Re: [PATCH v2 0/4] x86: sigcontext fixes, again

[Toshi Kani]

On Wed, 2015-10-28 at 13:22 -0600, Toshi Kani wrote:
> On Wed, 2015-10-28 at 10:34 -0600, Toshi Kani wrote:
> > On Wed, 2015-10-28 at 12:53 +0300, Stas Sergeev wrote:
> > > 28.10.2015 03:04, Toshi Kani ÐÐÑÐÑ:
> > > > On Wed, 2015-10-28 at 07:37 +0900, Linus Torvalds wrote:
> > > > > On Tue, Oct 27, 2015 at 11:05 PM, Stas Sergeev <stsp@xxxxxxx>
> > > > > wrote:
> > > > > > 
> > > > > > I can't easily post an Oops: under X it doesn't even appear -
> > > > > > machine freezes immediately, and under non-KMS console it is
> > > > > > possible to get one, but difficult to screen-shot (using bare
> > > > > > metal, not VM). Also the Oops was seemingly unrelated.
> > > > > > And if you run "dosemu -s" under non-KMS console, you'll also
> > > > > > reproduce this one:
> > > > > > https://bugzilla.kernel.org/show_bug.cgi?id=97321
> > > > > 
> > > > > Hmm. Andrew Morton responded to that initially, but then nothing
> > > > > happened, and now it's been another six months. Andrew?
> > > > > 
> > > > > The arch/x86/mm/pat.c error handling does seem to be suspect. This 
> > > > > is all code several years old, so none of this is new, and I think
> > > > > Suresh is gone.  Adding a few other people with recent sign-offs to 
> > > > > that file, in the hope that somebody feels like they own it..
> > > > 
> > > > In the case of PFNMAP, the range should always be mapped.  So, I 
> > > > wonder why follow_phys() failed with the !pte_present() check.
> > > > 
> > > > Stas, do you have a test program that can reproduce 97321?
> > > Get dosemu2 from here:
> > > https://github.com/stsp/dosemu2/releases
> > > or from git, or get dosemu1.
> > > Then boot your kernel with "nomodeset=1" to get a text console.
> > > Run
> > > 
> > > dosemu -s
> > > 
> > > and you'll get the bug.
> 
> I looked at the dosemu code and was able to reproduce the issue with a test
> program.  This problem happens when mremap() to /dev/mem (or PFNMAP) is
> called with MREMAP_FIXED.
> 
> In this case, mremap calls move_vma(), which first calls move_page_tables()
> to remap the translation and then calls do_munmap() to remove the original
> mapping.  Hence, when untrack_pfn() is called from do_munmap(), the
> original map is already removed, and follow_phys() fails with the
>  !pte_present() check.
> 
> I think there are a couple of issues:
>  - If untrack_pfn() ignores an error from follow_phys() and skips
> free_pfn_range(), PAT continues to track the original map that is removed.
>  - untrack_pfn() calls free_pfn_range() to untrack a given free range. 
>  However, rbt_memtype_erase() requires the free range match exactly to the
> tracked range.  This does not support mremap, which needs to free up part
> of the tracked range.
>  - PAT does not track a new translation specified by mremap() with MREMAP_F
> IXED.

Thinking further, I think the 1st and 3rd items are non-issues.  mremap remaps
virtual address, but keeps the same cache type and pfns.  So, PAT does not have
to change the tracked pfns in this case.  The 2nd item is still a problem,
though. 

Thanks,
-Toshi
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