[PATCH v2] userfaultfd: preserve user-supplied address tag in struct uffd_msg

[Catalin Marinas]

On Thu, Jul 01, 2021 at 10:27:31PM -0700, Lokesh Gidra wrote:
> On Thu, Jul 1, 2021 at 10:50 AM Peter Collingbourne <pcc at google.com> wrote:
> > On Thu, Jul 1, 2021 at 8:51 AM Catalin Marinas <catalin.marinas at arm.com> wrote:
> > > On Wed, Jun 30, 2021 at 04:29:31PM -0700, Peter Collingbourne wrote:
> > > > If a user program uses userfaultfd on ranges of heap memory, it may
> > > > end up passing a tagged pointer to the kernel in the range.start
> > > > field of the UFFDIO_REGISTER ioctl. This can happen when using an
> > > > MTE-capable allocator, or on Android if using the Tagged Pointers
> > > > feature for MTE readiness [1].
> > >
> > > When we added the tagged addr ABI, we realised it's nearly impossible to
> > > sort out all ioctls, so we added a note to the documentation that any
> > > address other than pointer to user structures as arguments to ioctl()
> > > should be untagged. Arguably, userfaultfd is not a random device but if
> > > we place it in the same category as mmap/mremap/brk, those don't allow
> > > tagged pointers either. And we do expect some apps to break when they
> > > rely on malloc() to return untagged pointers.
> >
> > Okay, so arguably another approach would be to make userfaultfd
> > consistent with mmap/mremap/brk and let the UFFDIO_REGISTER fail if
> > given a tagged address.
>
> This approach also seems reasonable. The problem, as things stand
> today, is that UFFDIO_REGISTER doesn't complain when a tagged pointer
> is used to register a memory range. But eventually the returned fault
> address in messages are untagged. If UFFDIO_REGISTER were to fail on
> passing a tagged pointer, then the userspace can address the issue.

On the mmap etc. functions we get an error as a side effect of addr
being larger than TASK_SIZE (unless explicitly untagged). The
userfaultfd_register() function had similar checks but they were relaxed
by commit 7d0325749a6c ("userfaultfd: untag user pointers").

I think we should revert the above, or part of it. We did something
similar for mmap/mremap/brk when untagging the address broke glibc:
commit dcde237319e6 ("mm: Avoid creating virtual address aliases in
brk()/mmap()/mremap()").

-- 
Catalin