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powerpc/mm: Fix possible out-of-bounds shift in arch_mmap_rnd()

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The recent patch to add runtime configuration of the ASLR limits added a bug in
arch_mmap_rnd() where we may shift an integer (32-bits) by up to 33 bits,
leading to undefined behaviour.

In practice it exhibits as every process seg faulting instantly, presumably
because the rnd value hasn't been restricited by the modulus at all. We didn't
notice because it only happens under certain kernel configurations and if the
number of bits is actually set to a large value.

Fix it by switching to unsigned long.

Fixes: 9fea59bd7c ("powerpc/mm: Add support for runtime configuration of ASLR limits")
Reported-by: Balbir Singh <bsingharora@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Michael Ellerman 2017-04-25 20:49:24 +10:00
parent 9765ad134a
commit b409946b2a
1 changed files with 1 additions and 1 deletions
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2
arch/powerpc/mm/mmap.c
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@ -66,7 +66,7 @@ unsigned long arch_mmap_rnd(void)
if (is_32bit_task())
shift = mmap_rnd_compat_bits;
#endif
rnd = get_random_long() % (1 << shift);
rnd = get_random_long() % (1ul << shift);
return rnd << PAGE_SHIFT;
}