Commit b409946b authored Apr 25, 2017 by Michael Ellerman

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



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: 9fea59bd ("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>

parent 9765ad13

arch/powerpc/mm/mmap.c

+1 −1

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

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