compat: cleanup coding in compat_get_bitmap() and compat_put_bitmap()

9b7b819c · Helge Deller <[email protected]> · 2015-06-04 23:57:18 +02:00

In the functions compat_get_bitmap() and compat_put_bitmap() the
variable nr_compat_longs stores how many compat_ulong_t words should be
copied in a loop.

The copy loop itself is this:
  if (nr_compat_longs-- > 0) {
      if (__get_user(um, umask)) return -EFAULT;
  } else {
      um = 0;
  }

Since nr_compat_longs gets unconditionally decremented in each loop and
since it's type is unsigned this could theoretically lead to out of
bounds accesses to userspace if nr_compat_longs wraps around to
(unsigned)(-1).

Although the callers currently do not trigger out-of-bounds accesses, we
should better implement the loop in a safe way to completely avoid such
warp-arounds.

Signed-off-by: Helge Deller <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Al Viro <[email protected]>

Commit: 9b7b819ca1e508195feed5ece558dca66adeef05
Parents: ff25ea8f

Changes

1 files changed · +4 −2

File	Status	Add	Del	Content
kernel/compat.c	modified	+4	−2	View file

kernel/compat.c

View file

diff --git a/kernel/compat.c b/kernel/compat.c
index 24f0061..333d364 100644
--- a/kernel/compat.c
+++ b/kernel/compat.c
@@ -912,7 +912,8 @@ long compat_get_bitmap(unsigned long *mask, const compat_ulong_t __user *umask,
 			 * bitmap. We must however ensure the end of the
 			 * kernel bitmap is zeroed.
 			 */
-			if (nr_compat_longs-- > 0) {
+			if (nr_compat_longs) {
+				nr_compat_longs--;
 				if (__get_user(um, umask))
 					return -EFAULT;
 			} else {
@@ -954,7 +955,8 @@ long compat_put_bitmap(compat_ulong_t __user *umask, unsigned long *mask,
 			 * We dont want to write past the end of the userspace
 			 * bitmap.
 			 */
-			if (nr_compat_longs-- > 0) {
+			if (nr_compat_longs) {
+				nr_compat_longs--;
 				if (__put_user(um, umask))
 					return -EFAULT;
 			}