Commit 9b7b819c authored Jun 04, 2015 by Helge Deller

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



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 <deller@gmx.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>

parent ff25ea8f

kernel/compat.c

+4 −2

Original line number	Diff line number	Diff line
		@@ -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;
		}

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