mm: remove gup_flags FOLL_WRITE games from __get_user_pages() (CVE-2016-5195)

debian/changelog

@@ -158,6 +158,8 @@ linux (4.7.8-1) UNRELEASED; urgency=medium
 
   [ Salvatore Bonaccorso ]
   * KEYS: Fix short sprintf buffer in /proc/keys show function (CVE-2016-7042)
+  * mm: remove gup_flags FOLL_WRITE games from __get_user_pages()
+    (CVE-2016-5195)
 
  -- Ben Hutchings <ben@decadent.org.uk>  Tue, 11 Oct 2016 22:43:14 +0100
 

debian/patches/bugfix/all/mm-remove-gup_flags-FOLL_WRITE-games-from-__get_user.patch

@@ -0,0 +1,83 @@
+From bd2f56577e4066fcd61eab60d817e86ae8d4c6bf Mon Sep 17 00:00:00 2001
+From: Linus Torvalds <torvalds@linux-foundation.org>
+Date: Thu, 13 Oct 2016 13:07:36 -0700
+Subject: [PATCH 01/11] mm: remove gup_flags FOLL_WRITE games from
+ __get_user_pages()
+
+This is an ancient bug that was actually already fixed once (badly) by
+me eleven years ago in commit 4ceb5db9757a ("Fix get_user_pages() race
+for write access") but that was then undone due to problems on s390 by
+commit f33ea7f404e5 ("fix get_user_pages bug").
+
+In the meantime, the s390 situation has long been fixed, and we can once
+more try to fix it by checking the pte_dirty() bit properly (and do it
+better).  We introduce a new internal FOLL_COW flag to mark the "yes, we
+already did a COW" rather than play racy games with FOLL_WRITE that is
+very fundamental.
+
+Reported-and-tested-by: Phil "not Paul" Oester <kernel@linuxace.com>
+Cc: Andy Lutomirski <luto@amacapital.net>
+Cc: Kees Cook <keescook@chromium.org>
+Cc: Oleg Nesterov <oleg@redhat.com>
+Cc: Willy Tarreau <w@1wt.eu>
+Cc: stable@vger.kernel.org
+Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
+---
+ include/linux/mm.h |  1 +
+ mm/gup.c           | 14 ++++++++++++--
+ 2 files changed, 13 insertions(+), 2 deletions(-)
+
+diff --git a/include/linux/mm.h b/include/linux/mm.h
+index e9caec6a51e9..ed85879f47f5 100644
+--- a/include/linux/mm.h
++++ b/include/linux/mm.h
+@@ -2232,6 +2232,7 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
+ #define FOLL_TRIED	0x800	/* a retry, previous pass started an IO */
+ #define FOLL_MLOCK	0x1000	/* lock present pages */
+ #define FOLL_REMOTE	0x2000	/* we are working on non-current tsk/mm */
++#define FOLL_COW	0x4000	/* internal GUP flag */
+ 
+ typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr,
+ 			void *data);
+diff --git a/mm/gup.c b/mm/gup.c
+index 96b2b2fd0fbd..22cc22e7432f 100644
+--- a/mm/gup.c
++++ b/mm/gup.c
+@@ -60,6 +60,16 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
+ 	return -EEXIST;
+ }
+ 
++/*
++ * FOLL_FORCE can write to even unwritable pte's, but only
++ * after we've gone through a COW cycle and they are dirty.
++ */
++static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
++{
++	return pte_write(pte) ||
++		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
++}
++
+ static struct page *follow_page_pte(struct vm_area_struct *vma,
+ 		unsigned long address, pmd_t *pmd, unsigned int flags)
+ {
+@@ -95,7 +105,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
+ 	}
+ 	if ((flags & FOLL_NUMA) && pte_protnone(pte))
+ 		goto no_page;
+-	if ((flags & FOLL_WRITE) && !pte_write(pte)) {
++	if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
+ 		pte_unmap_unlock(ptep, ptl);
+ 		return NULL;
+ 	}
+@@ -412,7 +422,7 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
+ 	 * reCOWed by userspace write).
+ 	 */
+ 	if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))
+-		*flags &= ~FOLL_WRITE;
++	        *flags |= FOLL_COW;
+ 	return 0;
+ }
+ 
+-- 
+2.10.1.562.ge11b675.dirty
+

debian/patches/series

@@ -118,6 +118,7 @@ bugfix/all/net-add-recursion-limit-to-gro.patch
 bugfix/all/posix_acl-clear-sgid-bit-when-setting-file-permissio.patch
 bugfix/all/scsi-arcmsr-buffer-overflow-in-arcmsr_iop_message_xf.patch
 bugfix/all/KEYS-Fix-short-sprintf-buffer-in-proc-keys-show-func.patch
+bugfix/all/mm-remove-gup_flags-FOLL_WRITE-games-from-__get_user.patch
 
 # ABI maintenance
 debian/i8042-revert-abi-break-in-4.7.3.patch