mm: add an explicit smp_wmb() to UFFDIO_CONTINUE

Users of UFFDIO_CONTINUE may reasonably assume that a write memory barrier
is included as part of UFFDIO_CONTINUE.  That is, a user may believe that
all writes it has done to a page that it is now UFFDIO_CONTINUE'ing are
guaranteed to be visible to anyone subsequently reading the page through
the newly mapped virtual memory region.

Today, such a user happens to be correct.  mmget_not_zero(), for example,
is called as part of UFFDIO_CONTINUE (and comes before any PTE updates),
and it implicitly gives us a write barrier.

To be resilient against future changes, include an explicit smp_wmb(). 
While we're at it, optimize the smp_wmb() that is already incidentally
present for the HugeTLB case.

Merely making a syscall does not generally imply the memory ordering
constraints that we need (including on x86).

Link: https://lkml.kernel.org/r/20240307010250.3847179-1-jthoughton@google.com
Signed-off-by: James Houghton <jthoughton@google.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 4744d6a96f96..a0331ba9ae2a 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -845,6 +845,15 @@ ssize_t mfill_atomic_zeropage(struct userfaultfd_ctx *ctx,
 ssize_t mfill_atomic_continue(struct userfaultfd_ctx *ctx, unsigned long start,
 			      unsigned long len, uffd_flags_t flags)
 {
+
+	/*
+	 * A caller might reasonably assume that UFFDIO_CONTINUE contains an
+	 * smp_wmb() to ensure that any writes to the about-to-be-mapped page by
+	 * the thread doing the UFFDIO_CONTINUE are guaranteed to be visible to
+	 * subsequent loads from the page through the newly mapped address range.
+	 */
+	smp_wmb();
+
 	return mfill_atomic(ctx, start, 0, len,
 			    uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
 }