linux-kernel - Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

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Date:   Mon, 8 Apr 2019 07:49:40 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Elena Reshetova <elena.reshetova@...el.com>
Cc:     luto@...nel.org, linux-kernel@...r.kernel.org, luto@...capital.net,
        keescook@...omium.org, jannh@...gle.com, enrico.perla@...el.com,
        mingo@...hat.com, bp@...en8.de, tglx@...utronix.de,
        peterz@...radead.org, gregkh@...uxfoundation.org
Subject: Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

On Mon, Apr 08, 2019 at 09:13:58AM +0300, Elena Reshetova wrote:
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 7bc105f47d21..38ddc213a5e9 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -35,6 +35,12 @@
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/syscalls.h>
>  
> +#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
> +#include <linux/random.h>
> +
> +void *alloca(size_t size);
> +#endif
> +
>  #ifdef CONFIG_CONTEXT_TRACKING
>  /* Called on entry from user mode with IRQs off. */
>  __visible inline void enter_from_user_mode(void)
> @@ -273,6 +279,13 @@ __visible void do_syscall_64(unsigned long nr, struct pt_regs *regs)
>  {
>  	struct thread_info *ti;
>  
> +#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
> +	size_t offset = ((size_t)prandom_u32()) % 256;
> +	char *ptr = alloca(offset);
> +
> +	asm volatile("":"=m"(*ptr));
> +#endif
> +
>  	enter_from_user_mode();
>  	local_irq_enable();
>  	ti = current_thread_info();

Would it make sense to also do this for the compat syscalls
(do_fast_syscall_32, do_int80_syscall_32)?

-- 
Josh