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>
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 @@
* 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 @@
* 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;
}