You are right there is a potential problem in the 64-bit kernel, when
emulating load/store instructions.
I have previously addressed this problem (and also send a patch).
The problem is the check_axs macro, that checks the access before
actually doing the emulation (any load/stores).
It's a simple copy from the 32-bit kernel, where you do a check for user
space by simple check the most significant bit.
That's fine in the 32-bit world, but it's not sufficient in the 64-bit
case.
Locally I have a check_axs that look like this:
/*
* User code may only access USEG;
* Kernel code may access the entire address space, except the area
between
* USEG (XUSEG) and KSEG0.
*/
#define check_axs(pc,a,s) \
if (((pc < KUSIZE) && (((a) | ((a)+(s))) >= KUSIZE)) || \
((((a) | ((a)+(s))) < K0BASE) && \
(((a) | ((a)+(s))) >= KUSIZE))) \
goto sigbus;
Hope that helps you.
/Carsten
Kip Walker wrote:
> After inspecting a strange case in the mips64 kernel with address
> errors, I'm starting to think there's a problem in the do_ade()
> implementation. I think the 32-bit kernel may have a similar problem,
> but I haven't really inspected it. The issue is where the kernel's
> emulation of an address error causes another address error (NOT a page
> fault).
>
> Basically, I don't see how the exception table stuff in
> emulate_load_store_insn is going to work. Consider this scenario:
>
> - user process does a 'sw' (for example) to an illegal address
> above xuseg but below xsseg
> - do_ade calls emulate_load_store_insn, which tries swl/swr
> - the swl again hits an illegal address, this time in the
> kernel's context
> - do_ade does NOT check the exception table for the swl
> - emulate_load_store_insn goes to the 'swl' part of the switch
> - die_if_kernel DOES __die before the SIGBUS is delivered.
>
> So I don't see how the ex_table stuff is useful at all. Shouldn't
> do_ade() do the exception table grovelling before calling
> emulate_load_store_insn?
>
> Kip
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