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SubjectRe: [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user recovery
On Fri, Aug 20, 2021 at 07:31:43PM +0200, Borislav Petkov wrote:
> On Tue, Aug 17, 2021 at 05:29:40PM -0700, Tony Luck wrote:
> > + /* Ten is likley overkill. Don't expect more than two faults before task_work() */
>
> "likely"

Oops.

>
> > + if (count > 10)
> > + mce_panic("Too many machine checks while accessing user data", m, msg);
>
> Ok, aren't we too nasty here? Why should we panic the whole box even
> with 10 MCEs? It is still user memory...
>
> IOW, why not:
>
> if (count > 10)
> current->mce_kill_me.func = kill_me_now;
>
> and when we return, that user process dies immediately.

It's the "when we return" part that is the problem here. Logical
trace looks like:

user-syscall:

kernel does get_user() or copyin(), hits user poison address

machine check
sees that this was kernel get_user()/copyin() and
uses extable to "return" to exception path

still in kernel, see that get_user() or copyin() failed

Kernel does another get_user() or copyin() (maybe the first
was inside a pagefault_disable() region, and kernel is trying
again to see if the error was a fixable page fault. But that
wasn't the problem so ...

machine check
sees that this was kernel get_user()/copyin() and
uses extable to "return" to exception path

still in kernel ... but persistently thinks that just trying again
might fix it.

machine check
sees that this was kernel get_user()/copyin() and
uses extable to "return" to exception path

still in kernel ... this time for sure! get_user()

machine check
sees that this was kernel get_user()/copyin() and
uses extable to "return" to exception path

still in kernel ... but you may see the pattern get_user()

machine check
sees that this was kernel get_user()/copyin() and
uses extable to "return" to exception path

I'm bored typing this, but the kernel may not ever give up

machine check
sees that this was kernel get_user()/copyin() and
uses extable to "return" to exception path

I.e. the kernel doesn't ever get to call current->mce_kill_me.func()

I do have tests that show as many as 4 consecutive machine checks
before the kernel gives up trying and returns to the user to complete
recovery.

Maybe the message could be clearer?

mce_panic("Too many consecutive machine checks in kernel while accessing user data", m, msg);

>
> > + /* Second or later call, make sure page address matches the one from first call */
> > + if (count > 1 && (current->mce_addr >> PAGE_SHIFT) != (m->addr >> PAGE_SHIFT))
> > + mce_panic("Machine checks to different user pages", m, msg);
>
> Same question here.

Not quite the same answer ... but similar. We could in theory handle
multiple different machine check addresses by turning the "mce_addr"
field in the task structure into an array and saving each address so
that when the kernel eventually gives up poking at poison and tries
to return to user kill_me_maybe() could loop through them and deal
with each poison page.

I don't think this can happen. Jue Wang suggested that multiple poisoned
pages passed to a single write(2) syscall might trigger this panic (and
because of a bug in my earlier version, he managed to trigger this
"different user pages" panic). But this fixed up version survives the
"Jue test".

-Tony

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