[PATCH 1/3] mm: avoid wrapping vm_pgoff in mremap() - CVE-2011-2496

[Paolo Pisati]

From: Linus Torvalds <torvalds at linux-foundation.org>

mm: avoid wrapping vm_pgoff in mremap()

The normal mmap paths all avoid creating a mapping where the pgoff
inside the mapping could wrap around due to overflow.  However, an
expanding mremap() can take such a non-wrapping mapping and make it
bigger and cause a wrapping condition.

Noticed by Robert Swiecki when running a system call fuzzer, where it
caused a BUG_ON() due to terminally confusing the vma_prio_tree code.  A
vma dumping patch by Hugh then pinpointed the crazy wrapped case.

Reported-and-tested-by: Robert Swiecki <robert at swiecki.net>
Acked-by: Hugh Dickins <hughd at google.com>
Cc: stable at kernel.org
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

CVE-2011-2496

BugLink: http://bugs.launchpad.net/bugs/869243

commit upstream 982134ba62618c2d69fbbbd166d0a11ee3b7e3d8

Signed-off-by: Paolo Pisati <paolo.pisati at canonical.com>
---
 mm/mremap.c |   11 +++++++++--
 1 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 0debf81..4be89e1a 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -263,9 +263,16 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
 	if (old_len > vma->vm_end - addr)
 		goto Efault;
 
-	if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP)) {
-		if (new_len > old_len)
+	/* Need to be careful about a growing mapping */
+	if (new_len > old_len) {
+		unsigned long pgoff;
+
+		if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))
 			goto Efault;
+		pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
+		pgoff += vma->vm_pgoff;
+		if (pgoff + (new_len >> PAGE_SHIFT) < pgoff)
+			goto Einval;
 	}
 
 	if (vma->vm_flags & VM_LOCKED) {
-- 
1.7.5.4