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Message-ID: <20160912162622.GD2492@e104818-lin.cambridge.arm.com>
Date: Mon, 12 Sep 2016 17:26:23 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Mark Rutland <mark.rutland@....com>
Cc: Kees Cook <keescook@...omium.org>, kernel-hardening@...ts.openwall.com,
Will Deacon <will.deacon@....com>,
AKASHI Takahiro <takahiro.akashi@...aro.org>,
James Morse <james.morse@....com>,
linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/7] arm64: Factor out PAN enabling/disabling into
separate uaccess_* macros
On Mon, Sep 12, 2016 at 04:09:59PM +0100, Mark Rutland wrote:
> On Mon, Sep 12, 2016 at 03:52:19PM +0100, Catalin Marinas wrote:
> > On Mon, Sep 05, 2016 at 04:38:28PM +0100, Mark Rutland wrote:
> > > On Fri, Sep 02, 2016 at 04:02:07PM +0100, Catalin Marinas wrote:
> > > > /*
> > > > + * User access enabling/disabling.
> > > > + */
> > > > +#define uaccess_disable(alt) \
> > > > +do { \
> > > > + asm(ALTERNATIVE("nop", SET_PSTATE_PAN(1), alt, \
> > > > + CONFIG_ARM64_PAN)); \
> > > > +} while (0)
> > > > +
> > > > +#define uaccess_enable(alt) \
> > > > +do { \
> > > > + asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), alt, \
> > > > + CONFIG_ARM64_PAN)); \
> > > > +} while (0)
> > >
> > > Passing the alternative down is somewhat confusing. e.g. in the futex
> > > case it looks like we're only doing something when PAN is present,
> > > whereas we'll manipulate TTBR0 in the absence of PAN.
> >
> > I agree it's confusing (I got it wrong first time as well and used the
> > wrong alternative for futex).
> >
> > > If I've understood correctly, we need this to distinguish regular
> > > load/store uaccess sequences (eg. the futex code) from potentially
> > > patched unprivileged load/store sequences (e.g. {get,put}_user) when
> > > poking PSTATE.PAN.
> > >
> > > So perhaps we could ahve something like:
> > >
> > > * privileged_uaccess_{enable,disable}()
> > > Which toggle TTBR0, or PAN (always).
> > > These would handle cases like the futex/swp code.
> > >
> > > * (unprivileged_)uaccess_{enable,disable}()
> > > Which toggle TTBR0, or PAN (in the absence of UAO).
> > > These would handle cases like the {get,put}_user sequences.
> > >
> > > Though perhaps that is just as confusing. ;)
> >
> > I find it more confusing.
>
> Fair enough. :)
>
> > In the non-UAO case, get_user etc. would
> > normally have to use privileged_uaccess_enable() since ldr is not
> > replaced with ldtr. Maybe uaccess_enable_for_exclusives() but it doesn't
> > look any better.
>
> I strongly prefer uaccess_enable_exclusives(), or something of that sort
> to both of the above. ;)
I think we would need a few more uaccess_enable_* variants (cache
maintenance, Xen) which makes this impractical.
We can consider the PAN_NOT_UAO the special case and if we assume that
UAO also implies PAN (ARMv8.2), we can define uaccess_enable_not_uao()
for the get_user etc. cases. We would use uaccess_enable() for the rest.
--
Catalin
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