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@google.com wrote:
On Thu, Jul 1, 2021 at 8:51 AM Catalin Marinas catalin.marinas@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()").