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

[Brian Gerst]

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