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Message-Id: <20200406231606.37619-6-keescook@chromium.org>
Date: Mon, 6 Apr 2020 16:16:06 -0700
From: Kees Cook <keescook@...omium.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Kees Cook <keescook@...omium.org>,
Elena Reshetova <elena.reshetova@...el.com>,
x86@...nel.org,
Andy Lutomirski <luto@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Alexander Potapenko <glider@...gle.com>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Jann Horn <jannh@...gle.com>,
kernel-hardening@...ts.openwall.com,
linux-arm-kernel@...ts.infradead.org,
linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: [PATCH v3 5/5] arm64: entry: Enable random_kstack_offset support
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@...omium.org>
---
arch/arm64/Kconfig | 1 +
arch/arm64/kernel/Makefile | 4 ++++
arch/arm64/kernel/syscall.c | 10 ++++++++++
3 files changed, 15 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/Makefile b/arch/arm64/kernel/Makefile
index fc6488660f64..b89005f125d6 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -11,6 +11,10 @@ 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_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 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
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