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Date:   Thu, 4 May 2023 09:28:27 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
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Subject: Re: [GIT PULL] x86/mm for 6.4
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On Wed, May 03, 2023 at 09:38:03AM -0700, Linus Torvalds wrote:
> > Why does it do that "shift-by-63" game there, instead of making
> > tlbstate_untag_mask just have bit #63 always set?
> 
> And it turns out that bit #63 really _is_ always set, so I think the
> solution to this all is to remove the sign games in untag_addr()
> entirely.

Untagging kernel pointer with LAM enabled will land it in the canonical
hole which is safe as it leads to #GP on dereference.

> Untagging a kernel address will "corrupt" it, but it will stay a
> kernel address (well, it will stay a "high bit set" address), which is
> all we care about anyway.

The interesting case to consider is untagging kernel pointer when LAM_U48
is enabled (not part of current LAM enabling). LAM_U48 would make the
untagging mask wider -- ~GENMASK(62, 48). With 5-level paging and LAM_SUP
enabled (also not supported yet) untagging kernel may transform it to
other valid kernel pointer.

So we cannot rely on #GP as backstop here. The kernel has to exclude
kernel pointer by other means. It can be fun to debug.

Looks like you are not worried about this kind of corruption. Hm.

Maybe it worth to indicate in the helper name that it is intended only for
userspace addresses? Rename it to untagged_uaddr() or something?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov