Re: [GIT PULL] x86/mm for 6.4

[Linus Torvalds]

On Tue, May 2, 2023 at 5:53 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> The fallout seems limited to (probably) perf and tracing poking at user
> stack frames.  But, yes, it definitely looks broken there.

So I ended up doing more cleanups - moving the 64-bit specific code to
'uaccess_64.h' etc.

And in the process I found another broken
thing:__untagged_addr_remote() is very very buggy.

The reason?

        long sign = addr >> 63;

that does *not* do at all what '__untagged_addr()' does, because while
'sign' is a signed long, 'addr' is an *unsigned* long.

So the actual shift ends up being done as an unsigned shift, and then
just the result is assigned to a signed variable.

End result? 'sign' ends up being 0 for user space (intentional) or 1
for kernel space (not intentional)..

It's not 0 or ~0 as the __untagged_addr() is, wh9ch uses a signed
shift in the inline asm ('sar')

That said, while this is all horribly buggy, I'm not actually 100%
sure that we care too much about untagging kernel addresses. So it's
an error case that doesn't get marked as an error.

So I guess in *practice* the horribly buggy code is actually just a
small harmless bug, and causes anybody who passes in an invalid
(kernel) address look like a possibly valid user address after all.

HOWEVER.

Why does it do that "shift-by-63" game there, instead of making
tlbstate_untag_mask just have bit #63 always set?

Untagging a kernel address is not a sensible operation, so the only
thing you want is to *keep* a kernel address as a bad address. You
don't have to actually keep it a *valid* kernel address, it just
should not become a (possibly valid) user address after untagging.

Hmmm? Am I missing something?

                  Linus