Re: [PATCH] x86/mm/vmfault: Make vmalloc_fault() handle large pages

[Toshi Kani <toshi.kani@hpe.com>]

On Tue, 2016-02-09 at 13:26 +0100, Henning Schild wrote:
> On Tue, 9 Feb 2016 11:22:35 +0100
> Ingo Molnar <mingo@kernel.org> wrote:
> 
> > * Henning Schild <henning.schild@siemens.com> wrote:
> > 
> > > On Tue, 9 Feb 2016 10:10:03 +0100
> > > Ingo Molnar <mingo@kernel.org> wrote:
> > >   
> > > > * Toshi Kani <toshi.kani@hpe.com> wrote:
> > > >   
> > > > > Since 4.1, ioremap() supports large page (pud/pmd) mappings in
> > > > > x86_64 and PAE. vmalloc_fault() however assumes that the vmalloc
> > > > > range is limited to pte mappings.
> > > > > 
> > > > > pgd_ctor() sets the kernel's pgd entries to user's during
> > > > > fork(), which makes user processes share the same page tables
> > > > > for the kernel ranges.  When a call to ioremap() is made at
> > > > > run-time that leads to allocate a new 2nd level table (pud in
> > > > > 64-bit and pmd in PAE), user process needs to re-sync with the
> > > > > updated kernel pgd entry with vmalloc_fault().
> > > > > 
> > > > > Following changes are made to vmalloc_fault().    
> > > > 
> > > > So what were the effects of this shortcoming? Were large page
> > > > ioremap()s unusable? Was this harmless because no driver used this
> > > > facility?  
> > > 
> > > Drivers do use huge ioremap()s. Now if a pre-existing mm is used to
> > > access the device memory a #PF and the call to vmalloc_fault would
> > > eventually make the kernel treat device memory as if it was a
> > > pagetable.
> > > The results are illegal reads/writes on iomem and dereferencing
> > > iomem content like it was a pointer to a lower level pagetable.
> > > - #PF if you are lucky

#PF -> vmalloc_fault -> oops

> > > - funny modification of arbitrary memory possible
> > > - can be abused with uio or regular userland ??   
> 
> Looking over the code again i am not sure the last two are even
> possible, it is just the pointer deref that can cause a #PF.
> If the pointer turns out to "work" the code will just read and
> eventually BUG().

The last two case are not possible.

> > Ok, so this is a serious live bug exposed to drivers, that also
> > requires a Cc: stable tag.

Yes, the fix should go to stable as well.

Thanks,
-Toshi

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