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usercopy: avoid potentially undefined behavior in pointer math

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check_bogus_address() checked for pointer overflow using this expression,
where 'ptr' has type 'const void *':

	ptr + n < ptr

Since pointer wraparound is undefined behavior, gcc at -O2 by default
treats it like the following, which would not behave as intended:

	(long)n < 0

Fortunately, this doesn't currently happen for kernel code because kernel
code is compiled with -fno-strict-overflow.  But the expression should be
fixed anyway to use well-defined integer arithmetic, since it could be
treated differently by different compilers in the future or could be
reported by tools checking for undefined behavior.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
This commit is contained in:
Eric Biggers 2016-08-19 12:15:22 -07:00 committed by Kees Cook
parent ef0e1ea885
commit 7329a65587
1 changed files with 1 additions and 1 deletions
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2
mm/usercopy.c
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@ -124,7 +124,7 @@ static inline const char *check_kernel_text_object(const void *ptr,
static inline const char *check_bogus_address(const void *ptr, unsigned long n) static inline const char *check_bogus_address(const void *ptr, unsigned long n)
{ {
/* Reject if object wraps past end of memory. */ /* Reject if object wraps past end of memory. */
if (ptr + n < ptr) if ((unsigned long)ptr + n < (unsigned long)ptr)
return "<wrapped address>"; return "<wrapped address>";
/* Reject if NULL or ZERO-allocation. */ /* Reject if NULL or ZERO-allocation. */