Re: [PATCH 3/4] mce/copyin: fix to not SIGBUS when copying from user hits poison

["Luck, Tony" <tony.luck@intel.com>]

On Wed, Apr 07, 2021 at 11:18:16PM +0200, Borislav Petkov wrote:
> On Thu, Mar 25, 2021 at 05:02:34PM -0700, Tony Luck wrote:
> > Andy Lutomirski pointed out that sending SIGBUS to tasks that
> > hit poison in the kernel copying syscall parameters from user
> > address space is not the right semantic.
> 
> What does that mean exactly?

Andy said that a task could check a memory range for poison by
doing:

	ret = write(fd, buf, size);
	if (ret == size) {
		memory range is all good
	}

That doesn't work if the kernel sends a SIGBUS.

It doesn't seem a likely scenario ... but Andy is correct that
the above ought to work.

> 
> From looking at the code, that is this conditional:
> 
>         if (t == EX_HANDLER_UACCESS && regs && is_copy_from_user(regs)) {
>                 m->kflags |= MCE_IN_KERNEL_RECOV;
>                 m->kflags |= MCE_IN_KERNEL_COPYIN;
> 
> so what does the above have to do with syscall params?

Most "copy from user" instances are the result of a system call parameter
(e.g. "buf" in the write(2) example above).

> If it is about us being in ring 0 and touching user memory and eating
> poison in same *user* memory while doing so, then sure, that makes
> sense.

Yes. This is for kernel reading memory belongng to "current" task.

> > So stop doing that. Add a new kill_me_never() call back that
> > simply unmaps and offlines the poison page.
> 
> Right, that's the same as handling poisoned user memory.

Same in that the page gets unmapped. Different in that there
is no SIGBUS if the kernel did the access for the user.

-Tony