60d5b473d61be61ac315e544fcd6a8234a79500e upstream.

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 hugetlb private
mappings, when someone firstly wr-protects a missing pte (which will
install a pte marker), then a write to the same page without any prior
access to the page.

Userfaultfd-wp trap for hugetlb was implemented in hugetlb_fault() before
reaching hugetlb_wp() to avoid taking more locks that userfault won't
need.  However there's one CoW optimization path that can trigger
hugetlb_wp() inside hugetlb_no_page(), which will bypass the trap.

This patch skips hugetlb_wp() for CoW and retries the fault if uffd-wp bit
is detected.  The new path will only trigger in the CoW optimization path
because generic hugetlb_fault() (e.g.  when a present pte was
wr-protected) will resolve the uffd-wp bit already.  Also make sure
anonymous UNSHARE won't be affected and can still be resolved, IOW only
skip CoW not CoR.

This patch will be needed for v5.19+ hence copy stable.

[peterx@xxxxxxxxxx: v2]
  Link: https://lkml.kernel.org/r/ZBzOqwF2wrHgBVZb@x1n
[peterx@xxxxxxxxxx: v3]
  Link: https://lkml.kernel.org/r/20230324142620.2344140-1-peterx@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20230321191840.1897940-1-peterx@xxxxxxxxxx
Fixes: 166f3ecc0daf ("mm/hugetlb: hook page faults for uffd write protection")
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
Reported-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
Tested-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
Cc: Nadav Amit <nadav.amit@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 mm/hugetlb.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5469,7 +5469,7 @@ static vm_fault_t hugetlb_wp(struct mm_s
 		       struct page *pagecache_page, spinlock_t *ptl)
 {
 	const bool unshare = flags & FAULT_FLAG_UNSHARE;
-	pte_t pte;
+	pte_t pte = huge_ptep_get(ptep);
 	struct hstate *h = hstate_vma(vma);
 	struct page *old_page, *new_page;
 	int outside_reserve = 0;
@@ -5481,6 +5481,17 @@ static vm_fault_t hugetlb_wp(struct mm_s
 	VM_BUG_ON(!unshare && !(flags & FOLL_WRITE));
 
 	/*
+	 * Never handle CoW for uffd-wp protected pages.  It should be only
+	 * handled when the uffd-wp protection is removed.
+	 *
+	 * Note that only the CoW optimization path (in hugetlb_no_page())
+	 * can trigger this, because hugetlb_fault() will always resolve
+	 * uffd-wp bit first.
+	 */
+	if (!unshare && huge_pte_uffd_wp(pte))
+		return 0;
+
+	/*
 	 * hugetlb does not support FOLL_FORCE-style write faults that keep the
 	 * PTE mapped R/O such as maybe_mkwrite() would do.
 	 */
@@ -5495,7 +5506,6 @@ static vm_fault_t hugetlb_wp(struct mm_s
 		return 0;
 	}
 
-	pte = huge_ptep_get(ptep);
 	old_page = pte_page(pte);
 
 	delayacct_wpcopy_start();


Patches currently in stable-queue which might be from peterx@xxxxxxxxxx are

queue-6.1/mm-hugetlb-fix-uffd-wr-protection-for-cow-optimization-path.patch



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