linux-kernel - Re: [PATCH] MIPS: Handle address errors for accesses above CPU max virtual user address

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Date:   Tue, 22 Feb 2022 20:58:06 +0100
From:   Thomas Bogendoerfer <tsbogend@...ha.franken.de>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     linux-mips <linux-mips@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] MIPS: Handle address errors for accesses above CPU max
 virtual user address

On Tue, Feb 22, 2022 at 06:04:07PM +0100, Arnd Bergmann wrote:
> On Tue, Feb 22, 2022 at 4:53 PM Thomas Bogendoerfer
> <tsbogend@...ha.franken.de> wrote:
> >
> > Address errors have always been treated as unaliged accesses and handled
> > as such. But address errors are also issued for illegal accesses like
> > user to kernel space or accesses outside of implemented spaces. This
> > change implements Linux exception handling for accesses to the illegal
> > space above the CPU implemented maximum virtual user address and the
> > MIPS 64bit architecture maximum. With this we can now use a fixed value
> > for the maximum task size on every MIPS CPU and get a more optimized
> > access_ok().
> >
> > Signed-off-by: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
> 
> Thank you for addressing this. Should I add this patch to my series
> ahead of "mips: use simpler access_ok()"? That way I can keep it all
> in my asm-generic tree as a series for 5.18.

yes please add it to your series.

> >  arch/mips/kernel/unaligned.c | 17 +++++++++++++++++
> >  1 file changed, 17 insertions(+)
> >
> > diff --git a/arch/mips/kernel/unaligned.c b/arch/mips/kernel/unaligned.c
> > index df4b708c04a9..7b5aba5df02e 100644
> > --- a/arch/mips/kernel/unaligned.c
> > +++ b/arch/mips/kernel/unaligned.c
> > @@ -1480,6 +1480,23 @@ asmlinkage void do_ade(struct pt_regs *regs)
> >         prev_state = exception_enter();
> >         perf_sw_event(PERF_COUNT_SW_ALIGNMENT_FAULTS,
> >                         1, regs, regs->cp0_badvaddr);
> > +
> > +#ifdef CONFIG_64BIT
> > +       /*
> > +        * check, if we are hitting space between CPU implemented maximum
> > +        * virtual user address and 64bit maximum virtual user address
> > +        * and do exception handling to get EFAULTs for get_user/put_user
> > +        */
> > +       if ((regs->cp0_badvaddr >= (1UL << cpu_vmbits)) &&
> > +           (regs->cp0_badvaddr < XKSSEG)) {
> 
> It might be clearer to use TASK_SIZE_MAX here instead of XKSSEG,
> to match the check in access_ok(). If you like, I can change that while
> applying.

I had TASK_SIZE_MAX in an intermediate version, but decided to go with XKSSEG,
because it's what this check is about. It's about checking what the MIPS
architecture defined.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]