generic/4.4: add fix for CVE-2016-5195

Fixes CVE-2016-5195, backport the mm patch into 097- until the kernel
version is upgraded.
Compile-tested on ar71xx, ramips.
Runtime-tested on ramips.

Signed-off-by: Zoltan HERPAI <[email protected]>

target/linux/generic/patches-4.4/097-mm-remove-gup_flags-FOLL_WRITE-games-from-__get_user.patch

@@ -0,0 +1,96 @@
+From 1294d355881cc5c3421d24fee512f16974addb6c Mon Sep 17 00:00:00 2001
+From: Linus Torvalds <[email protected]>
+Date: Thu, 13 Oct 2016 13:07:36 -0700
+Subject: mm: remove gup_flags FOLL_WRITE games from __get_user_pages()
+
+commit 19be0eaffa3ac7d8eb6784ad9bdbc7d67ed8e619 upstream.
+
+This is an ancient bug that was actually attempted to be 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 now
+fix it by checking the pte_dirty() bit properly (and do it better).  The
+s390 dirty bit was implemented in abf09bed3cce ("s390/mm: implement
+software dirty bits") which made it into v3.9.  Earlier kernels will
+have to look at the page state itself.
+
+Also, the VM has become more scalable, and what used a purely
+theoretical race back then has become easier to trigger.
+
+To fix it, 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, and then use the pte dirty flag to validate that
+the FOLL_COW flag is still valid.
+
+Reported-and-tested-by: Phil "not Paul" Oester <[email protected]>
+Acked-by: Hugh Dickins <[email protected]>
+Reviewed-by: Michal Hocko <[email protected]>
+Cc: Andy Lutomirski <[email protected]>
+Cc: Kees Cook <[email protected]>
+Cc: Oleg Nesterov <[email protected]>
+Cc: Willy Tarreau <[email protected]>
+Cc: Nick Piggin <[email protected]>
+Cc: Greg Thelen <[email protected]>
+Signed-off-by: Linus Torvalds <[email protected]>
+Signed-off-by: Greg Kroah-Hartman <[email protected]>
+---
+ 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 cfebb74..f0ffa01 100644
+--- a/include/linux/mm.h
++++ b/include/linux/mm.h
+@@ -2112,6 +2112,7 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
+ #define FOLL_MIGRATION	0x400	/* wait for page to replace migration entry */
+ #define FOLL_TRIED	0x800	/* a retry, previous pass started an IO */
+ #define FOLL_MLOCK	0x1000	/* lock present pages */
++#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 deafa2c..4b0b7e7 100644
+--- a/mm/gup.c
++++ b/mm/gup.c
+@@ -58,6 +58,16 @@ 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)
+ {
+@@ -92,7 +102,7 @@ retry:
+ 	}
+ 	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;
+ 	}
+@@ -352,7 +362,7 @@ 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;
+ }
+
+-- 
+cgit v0.12
+