Re: [PATCH] riscv: Support RANDOMIZE_KSTACK_OFFSET

[Kees Cook]

On Wed, Nov 01, 2023 at 02:44:23PM +0800, Song Shuai wrote:
> Inspired from arm64's implement -- commit 70918779aec9
> ("arm64: entry: Enable random_kstack_offset support")
> 
> Add support of kernel stack offset randomization while handling syscall,
> the offset is defaultly limited by KSTACK_OFFSET_MAX() (i.e. 10 bits).
> 
> In order to avoid trigger stack canaries (due to __builtin_alloca) and
> slowing down the entry path, use __no_stack_protector attribute to
> disable stack protector for do_trap_ecall_u() at the function level.
> 
> Signed-off-by: Song Shuai <songshuaishuai@xxxxxxxxxxx>

I can't speak to the correctness of the entropy level, but the usage of
the helpers looks correct to me.

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
> Testing with randomize_kstack_offset=y cmdline, lkdtm/stack-entropy.sh
> showed appropriate stack offset instead of zero.
> ---
>  arch/riscv/Kconfig        |  1 +
>  arch/riscv/kernel/traps.c | 18 +++++++++++++++++-
>  2 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index d607ab0f7c6d..0e843de33f0c 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -100,6 +100,7 @@ config RISCV
>  	select HAVE_ARCH_KGDB_QXFER_PKT
>  	select HAVE_ARCH_MMAP_RND_BITS if MMU
>  	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
> +	select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
>  	select HAVE_ARCH_SECCOMP_FILTER
>  	select HAVE_ARCH_THREAD_STRUCT_WHITELIST
>  	select HAVE_ARCH_TRACEHOOK
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index 19807c4d3805..3f869b2d47c3 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -6,6 +6,7 @@
>  #include <linux/cpu.h>
>  #include <linux/kernel.h>
>  #include <linux/init.h>
> +#include <linux/randomize_kstack.h>
>  #include <linux/sched.h>
>  #include <linux/sched/debug.h>
>  #include <linux/sched/signal.h>
> @@ -296,9 +297,11 @@ asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs)
>  	}
>  }
>  
> -asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
> +asmlinkage __visible __trap_section  __no_stack_protector
> +void do_trap_ecall_u(struct pt_regs *regs)
>  {
>  	if (user_mode(regs)) {
> +
>  		long syscall = regs->a7;
>  
>  		regs->epc += 4;
> @@ -308,10 +311,23 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
>  
>  		syscall = syscall_enter_from_user_mode(regs, syscall);
>  
> +		add_random_kstack_offset();
> +
>  		if (syscall >= 0 && syscall < NR_syscalls)
>  			syscall_handler(regs, syscall);
>  		else if (syscall != -1)
>  			regs->a0 = -ENOSYS;
> +		/*
> +		 * Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),
> +		 * so the maximum stack offset is 1k bytes (10 bits).
> +		 *
> +		 * The actual entropy will be further reduced by the compiler when
> +		 * applying stack alignment constraints: 16-byte (i.e. 4-bit) aligned
> +		 * for RV32I or RV64I.
> +		 *
> +		 * The resulting 6 bits of entropy is seen in SP[9:4].
> +		 */
> +		choose_random_kstack_offset(get_random_u16());
>  
>  		syscall_exit_to_user_mode(regs);
>  	} else {
> -- 
> 2.20.1
> 

-- 
Kees Cook