Re: [Xen-devel] Patch "x86/entry/64: Remove %ebx handling from error_entry/exit" has been added to the 4.9-stable tree

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On Wed, 2018-11-28 at 08:44 -0800, Andy Lutomirski wrote:
> > Can we assume it's always from kernel? The Xen code definitely seems to
> > handle invoking this from both kernel and userspace contexts.
> 
> I learned that my comment here was wrong shortly after the patch landed :(

Turns out the only place I see it getting called from is under
__context_switch().

 #7 [ffff8801144a7cf0] new_xen_failsafe_callback at ffffffffa028028a 
[kmod_ebxfix]
 #8 [ffff8801144a7d90] xen_hypercall_update_descriptor at ffffffff8100114a
 #9 [ffff8801144a7db8] xen_hypercall_update_descriptor at ffffffff8100114a
#10 [ffff8801144a7df0] xen_mc_flush at ffffffff81006ab9
#11 [ffff8801144a7e30] xen_end_context_switch at ffffffff81004e12
#12 [ffff8801144a7e48] __switch_to at ffffffff81016582
#13 [ffff8801144a7ea0] __schedule at ffffffff815d2b37

That …114a in xen_hypercall_update_descriptor is the 'pop' instruction
right after the syscall; it's happening when Xen is preempting the
domain in the hypercall and then reloads the segment registers to run
that vCPU again later.

[  44185.225289]   WARN: RDX: 0000000000000000 RSI: 0000000000000000 RDI: 
0000000abbd76060

The update_descriptor hypercall args (rdi, rsi) were 0xabbd76060 and 0
respectively — it was setting a descriptor at that address, to zero.

Xen then failed to load the selector 0x63 into the %gs register (since
that descriptor has just been wiped?), leaving it zero.

[  44185.225256]   WARN: xen_failsafe_callback from 
xen_hypercall_update_descriptor+0xa/0x40
[  44185.225263]   WARN: DS: 2b/2b ES: 2b/2b FS: 0/0 GS:0/63

This is on context switch from a 32-bit task to idle. So
xen_failsafe_callback is returning to the "faulting" instruction, with
a comment saying "Retry the IRET", but in fact is just continuing on
its merry way with %gs unexpectedly set to zero.

In fact I think this is probably fine in practice, since it's about to
get explicitly set a few lines further down in __context_switch(). But
it's odd enough, and far enough away from what's actually said by the
comments, that I'm utterly unsure.

In xen_load_tls() we explicitly only do the lazy_load_gs(0) for the
32-bit kernel. Is that really right?

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