Re: [patch 13/16] x86/ldt: Introduce LDT write fault handler

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, 12 Dec 2017, Thomas Gleixner wrote:

> On Tue, 12 Dec 2017, Thomas Gleixner wrote:
> > On Tue, 12 Dec 2017, Dave Hansen wrote:
> > 
> > > On 12/12/2017 11:21 AM, Thomas Gleixner wrote:
> > > > The only critical interaction is the return to user path (user CS/SS) and
> > > > we made sure with the LAR touching that these are precached in the CPU
> > > > before we go into fragile exit code.
> > > 
> > > How do we make sure that it _stays_ cached?
> > > 
> > > Surely there is weird stuff like WBINVD or SMI's that can come at very
> > > inconvenient times and wipe it out of the cache.
> > 
> > This does not look like cache in the sense of memory cache. It seems to be
> > CPU internal state and I just stuffed WBINVD and alternatively CLFLUSH'ed
> > the entries after the 'touch' via LAR. Still works.
> 
> Dave pointed me once more to the following paragraph in the SDM, which
> Peter and I looked at before and we tried that w/o success:
> 
>     If the segment descriptors in the GDT or an LDT are placed in ROM, the
>     processor can enter an indefinite loop if software or the processor
>     attempts to update (write to) the ROM-based segment descriptors. To
>     prevent this problem, set the accessed bits for all segment descriptors
>     placed in a ROM. Also, remove operating-system or executive code that
>     attempts to modify segment descriptors located in ROM.
> 
> Now that made me go back to the state of the patch series which made us
> make that magic 'touch' and write fault handler. The difference to the code
> today is that it did not prepopulate the user visible mapping.
> 
> We added that later because we were worried about not being able to
> populate it in the #PF due to memory pressure without ripping out the magic
> cure again.
> 
> But I did now and actually removing both the user exit magic 'touch' code
> and the write fault handler keeps it working.
> 
> Removing the prepopulate code makes it break again with a #GP in
> IRET/SYSRET.
> 
> What happens there is that the IRET pops SS (with a minimal testcase) which
> causes the #PF. That populates the PTE and returns happily. Right after
> that the #GP comes in with IP pointing to the user space instruction right
> after the syscall.
> 
> That simplifies and descaryfies that code massively.
> 
> Darn, I should have gone back and check every part again as I usually do,
> but my fried brain failed.

The magic write ACCESS bit handler is a left over from the early attempts
not to force ACCESS=1 when setting up the descriptor entry.

Bah. My patch stack history proves where the 3 cross roads are where I took
the wrong turn.

Thanks,

	tglx

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