Re: [GIT PULL] x86/mm for 6.4

[Linus Torvalds]

On Fri, Apr 28, 2023 at 5:38 PM Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
>
> BTW, I think the static check can be relaxed. Checking size against
> PAGE_SIZE is rather conservative: there's 8 TB (or 4 PB for 5-level
> paging) guard hole at the begging of kernel address space.

So I don't worry about the size per se - we just don't have any
constant sized accesses that are bigger than a page.

The constant-sized case is for things like structures being copied to
user space.

And having a bug gap is nice for the suzkaller case, although I don't
think that GP fault has triggered lately (or ever, I don't remember).
Having random system call arguments that trigger "oh, this is in the
non-canonical region" is a good thing.

> > So being careful about the range is kind of annoying, when we don't
> > really need it.
>
> Hm. Is there anybody who access high to low after the check (glibc
> memcpy() bug flashbacks)? Or not in any particular order?

Yeah, I can't think of a single case, which is why it seems so silly
to even bother.

Almost all real life cases end up being limited by things like the
page/folio size.

We do have exceptions, like module loading etc that might copy a
bigger area from user space, but no, we don't have any backwards
copies.

So you'd almost have to have some "access_ok()" followed by random
access with a user-controlled offset, and that seems nonsensical and
fundamentally impossible anyway.

But just because I can't think of it, and go "that would be insane"
doesn't mean that some driver ioctl interface might not try it.

Which is why I think having others look at it would be a good idea.

               Linus