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Date:   Thu, 21 Oct 2021 09:15:06 -0700
From:   Kees Cook <keescook@chromium.org>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, H Peter Anvin <hpa@zytor.com>
Subject: Re: [PATCH 09/20] signal/vm86_32: Replace open coded BUG_ON with an
 actual BUG_ON
Message-ID: <202110210914.59245E29CF@keescook>
References: <87y26nmwkb.fsf@disp2133>
 <20211020174406.17889-9-ebiederm@xmission.com>
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On Wed, Oct 20, 2021 at 12:43:55PM -0500, Eric W. Biederman wrote:
> The function save_v86_state is only called when userspace was
> operating in vm86 mode before entering the kernel.  Not having vm86
> state in the task_struct should never happen.  So transform the hand
> rolled BUG_ON into an actual BUG_ON to make it clear what is
> happening.

If this is actually not a state userspace can put itself into:

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

Otherwise, this should be a WARN+kill.

> 
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: x86@kernel.org
> Cc: H Peter Anvin <hpa@zytor.com>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
>  arch/x86/kernel/vm86_32.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
> index e5a7a10a0164..63486da77272 100644
> --- a/arch/x86/kernel/vm86_32.c
> +++ b/arch/x86/kernel/vm86_32.c
> @@ -106,10 +106,8 @@ void save_v86_state(struct kernel_vm86_regs *regs, int retval)
>  	 */
>  	local_irq_enable();
>  
> -	if (!vm86 || !vm86->user_vm86) {
> -		pr_alert("no user_vm86: BAD\n");
> -		do_exit(SIGSEGV);
> -	}
> +	BUG_ON(!vm86 || !vm86->user_vm86);
> +
>  	set_flags(regs->pt.flags, VEFLAGS, X86_EFLAGS_VIF | vm86->veflags_mask);
>  	user = vm86->user_vm86;
>  
> -- 
> 2.20.1
> 

-- 
Kees Cook