Re: save_xstate_sig (Re: frequent lockups in 3.18rc4)

[Andy Lutomirski]

On Thu, Dec 18, 2014 at 1:34 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Dec 18, 2014 at 1:17 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>
>> I admit that my understanding of the disaster that is x86's FPU handling is
>> limited, but I'm moderately confident that save_xstate_sig is broken.
>
> Very possible. The FPU code *is* nasty.
>
>> The code is:
>>
>>         if (user_has_fpu()) {
>>                 /* Save the live register state to the user directly. */
>>                 if (save_user_xstate(buf_fx))
>>                         return -1;
>>                 /* Update the thread's fxstate to save the fsave header. */
>>                 if (ia32_fxstate)
>>                         fpu_fxsave(&tsk->thread.fpu);
>>         } else {
>>                 sanitize_i387_state(tsk);
>>                 if (__copy_to_user(buf_fx, xsave, xstate_size))
>>                         return -1;
>>         }
>>
>> Suppose that user_has_fpu() returns true, we call save_user_xstate, and the
>> xsave instruction (or anything else in there, for that matter) causes a page
>> fault.
>>
>> The page fault handler is well within its rights to schedule.
>
> You don't even have to page fault. Preemption..
>
> But that shouldn't actually be the bug. This is just an optimization.
> If we have the FPU, we save it from the FP state, rather than copying
> it from our kernel copy. If we schedule (page fault, preemption,
> whatever) and lose the FPU, the code still works - we'll just take a
> TS fault, and have to reload the information.
>

Not if this happens:

    /*
     * Paranoid restore. send a SIGSEGV if we fail to restore the state.
     */
    if (unlikely(restore_fpu_checking(tsk))) {
        drop_init_fpu(tsk);
        force_sig_info(SIGSEGV, SEND_SIG_PRIV, tsk);
        return;
    }

I have no idea what, if anything, can cause FPU restore to fail, but
that looks like an infinite loop to me.

And the fact that we have an xsave instruction that can cause page
faults *and* has an extable fixup doesn't exactly inspire confidence,
but the code looks correct.

If this is easy enough for Dave to trigger, it could be worth
instrumenting __do_page_fault to log when a fault happens on that
xsave instruction and to maybe also log the outcome.  Do we know
whether your fault retry fixes solved the problem yet?


FWIW, Dave's run of my test seems to rule out easy bugs in his CPU,
and I couldn't trigger a bogus cr2 value on Sandy Bridge or Core 2
Quad.

--Andy

> So I'm with you in that there can certainly be bugs in the FPU
> handling, but I don't think this is one.
>
>                         Linus



-- 
Andy Lutomirski
AMA Capital Management, LLC
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