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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Fri, 12 Nov 2021 12:51:32 -0500
From:   Brian Gerst <brgerst@...il.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        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>,
        "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

On Fri, Nov 12, 2021 at 10:41 AM Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
> "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

It's possible that a null pointer was passed to the vm86 syscall.
Since vm86 mode usually requires memory to be mapped at address 0 this
wouldn't trigger a fault when reading the vm86_struct data.  It should
be fine to remove !vm86->user_vm86 from the BUG_ON(), since the write
to userspace can handle a fault.

--
Brian Gerst