+ userfaultfd-provide-properly-masked-address-for-huge-pages.patch added to mm-hotfixes-unstable branch

+ userfaultfd-provide-properly-masked-address-for-huge-pages.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: userfaultfd: provide properly masked address for huge-pages
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     userfaultfd-provide-properly-masked-address-for-huge-pages.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/userfaultfd-provide-properly-masked-address-for-huge-pages.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Nadav Amit <namit@vmware.com>
Subject: userfaultfd: provide properly masked address for huge-pages
Date: Mon, 11 Jul 2022 09:59:06 -0700

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>
---

 fs/userfaultfd.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

--- a/fs/userfaultfd.c~userfaultfd-provide-properly-masked-address-for-huge-pages
+++ a/fs/userfaultfd.c
@@ -192,17 +192,19 @@ 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.
@@ -488,8 +490,8 @@ 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;
 
_

Patches currently in -mm which might be from namit@vmware.com are

userfaultfd-provide-properly-masked-address-for-huge-pages.patch