linux-kernel - Re: [PATCH 09/20] signal/vm86_32: Replace open coded BUG_ON with an actual BUG

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Message-ID: <874k8htmb2.fsf@email.froward.int.ebiederm.org>
Date:   Fri, 12 Nov 2021 09:40:01 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     linux-kernel@...r.kernel.org
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Al Viro <viro@...IV.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, H Peter Anvin <hpa@...or.com>,
        "Andy Lutomirski" <luto@...nel.org>
Subject: Re: [PATCH 09/20] signal/vm86_32: Replace open coded BUG_ON with an actual BUG_ON

"Eric W. Biederman" <ebiederm@...ssion.com> writes:

> 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.

Now that this change has been merged into Linus' tree I have a report
that it is possible to trigger this new BUG_ON.  Which obviously is not
good.

We could revert the change but I think that would just be shooting the
messenger.

Does anyone have an idea where to start to track down what is going on?

A very quick skim through the code suggests that the only code path
that calls save_v86_state that has not already accessed is
current->thread.vm86 is handle_signal.

Another quick look suggests that the only place where X86_VM_MASK gets
set in eflags is in do_sys_vm86.  So it appears do_sys_vm86 must
be called before v8086_mode returns true in handle_signal.

Which seems to suggest that the bug on can't trigger.

But that is obviously wrong.

I will keep digging but if anyone has some ideas that would be appreciated.

Eric

> 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/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;