Re: [GIT PULL] x86/mm for 6.4

[Linus Torvalds]

On Wed, May 3, 2023 at 9:45 AM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> On 5/3/23 09:38, Linus Torvalds wrote:
> > If somebody actually tries to untag a kernel address, that would be a
> > bug anyway, as far as I can tell.
>
> Is it a bug?  The do_madvise() path, for instance, is passing a value in
> there that came right from userspace.

That's still a "user address" - just not a *valid* one.

So we do not want to mask the high bit off - because that is what will
catch people later doing things like vma address range comparisons on
it and notice "that's not a valid address", but it's also not a
"kernel address" that we need to preserve as such.

So yeah, it's a bit confusing in that it's _also_ true that "kernel
addresses have the high bit set" and "user addresses have the high bit
clear", and I'm basically using two different semantics for "kernel
address".

IOW: what I mean by "it's not valid to do 'untagged_addr()' on a
kernel address" is that you can't take a (valid) kernel address, do
'untagged_addr()' on it, and expect it to still work as a kernel
address.

But at the same time you *are* supposed to be able to use
'untagged_addr()' on a - untrusted and possibly invalid - user
pointer, and it's supposed to end up having the tag bits clear and
still be usable as a user pointer. And yet still also be caught by any
validity checks (ie a high bit set would never be a valid user
pointer, not even after doing 'untagged_addr()' on it).

                Linus