[PATCH 5.18 06/88] userfaultfd: provide properly masked address for huge-pages

[Greg Kroah-Hartman]

From: Nadav Amit <namit@xxxxxxxxxx>

commit d172b1a3bd065dd89234eac547fc62cf80681631 upstream.

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@xxxxxxxxxx
Fixes: 824ddc601adc ("userfaultfd: provide unmasked address on page-fault")
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
Reported-by: James Houghton <jthoughton@xxxxxxxxxx>
Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
Reviewed-by: James Houghton <jthoughton@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/userfaultfd.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -191,17 +191,19 @@ static inline void msg_init(struct uffd_
 }
 
 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.
@@ -485,8 +487,8 @@ vm_fault_t handle_userfault(struct vm_fa
 
 	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;