Commit d172b1a3 authored by Nadav Amit's avatar Nadav Amit Committed by Andrew Morton
Browse files

userfaultfd: provide properly masked address for huge-pages 

Commit 824ddc60 ("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: 824ddc60 ("userfaultfd: provide unmasked address on page-fault")
Signed-off-by: default avatarNadav Amit <namit@vmware.com>
Reported-by: default avatarJames Houghton <jthoughton@google.com>
Reviewed-by: default avatarMike Rapoport <rppt@linux.ibm.com>
Reviewed-by: default avatarPeter Xu <peterx@redhat.com>
Reviewed-by: default avatarJames 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: default avatarAndrew Morton <akpm@linux-foundation.org>
parent c80af0c2

fs/userfaultfd.c

+7 −5
Original line number Diff line number Diff line
@@ -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;