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

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Message-ID: <87ee7lqe6k.fsf@email.froward.int.ebiederm.org>
Date:   Fri, 12 Nov 2021 15:03:47 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Brian Gerst <brgerst@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...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>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 09/20] signal/vm86_32: Replace open coded BUG_ON with an actual BUG_ON

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

> On Fri, Nov 12, 2021 at 11:57 AM Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>>
>> Still user space would have had to have mapped address 0 to get that
>> value set in do_sys_vm86.
>
> You have to map address 0 anyway just to get vm86 mode to work.
>
> vm86 mode fundamentally requires the low 1MB of virtual memory to be
> mapped, since there is no virtual memory offset in the vm86 model.

True.

However that also means if struct vm86plus_struct is at address 0
instead of the 16bit interrupt table something is badly wrong.

Still if we are going to check for userspace being silly that it should
be in do_sys_vm86.

I have managed to get the fuzzer that hit the problem to run and with
the test for !vm86->user_vm86 removed the BUG_ON is not being hit.

I am going to keep running it for a bit just to make certain, and
then I will put together a proper patch.

Eric