Re: [PATCH] loongarch: Support RANDOMIZE_KSTACK_OFFSET

[Huacai Chen]

Hi, Jinjie,

Thank you for your patch, but I think it is better to use drdtime()
instead of get_random_u16()? drdtime() is similar to rdtsc() of x86
which is defined in arch/loongarch/include/asm/loongarch.h.

Huacai

On Thu, Jul 18, 2024 at 8:42 PM Jinjie Ruan <ruanjinjie@xxxxxxxxxx> wrote:
>
> Add support of kernel stack offset randomization while handling syscall,
> the offset is defaultly limited by KSTACK_OFFSET_MAX().
>
> 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_syscall() at function level.
>
> With this patch, the REPORT_STACK test show that:
>         `loongarch64 bits of stack entropy: 7`
>
> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
> ---
>  arch/loongarch/Kconfig          |  1 +
>  arch/loongarch/kernel/syscall.c | 16 +++++++++++++++-
>  2 files changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
> index ddc042895d01..fcf6451b4e38 100644
> --- a/arch/loongarch/Kconfig
> +++ b/arch/loongarch/Kconfig
> @@ -106,6 +106,7 @@ config LOONGARCH
>         select HAVE_ARCH_KFENCE
>         select HAVE_ARCH_KGDB if PERF_EVENTS
>         select HAVE_ARCH_MMAP_RND_BITS if MMU
> +       select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
>         select HAVE_ARCH_SECCOMP
>         select HAVE_ARCH_SECCOMP_FILTER
>         select HAVE_ARCH_TRACEHOOK
> diff --git a/arch/loongarch/kernel/syscall.c b/arch/loongarch/kernel/syscall.c
> index ec17cd5163b7..a332c6cb76ec 100644
> --- a/arch/loongarch/kernel/syscall.c
> +++ b/arch/loongarch/kernel/syscall.c
> @@ -9,6 +9,7 @@
>  #include <linux/entry-common.h>
>  #include <linux/errno.h>
>  #include <linux/linkage.h>
> +#include <linux/randomize_kstack.h>
>  #include <linux/syscalls.h>
>  #include <linux/unistd.h>
>
> @@ -39,7 +40,7 @@ void *sys_call_table[__NR_syscalls] = {
>  typedef long (*sys_call_fn)(unsigned long, unsigned long,
>         unsigned long, unsigned long, unsigned long, unsigned long);
>
> -void noinstr do_syscall(struct pt_regs *regs)
> +__no_stack_protector void noinstr do_syscall(struct pt_regs *regs)
>  {
>         unsigned long nr;
>         sys_call_fn syscall_fn;
> @@ -55,11 +56,24 @@ void noinstr do_syscall(struct pt_regs *regs)
>
>         nr = syscall_enter_from_user_mode(regs, nr);
>
> +       add_random_kstack_offset();
> +
>         if (nr < NR_syscalls) {
>                 syscall_fn = sys_call_table[nr];
>                 regs->regs[4] = syscall_fn(regs->orig_a0, regs->regs[5], regs->regs[6],
>                                            regs->regs[7], regs->regs[8], regs->regs[9]);
>         }
>
> +       /*
> +        * This value will get limited by KSTACK_OFFSET_MAX(), which is 10
> +        * bits. The actual entropy will be further reduced by the compiler
> +        * when applying stack alignment constraints: 16-byte (i.e. 4-bit)
> +        * aligned, which will remove the 4 low bits from any entropy chosen
> +        * here.
> +        *
> +        * 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);
>  }
> --
> 2.34.1
>
>