On Thu, Nov 01, 2018 at 04:06:19PM -0700, Linus Torvalds wrote:

On Thu, Nov 1, 2018 at 4:00 PM Kees Cook [off-list ref] wrote:

quoted

+       memset(&rsp, 0, sizeof(rsp));
+       rsp.response = VIRTIO_SCSI_S_FUNCTION_REJECTED;
+       resp = vq->iov[out].iov_base;
+       ret = __copy_to_user(resp, &rsp, sizeof(rsp));

Is it actually safe to trust that iov_base has passed an earlier
access_ok() check here? Why not just use copy_to_user() instead?

Good point.

We really should have removed those double-underscore things ages ago.

FWIW, on arm64 we always check/sanitize the user address as a result of
our sanitization of speculated values. Almost all of our uaccess
routines have an explicit access_ok().

All our uaccess routines mask the user pointer based on addr_limit,
which prevents speculative or architectural uaccess to kernel addresses
when addr_limit it USER_DS:

    4d8efc2d5ee4c9cc ("arm64: Use pointer masking to limit uaccess speculation")

We also inhibit speculative stores to addr_limit being forwarded under
speculation:

    c2f0ad4fc089cff8 ("arm64: uaccess: Prevent speculative use of the current addr_limit")

... and given all that, we folded explicit access_ok() checks into
__{get,put}_user():

    84624087dd7e3b48 ("arm64: uaccess: Don't bother eliding access_ok checks in __{get, put}_user")

IMO we could/should do the same for __copy_{to,from}_user().

Thanks,
Mark.