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

[Carsten Langgaard <carstenl@mips.com>]

Dominic Sweetman wrote:

> > > > The __access_ok macro in include/asm-mips64/uaccess.h and the
> > > > check_axs macro in arch/mips64/kernel/unaligned.c ... is a copy
> > > > from the 32-bit kernel...
> > > >
> > > > The area between USEG (XUSEG) and KSEG0 will in 64-bit
> > > > addressing mode generate an address error, if accessed.
>
> 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?

Yes, that's the case.
It's been a while since I fixed it locally, but if I ran something like
crashme, I could end up, in a situation where the kernel tries (on the
behalf of the user) to access an address like 2**41 in a CPU supporting
40-bit user virtual addresses, which generate an address error and
because we are in kernel mode we die.


>
> 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?
>
> --
> Dominic Sweetman
> MIPS Technologies
> The Fruit Farm, Ely Road, Chittering, CAMBS CB5 9PH, ENGLAND
> phone +44 1223 706205/fax +44 1223 706250/swbrd +44 1223 706200
> http://www.algor.co.uk

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