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

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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>
This commit is contained in:
Helge Deller 2015-06-04 23:57:18 +02:00
parent ff25ea8f4e
commit 9b7b819ca1
1 changed files with 4 additions and 2 deletions
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6
kernel/compat.c
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@ -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;
}