userfaultfd: provide properly masked address for huge-pages

Commit 824ddc601adc ("userfaultfd: provide unmasked address on
page-fault") was introduced to fix an old bug, in which the offset in the
address of a page-fault was masked.  Concerns were raised - although were
never backed by actual code - that some userspace code might break because
the bug has been around for quite a while.  To address these concerns a
new flag was introduced, and only when this flag is set by the user,
userfaultfd provides the exact address of the page-fault.

The commit however had a bug, and if the flag is unset, the offset was
always masked based on a base-page granularity.  Yet, for huge-pages, the
behavior prior to the commit was that the address is masked to the
huge-page granulrity.

While there are no reports on real breakage, fix this issue.  If the flag
is unset, use the address with the masking that was done before.

Link: https://lkml.kernel.org/r/20220711165906.2682-1-namit@vmware.com
Fixes: 824ddc601adc ("userfaultfd: provide unmasked address on page-fault")
Signed-off-by: Nadav Amit <namit@vmware.com>
Reported-by: James Houghton <jthoughton@google.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: James Houghton <jthoughton@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index e943370..de86f5b 100644 (file)
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -192,17 +192,19 @@ static inline void msg_init(struct uffd_msg *msg)
 }
 
 static inline struct uffd_msg userfault_msg(unsigned long address,
+                                           unsigned long real_address,
                                            unsigned int flags,
                                            unsigned long reason,
                                            unsigned int features)
 {
        struct uffd_msg msg;
+
        msg_init(&msg);
        msg.event = UFFD_EVENT_PAGEFAULT;
 
-       if (!(features & UFFD_FEATURE_EXACT_ADDRESS))
-               address &= PAGE_MASK;
-       msg.arg.pagefault.address = address;
+       msg.arg.pagefault.address = (features & UFFD_FEATURE_EXACT_ADDRESS) ?
+                                   real_address : address;
+
        /*
         * These flags indicate why the userfault occurred:
         * - UFFD_PAGEFAULT_FLAG_WP indicates a write protect fault.
@@ -488,8 +490,8 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 
        init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
        uwq.wq.private = current;
-       uwq.msg = userfault_msg(vmf->real_address, vmf->flags, reason,
-                       ctx->features);
+       uwq.msg = userfault_msg(vmf->address, vmf->real_address, vmf->flags,
+                               reason, ctx->features);
        uwq.ctx = ctx;
        uwq.waken = false;