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Re: The 64-bit version of __access_ok is broken.

To: Ralf Baechle <>
Subject: Re: The 64-bit version of __access_ok is broken.
From: Carsten Langgaard <>
Date: Tue, 10 Dec 2002 08:50:55 +0100
Cc: Dominic Sweetman <>, Dominic Sweetman <>,,,
Original-recipient: rfc822;
References: <> <> <> <> <>
Ralf Baechle wrote:

> On Mon, Dec 09, 2002 at 11:54:20AM +0000, Dominic Sweetman wrote:
> > I'd like to be clear about the consequences of this.  Presumably the
> > 'access_ok()' macro is used to check addresses which were (originally)
> > provided by a user program's system call.
> >
> > Carsten, are you saying that if such an address is set to say 2**41 in
> > a CPU supporting 40-bit user virtual addresses, that the kernel will
> > crash?
> That's correct.  The problem which Carsten diagnosed correctly was the
> assumption which has been inherited from the 32-bit kernel that the sign-
> bit makes the difference between valid userspace and kernelspace
> addresses.
> Linux doesn't use the supervisor mode so basically that assumption is still
> true with the except of the area 2^PHYSBITS ... 2^63-1.
> > If so, that seems to require a fix, even if we don't know a very
> > efficient one.  But perhaps any problem is a bit more subtle than
> > that?
> Access_ok is a macro which depending on kernel configuration is expanded
> hundreds, if not thousands of times throughout the kernel.  So every single
> machine instruction in access_ok will make a size difference of several
> kB.  Carsten's patch was performing pretty badly in that cathegory.  If
> access_ok wasn't used that often the issue certainly wasn't worth the fuzz.
> Access_ok is of course only usable in C code.  We also have a few piece of
> assembler code that access userspace and need to perform the same kind of
> address validation tests.  Carsten's patch was missing these completly.  As
> such it did only reduce the window of this bug from huge to "just" big.

At least I haven't hit those holes, the would have been fixed otherwise, too

> An efficient solution only requires fairly minor changes as you can see in
> the patch I just posted.  It doesn't even require thinking, it can be
> obtained by cut'n'paste from the Alpha code.  Alternatively the problem
> could also have been solved by forwarding address errors for the address
> range in question to the page fault handler which would have served the
> same purpose, maybe even a tad more efficient but ofuscated ...

I absolutely agree that we should go for an optimized solution, but we discuss
this issue 1/2 year ago, none of us, had the time to come up with a better fix
than the one I send. I'm going through my to-do list and came across this
issue again, and I just wanted to reopen the case again.
This time it annoyed you enough, so you came up with a better solution and
I achieved what I came for, so that's great ;-)
Thanks a lot. I will try you patch right away.

>   Ralf

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