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

[Ingo Molnar]

* Henning Schild <henning.schild@xxxxxxxxxxx> wrote:

> On Tue, 9 Feb 2016 10:10:03 +0100
> Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> 
> > * Toshi Kani <toshi.kani@xxxxxxx> 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
> - funny modification of arbitrary memory possible
> - can be abused with uio or regular userland ?? 

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

All of this should have been in the changelog!

Thanks,

	Ingo