[kernel-hardening] Re: [PATCHv2 2/2] x86: SROP mitigation: implement signal cookies

["Mika Penttilä" <mika.penttila@nextfour.com>]

Hi,


On 07.02.2016 01:39, Scott Bauer wrote:
> This patch adds SROP mitigation logic to the x86 signal delivery
> and sigreturn code. The cookie is placed in the unused alignment
> space above the saved FP state, if it exists. If there is no FP
> state to save then the cookie is placed in the alignment space above
> the sigframe.
>
> Cc: Abhiram Balasubramanian <abhiram@cs.utah.edu>
> Signed-off-by: Scott Bauer <sbauer@eng.utah.edu>
> ---
>  arch/x86/ia32/ia32_signal.c        | 63 +++++++++++++++++++++++++---
>  arch/x86/include/asm/fpu/signal.h  |  1 +
>  arch/x86/include/asm/sighandling.h |  5 ++-
>  arch/x86/kernel/fpu/signal.c       | 10 +++++
>  arch/x86/kernel/signal.c           | 86 +++++++++++++++++++++++++++++++++-----
>  5 files changed, 146 insertions(+), 19 deletions(-)
>
> diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
> index 0552884..2751f47 100644
> --- a/arch/x86/ia32/ia32_signal.c
> +++ b/arch/x86/ia32/ia32_signal.c
> @@ -68,7 +68,8 @@
>  }
>  
>  static int ia32_restore_sigcontext(struct pt_regs *regs,
> -				   struct sigcontext_32 __user *sc)
> +				   struct sigcontext_32 __user *sc,
> +				   void __user **user_cookie)
>  {
>  	unsigned int tmpflags, err = 0;
>  	void __user *buf;
> @@ -105,6 +106,16 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
>  		buf = compat_ptr(tmp);
>  	} get_user_catch(err);
>  
> +	/*
> +	 * If there is fp state get cookie from the top of the fp state,
> +	 * else get it from the top of the sig frame.
> +	 */
> +
> +	if (tmp != 0)
> +		*user_cookie = compat_ptr(tmp + fpu__getsize(1));
> +	else
> +		*user_cookie = NULL;
> +
>  	err |= fpu__restore_sig(buf, 1);
>  
>  	force_iret();
> @@ -117,6 +128,7 @@ asmlinkage long sys32_sigreturn(void)
>  	struct pt_regs *regs = current_pt_regs();
>  	struct sigframe_ia32 __user *frame = (struct sigframe_ia32 __user *)(regs->sp-8);
>  	sigset_t set;
> +	void __user *user_cookie;
>  
>  	if (!access_ok(VERIFY_READ, frame, sizeof(*frame)))
>  		goto badframe;
> @@ -129,8 +141,15 @@ asmlinkage long sys32_sigreturn(void)
>  
>  	set_current_blocked(&set);
>  
> -	if (ia32_restore_sigcontext(regs, &frame->sc))
> +	if (ia32_restore_sigcontext(regs, &frame->sc, &user_cookie))
> +		goto badframe;
> +
> +	if (user_cookie == NULL)
> +		user_cookie = compat_ptr((regs->sp - 8) + sizeof(*frame));
> +
> +	if (verify_clear_sigcookie(user_cookie))
>  		goto badframe;
> +
>  	return regs->ax;
>  
>  badframe:
> @@ -142,6 +161,7 @@ asmlinkage long sys32_rt_sigreturn(void)
>  {
>  	struct pt_regs *regs = current_pt_regs();
>  	struct rt_sigframe_ia32 __user *frame;
> +	void __user *user_cookie;
>  	sigset_t set;
>  
>  	frame = (struct rt_sigframe_ia32 __user *)(regs->sp - 4);
> @@ -153,7 +173,13 @@ asmlinkage long sys32_rt_sigreturn(void)
>  
>  	set_current_blocked(&set);
>  
> -	if (ia32_restore_sigcontext(regs, &frame->uc.uc_mcontext))
> +	if (ia32_restore_sigcontext(regs, &frame->uc.uc_mcontext, &user_cookie))
> +		goto badframe;
> +
> +	if (user_cookie == NULL)
> +		user_cookie = (void __user *)((regs->sp - 4) + sizeof(*frame));
regs->sp is already restored so you should use frame instead.

--Mika