We used to have a report that pte-marker code can be reached even when
uffd-wp is not compiled in for file memories, here:

https://lore.kernel.org/all/YzeR+R6b4bwBlBHh@x1n/T/#u

I just got time to revisit this and found that the root cause is we simply
messed up with the vma check, so that for !PTE_MARKER_UFFD_WP system, we
will allow UFFDIO_REGISTER of MINOR & WP upon shmem as the check was wrong:

    if (vm_flags & VM_UFFD_MINOR)
        return is_vm_hugetlb_page(vma) || vma_is_shmem(vma);

Where we'll allow anything to pass on shmem as long as minor mode is
requested.

Axel did it right when introducing minor mode but I messed it up in
b1f9e876862d when moving code around.  Fix it.

Cc: Axel Rasmussen <axelrasmussen@google.com>
Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs")
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/linux/userfaultfd_k.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index f07e6998bb68..9df0b9a762cc 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -146,9 +146,9 @@ static inline bool userfaultfd_armed(struct vm_area_struct *vma)
 static inline bool vma_can_userfault(struct vm_area_struct *vma,
 				     unsigned long vm_flags)
 {
-	if (vm_flags & VM_UFFD_MINOR)
-		return is_vm_hugetlb_page(vma) || vma_is_shmem(vma);
-
+	if ((vm_flags & VM_UFFD_MINOR) &&
+	    (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)))
+		return false;
 #ifndef CONFIG_PTE_MARKER_UFFD_WP
 	/*
 	 * If user requested uffd-wp but not enabled pte markers for
-- 
2.37.3

On Mon, Oct 24, 2022 at 03:33:35PM -0400, Peter Xu wrote:
> We used to have a report that pte-marker code can be reached even when
> uffd-wp is not compiled in for file memories, here:
> 
> https://lore.kernel.org/all/YzeR+R6b4bwBlBHh@x1n/T/#u
> 
> I just got time to revisit this and found that the root cause is we simply
> messed up with the vma check, so that for !PTE_MARKER_UFFD_WP system, we
> will allow UFFDIO_REGISTER of MINOR & WP upon shmem as the check was wrong:
> 
>     if (vm_flags & VM_UFFD_MINOR)
>         return is_vm_hugetlb_page(vma) || vma_is_shmem(vma);
> 
> Where we'll allow anything to pass on shmem as long as minor mode is
> requested.
> 
> Axel did it right when introducing minor mode but I messed it up in
> b1f9e876862d when moving code around.  Fix it.
> 
> Cc: Axel Rasmussen <axelrasmussen@google.com>
> Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs")

Should also have had:

Cc: stable@vger.kernel.org

> Signed-off-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu