linux-kernel - Re: [PATCH 10/20] signal/vm86_32: Properly send SIGSEGV when the vm86 state cannot be saved.

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Message-ID: <202110210915.BF17C14980@keescook>
Date:   Thu, 21 Oct 2021 09:16:02 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, H Peter Anvin <hpa@...or.com>
Subject: Re: [PATCH 10/20] signal/vm86_32: Properly send SIGSEGV when the
 vm86 state cannot be saved.

On Wed, Oct 20, 2021 at 12:43:56PM -0500, Eric W. Biederman wrote:
> Instead of pretending to send SIGSEGV by calling do_exit(SIGSEGV)
> call force_sigsegv(SIGSEGV) to force the process to take a SIGSEGV
> and terminate.
> 
> Update handle_signal to return immediately when save_v86_state fails
> and kills the process.  Returning immediately without doing anything
> except killing the process with SIGSEGV is also what signal_setup_done
> does when setup_rt_frame fails.  Plus it is always ok to return
> immediately without delivering a signal to a userspace handler when a
> fatal signal has killed the current process.

Do the tools/testing/selftests/x86 tests all pass after these changes? I
know Andy has a bunch of weird corner cases in there.

Reviewed-by: Kees Cook <keescook@...omium.org>

> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: x86@...nel.org
> Cc: H Peter Anvin <hpa@...or.com>
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> ---
>  arch/x86/kernel/signal.c  | 6 +++++-
>  arch/x86/kernel/vm86_32.c | 2 +-
>  2 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index f4d21e470083..25a230f705c1 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -785,8 +785,12 @@ handle_signal(struct ksignal *ksig, struct pt_regs *regs)
>  	bool stepping, failed;
>  	struct fpu *fpu = &current->thread.fpu;
>  
> -	if (v8086_mode(regs))
> +	if (v8086_mode(regs)) {
>  		save_v86_state((struct kernel_vm86_regs *) regs, VM86_SIGNAL);
> +		/* Has save_v86_state failed and killed the process? */
> +		if (fatal_signal_pending(current))
> +			return;
> +	}
>  
>  	/* Are we from a system call? */
>  	if (syscall_get_nr(current, regs) != -1) {
> diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
> index 63486da77272..040fd01be8b3 100644
> --- a/arch/x86/kernel/vm86_32.c
> +++ b/arch/x86/kernel/vm86_32.c
> @@ -159,7 +159,7 @@ void save_v86_state(struct kernel_vm86_regs *regs, int retval)
>  	user_access_end();
>  Efault:
>  	pr_alert("could not access userspace vm86 info\n");
> -	do_exit(SIGSEGV);
> +	force_sigsegv(SIGSEGV);
>  }
>  
>  static int do_vm86_irq_handling(int subfunction, int irqnumber);
> -- 
> 2.20.1
> 

-- 
Kees Cook