[PATCH v2 5/5] arm64: entry: Enable random_kstack_offset support

Kees Cook
2020-03-24T20:33:45Z

Allow for a randomized stack offset on a per-syscall basis, with roughly
5 bits of entropy.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
arch/arm64/Kconfig | 1 +
arch/arm64/kernel/syscall.c | 10 ++++++++++
2 files changed, 11 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 0b30e884e088..4d5aa4959f72 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -127,6 +127,7 @@ config ARM64
select HAVE_ARCH_MMAP_RND_BITS
select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
select HAVE_ARCH_PREL32_RELOCATIONS
+ select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
select HAVE_ARCH_SECCOMP_FILTER
select HAVE_ARCH_STACKLEAK
select HAVE_ARCH_THREAD_STRUCT_WHITELIST
diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index a12c0c88d345..238dbd753b44 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -5,6 +5,7 @@
#include <linux/errno.h>
#include <linux/nospec.h>
#include <linux/ptrace.h>
+#include <linux/randomize_kstack.h>
#include <linux/syscalls.h>

#include <asm/daifflags.h>
@@ -42,6 +43,8 @@ static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
{
long ret;

+ add_random_kstack_offset();
+
if (scno < sc_nr) {
syscall_fn_t syscall_fn;
syscall_fn = syscall_table[array_index_nospec(scno, sc_nr)];
@@ -51,6 +54,13 @@ static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
}

regs->regs[0] = ret;
+
+ /*
+ * Since the compiler chooses a 4 bit alignment for the stack,
+ * let's save one additional bit (9 total), which gets us up
+ * near 5 bits of entropy.
+ */
+ choose_random_kstack_offset(get_random_int() & 0x1FF);
}

static inline bool has_syscall_work(unsigned long flags)
--
2.20.1