arm64: mm: support ARCH_MMAP_RND_BITS

arm64: arch_mmap_rnd() uses STACK_RND_MASK to generate the random offset
for the mmap base address.  This value represents a compromise between
increased ASLR effectiveness and avoiding address-space fragmentation.
Replace it with a Kconfig option, which is sensibly bounded, so that
platform developers may choose where to place this compromise.  Keep
default values as new minimums.

Signed-off-by: Daniel Cashman <[email protected]>
Cc: Russell King <[email protected]>
Acked-by: Kees Cook <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Don Zickus <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: Heinrich Schuchardt <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Mark Salyzyn <[email protected]>
Cc: Jeff Vander Stoep <[email protected]>
Cc: Nick Kralevich <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Hector Marco-Gisbert <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 6 insertions, 2 deletions

diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
index ed177475dd8c..4c893b5189dd 100644
--- a/arch/arm64/mm/mmap.c
+++ b/arch/arm64/mm/mmap.c
@@ -51,8 +51,12 @@ unsigned long arch_mmap_rnd(void)
 {
 	unsigned long rnd;
 
-	rnd = (unsigned long)get_random_int() & STACK_RND_MASK;
-
+#ifdef CONFIG_COMPAT
+	if (test_thread_flag(TIF_32BIT))
+		rnd = (unsigned long)get_random_int() & ((1 << mmap_rnd_compat_bits) - 1);
+	else
+#endif
+		rnd = (unsigned long)get_random_int() & ((1 << mmap_rnd_bits) - 1);
 	return rnd << PAGE_SHIFT;
 }