Allow for a randomized stack offset on a per-syscall basis, with roughly
5 bits of entropy.
In order to avoid unconditional stack canaries on syscall entry, also
downgrade from -fstack-protector-strong to -fstack-protector to avoid
triggering checks due to alloca(). Examining the resulting syscall.o,
sees no changes in canary coverage (none before, none now).
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
arch/arm64/Kconfig | 1 +
arch/arm64/kernel/Makefile | 5 +++++
arch/arm64/kernel/syscall.c | 10 ++++++++++
3 files changed, 16 insertions(+)
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index a4a094bedcb2..2902e5316e1a 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -135,6 +135,7 @@ 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/Makefile b/arch/arm64/kernel/Makefile
index 151f28521f1e..39fc23d3770b 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -11,6 +11,11 @@ CFLAGS_REMOVE_ftrace.o = $(CC_FLAGS_FTRACE)
CFLAGS_REMOVE_insn.o = $(CC_FLAGS_FTRACE)
CFLAGS_REMOVE_return_address.o = $(CC_FLAGS_FTRACE)
+# Downgrade to -fstack-protector to avoid triggering unneeded stack canary
+# checks due to randomize_kstack_offset.
+CFLAGS_REMOVE_syscall.o += -fstack-protector-strong
+CFLAGS_syscall.o += $(subst -fstack-protector-strong,-fstack-protector,$(filter -fstack-protector-strong,$(KBUILD_CFLAGS)))
+
# Object file lists.
obj-y := debug-monitors.o entry.o irq.o fpsimd.o \
entry-common.o entry-fpsimd.o process.o ptrace.o \
diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index 5f5b868292f5..00d3c84db9cd 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)
--
2.25.1
