powerpc/mm: Add support for runtime configuration of ASLR limits

Add powerpc support for mmap_rnd_bits and mmap_rnd_compat_bits, which are two
sysctls that allow a user to configure the number of bits of randomness used for
ASLR.

Because of the way the Kconfig for ARCH_MMAP_RND_BITS is defined, we have to
construct at least the MIN value in Kconfig, vs in a header which would be more
natural. Given that we just go ahead and do it all in Kconfig.

At least according to the code (the documentation makes no mention of it), the
value is defined as the number of bits of randomisation *of the page*, not the
address. This makes some sense, with larger page sizes more of the low bits are
forced to zero, which would reduce the randomisation if we didn't take the
PAGE_SIZE into account. However it does mean the min/max values have to change
depending on the PAGE_SIZE in order to actually limit the amount of address
space consumed by the randomisation.

The result of that is that we have to define the default values based on both
32-bit vs 64-bit, but also the configured PAGE_SIZE. Furthermore now that we
have 128TB address space support on Book3S, we also have to take that into
account.

Finally we can wire up the value in arch_mmap_rnd().

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
Tested-by: Bhupesh Sharma <bhsharma@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

arch/powerpc/Kconfig

@@ -22,6 +22,48 @@ config MMU
 	bool
 	default y
 
+config ARCH_MMAP_RND_BITS_MAX
+	# On Book3S 64, the default virtual address space for 64-bit processes
+	# is 2^47 (128TB). As a maximum, allow randomisation to consume up to
+	# 32T of address space (2^45), which should ensure a reasonable gap
+	# between bottom-up and top-down allocations for applications that
+	# consume "normal" amounts of address space. Book3S 64 only supports 64K
+	# and 4K page sizes.
+	default 29 if PPC_BOOK3S_64 && PPC_64K_PAGES # 29 = 45 (32T) - 16 (64K)
+	default 33 if PPC_BOOK3S_64		     # 33 = 45 (32T) - 12 (4K)
+	#
+	# On all other 64-bit platforms (currently only Book3E), the virtual
+	# address space is 2^46 (64TB). Allow randomisation to consume up to 16T
+	# of address space (2^44). Only 4K page sizes are supported.
+	default 32 if 64BIT	# 32 = 44 (16T) - 12 (4K)
+	#
+	# For 32-bit, use the compat values, as they're the same.
+	default ARCH_MMAP_RND_COMPAT_BITS_MAX
+
+config ARCH_MMAP_RND_BITS_MIN
+	# Allow randomisation to consume up to 1GB of address space (2^30).
+	default 14 if 64BIT && PPC_64K_PAGES	# 14 = 30 (1GB) - 16 (64K)
+	default 18 if 64BIT			# 18 = 30 (1GB) - 12 (4K)
+	#
+	# For 32-bit, use the compat values, as they're the same.
+	default ARCH_MMAP_RND_COMPAT_BITS_MIN
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+	# Total virtual address space for 32-bit processes is 2^31 (2GB).
+	# Allow randomisation to consume up to 512MB of address space (2^29).
+	default 11 if PPC_256K_PAGES	# 11 = 29 (512MB) - 18 (256K)
+	default 13 if PPC_64K_PAGES	# 13 = 29 (512MB) - 16 (64K)
+	default 15 if PPC_16K_PAGES 	# 15 = 29 (512MB) - 14 (16K)
+	default 17			# 17 = 29 (512MB) - 12 (4K)
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+	# Total virtual address space for 32-bit processes is 2^31 (2GB).
+	# Allow randomisation to consume up to 8MB of address space (2^23).
+	default 5 if PPC_256K_PAGES	#  5 = 23 (8MB) - 18 (256K)
+	default 7 if PPC_64K_PAGES	#  7 = 23 (8MB) - 16 (64K)
+	default 9 if PPC_16K_PAGES	#  9 = 23 (8MB) - 14 (16K)
+	default 11			# 11 = 23 (8MB) - 12 (4K)
+
 config HAVE_SETUP_PER_CPU_AREA
 	def_bool PPC64
 
@@ -120,6 +162,8 @@ config PPC
 	select HAVE_ARCH_HARDENED_USERCOPY
 	select HAVE_ARCH_JUMP_LABEL
 	select HAVE_ARCH_KGDB
+	select HAVE_ARCH_MMAP_RND_BITS
+	select HAVE_ARCH_MMAP_RND_COMPAT_BITS	if COMPAT
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_CBPF_JIT			if !PPC64

arch/powerpc/mm/mmap.c

@@ -59,13 +59,14 @@ static inline int mmap_is_legacy(void)
 
 unsigned long arch_mmap_rnd(void)
 {
-	unsigned long rnd;
+	unsigned long shift, rnd;
 
-	/* 8MB for 32bit, 1GB for 64bit */
+	shift = mmap_rnd_bits;
+#ifdef CONFIG_COMPAT
 	if (is_32bit_task())
-		rnd = get_random_long() % (1<<(23-PAGE_SHIFT));
-	else
-		rnd = get_random_long() % (1UL<<(30-PAGE_SHIFT));
+		shift = mmap_rnd_compat_bits;
+#endif
+	rnd = get_random_long() % (1 << shift);
 
 	return rnd << PAGE_SHIFT;
 }