Messages in this thread | | | From | Linus Torvalds <> | Date | Tue, 2 May 2023 18:17:15 -0700 | Subject | Re: [GIT PULL] x86/mm for 6.4 |
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On Tue, May 2, 2023 at 5:53 PM Dave Hansen <dave.hansen@intel.com> 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
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