linux-kernel - Re: [PATCH v2 10/32] signal/vm86_32: Properly send SIGSEGV when the vm86 state cannot be saved.

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Date:   Mon, 25 Oct 2021 16:28:22 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.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>,
        "the arch\/x86 maintainers" <x86@...nel.org>,
        H Peter Anvin <hpa@...or.com>
Subject: Re: [PATCH v2 10/32] signal/vm86_32: Properly send SIGSEGV when the vm86 state cannot be saved.

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Mon, Oct 25, 2021 at 1:54 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>
>> Update save_v86_state to always complete all of it's work except
>> possibly some of the copies to userspace even if save_v86_state takes
>> a fault.  This ensures that the kernel is always in a sane state, even
>> if userspace has done something silly.
>
> Well, honestly, with this change, you might as well replace the
> force_sigsegv() with just a plain "force_sig()", and make it something
> the process can catch.

The trouble is I don't think there is enough information made available
for user space to do anything with the SIGSEGV.  My memory is that
applications like dosemu very much have a SIGSEGV handler.

So I think if it ever happened it could be quite confusing.  Not to
mention the pr_alert message.

But I guess if a test is written like you suggest we can include enough
information for someone to make sense of things.

> The only thing that "force_sigsgv()" does is to make SIGSEGV
> uncatchable. In contrast, a plain "force_sig()" just means that it
> can't be ignored - but it can be caught, and it is fatal only when not
> caught.
>
> And with the "always complete the non-vm86 state restore" part change,
> there's really no reason for it to not be caught.
>
> Of course, the other case (where we have no state information for the
> "enter vm86 mode" case) is still fatal, and is a "this should never
> happen". But the "cannot write to the vm86 save state" thing isn't
> technically fatal.
>
> It should even be possible to write a test for it: passing a read-only
> pointer to the vm86() system call. The vm86 entry will work (because
> it only reads the vm86 state from it), but then at vm86 exit, writing
> the state back will fail.
>
> Anybody?

I am enthusiastic about writing a test, but I will plod in that
direction just so I can get this sorted out.

Eric