userfaultfd: provide properly masked address for huge-pages

Commit 824ddc6 ("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/[email protected]
Fixes: 824ddc6 ("userfaultfd: provide unmasked address on page-fault")
Signed-off-by: Nadav Amit <[email protected]>
Reported-by: James Houghton <[email protected]>
Reviewed-by: Mike Rapoport <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Reviewed-by: James Houghton <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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;