11:26 p.m.
On Tue, 9 Feb 2016 11:22:35 +0100
Ingo Molnar <mingo(a)kernel.org> wrote:
* Henning Schild <henning.schild(a)siemens.com> wrote:
> On Tue, 9 Feb 2016 10:10:03 +0100
> Ingo Molnar <mingo(a)kernel.org> wrote:
>
> > * Toshi Kani <toshi.kani(a)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
> - 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().
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