Re: [PATCH v7 5/6] arm64: entry: Enable random_kstack_offset support

[Will Deacon]

On Fri, Mar 19, 2021 at 02:28:34PM -0700, Kees Cook wrote:
> Allow for a randomized stack offset on a per-syscall basis, with roughly
> 5 bits of entropy. (And include AAPCS rationale AAPCS thanks to Mark
> Rutland.)
> 
> In order to avoid unconditional stack canaries on syscall entry (due to
> the use of alloca()), also disable stack protector to avoid triggering
> needless checks and slowing down the entry path. As there is no general
> way to control stack protector coverage with a function attribute[1],
> this must be disabled at the compilation unit level. This isn't a problem
> here, though, since stack protector was not triggered before: examining
> the resulting syscall.o, there are no changes in canary coverage (none
> before, none now).
> 
> [1] a working __attribute__((no_stack_protector)) has been added to GCC
> and Clang but has not been released in any version yet:
> https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=346b302d09c1e6db56d9fe69048acb32fbb97845
> https://reviews.llvm.org/rG4fbf84c1732fca596ad1d6e96015e19760eb8a9b
> 
> 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 1f212b47a48a..2d0e5f544429 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -146,6 +146,7 @@ config ARM64
>  	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
>  	select HAVE_ARCH_PFN_VALID
>  	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 ed65576ce710..6cc97730790e 100644
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -9,6 +9,11 @@ CFLAGS_REMOVE_ftrace.o = $(CC_FLAGS_FTRACE)
>  CFLAGS_REMOVE_insn.o = $(CC_FLAGS_FTRACE)
>  CFLAGS_REMOVE_return_address.o = $(CC_FLAGS_FTRACE)
>  
> +# Remove stack protector to avoid triggering unneeded stack canary
> +# checks due to randomize_kstack_offset.
> +CFLAGS_REMOVE_syscall.o	 = -fstack-protector -fstack-protector-strong
> +CFLAGS_syscall.o	+= -fno-stack-protector
> +
>  # 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 b9cf12b271d7..58227a1c207e 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>
> @@ -43,6 +44,8 @@ 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)];
> @@ -55,6 +58,13 @@ static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
>  		ret = lower_32_bits(ret);
>  
>  	regs->regs[0] = ret;
> +
> +	/*
> +	 * The AAPCS mandates a 16-byte (i.e. 4-bit) aligned SP at
> +	 * function boundaries. We want at least 5 bits of entropy so we
> +	 * must randomize at least SP[8:4].
> +	 */
> +	choose_random_kstack_offset(get_random_int() & 0x1FF);

Not sure about either of these new calls -- aren't we preemptible in
invoke_syscall()?

Will