On Tue, 2016-02-09 at 13:26 +0100, Henning Schild wrote:
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
#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
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.