Re: Re: Splitting the mmap_sem

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Feb 06, 2020 at 01:20:24PM -0800, Matthew Wilcox wrote:
> On Thu, Feb 06, 2020 at 09:55:29PM +0100, Peter Zijlstra wrote:
> > On Thu, Feb 06, 2020 at 12:15:36PM -0800, Matthew Wilcox wrote:
> > > then, at the beginning of a page fault call srcu_read_lock(&vma_srcu);
> > > walk the tree as we do now, allocate memory for PTEs, sleep waiting for
> > > pages to arrive back from disc, etc, etc, then at the end of the fault,
> > > call srcu_read_unlock(&vma_srcu). 
> > 
> > So far so good,...
> > 
> > > munmap() would consist of removing the
> > > VMA from the tree, then calling synchronize_srcu() to wait for all faults
> > > to finish, then putting the backing file, etc, etc and freeing the VMA.
> > 
> > call_srcu(), and the (s)rcu callback will then fput() and such things
> > more.
> > 
> > synchronize_srcu() (like synchronize_rcu()) is stupid slow and would
> > make munmap()/exit()/etc.. unusable.
> 
> I'll need to think about that a bit.  I was convinced we needed to wait
> for the current pagefaults to finish before we could return from munmap().
> I need to convince myself that it's OK to return to userspace while the
> page faults for that range are still proceeding on other CPUs.

File-io might be in progress, any actual faults will result in SIGFAULT
instead of installing a PTE.

It is not fundamentally different from any threaded uaf race.

> > > This seems pretty reasonable, and investigation could actually proceed
> > > before the Maple tree work lands.  Today, that would be:
> > > 
> > > srcu_read_lock(&vmas_srcu);
> > > down_read(&mm->mmap_sem);
> > > find_vma(mm, address);
> > > up_read(&mm->mmap_sem);
> > > ... rest of fault handler path ...
> > > srcu_read_unlock(&vmas_srcu);
> > > 
> > > Kind of a pain because we still call find_vma() in the per-arch page
> > > fault handler, but for prototyping, we'd only have to do one or two
> > > architectures.
> > 
> > If you look at the earlier speculative page-fault patches by Laurent,
> > which were based on my still earlier patches, you'll find most of this
> > there.
> > 
> > The tricky bit was validating everything on the second page-table walk,
> > so see if nothing had fundamentally changed, specifically the VMA,
> > before installing the PTE. If you do this without mmap_sem, you need to
> > hold ptlock to pin stuff while validating everything you did earlier.
> 
> The patches Laurent posted used regular RCU and a per-VMA refcount, not
> SRCU.

That are his later patches, and I distinctly disagree with that
approach.

If you look at the patches here:

  https://lkml.kernel.org/r/cover.1479465699.git.ldufour@linux.vnet.ibm.com

you'll find it uses SRCU.

> If you use SRCU, why would you need a second page table walk?

Because SRCU only ensures the VMA object remains extant, it does not
prevent modification of it, normally that guarantee is provided by
mmap_sem, but we're not going to use that.

Instead, what we serialize on is the (split) ptlock. So we do the first
page-walk and ptlock to verify the vma-lookup, then we drop ptlock and
do the file-io, then we page-walk and take ptlock again, verify the vma
(again) and install the PTE. If anything goes wrong, we bail.

See this patch:

  https://lkml.kernel.org/r/301fb863785f37c319b493bd0d43167353871804.1479465699.git.ldufour@linux.vnet.ibm.com