+ userfaultfd-avoid-huge_zero_page-in-uffdio_move.patch added to mm-hotfixes-unstable branch

+ userfaultfd-avoid-huge_zero_page-in-uffdio_move.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: userfaultfd: avoid huge_zero_page in UFFDIO_MOVE
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     userfaultfd-avoid-huge_zero_page-in-uffdio_move.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/userfaultfd-avoid-huge_zero_page-in-uffdio_move.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Suren Baghdasaryan <surenb@google.com>
Subject: userfaultfd: avoid huge_zero_page in UFFDIO_MOVE
Date: Thu, 11 Jan 2024 17:39:35 -0800

While testing UFFDIO_MOVE ioctl, syzbot triggered VM_BUG_ON_PAGE caused by
a call to PageAnonExclusive() with a huge_zero_page as a parameter. 
UFFDIO_MOVE does not yet handle zeropages and returns EBUSY when one is
encountered.  Add an early huge_zero_page check in the PMD move path to
avoid this situation.

Link: https://lkml.kernel.org/r/20240112013935.1474648-1-surenb@google.com
Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
Reported-by: syzbot+705209281e36404998f6@syzkaller.appspotmail.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/userfaultfd.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/mm/userfaultfd.c~userfaultfd-avoid-huge_zero_page-in-uffdio_move
+++ a/mm/userfaultfd.c
@@ -1393,6 +1393,12 @@ ssize_t move_pages(struct userfaultfd_ct
 				err = -ENOENT;
 				break;
 			}
+			/* Avoid moving zeropages for now */
+			if (is_huge_zero_pmd(*src_pmd)) {
+				spin_unlock(ptl);
+				err = -EBUSY;
+				break;
+			}
 
 			/* Check if we can move the pmd without splitting it. */
 			if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
_

Patches currently in -mm which might be from surenb@google.com are

userfaultfd-avoid-huge_zero_page-in-uffdio_move.patch