[kernel-hardening] Re: [PATCH v3 3/7] arm64: Introduce uaccess_{disable,enable} functionality based on TTBR0_EL1

[Kees Cook]

On Wed, Sep 14, 2016 at 1:52 AM, Mark Rutland <mark.rutland at arm.com> wrote:
> On Tue, Sep 13, 2016 at 01:45:21PM -0700, Kees Cook wrote:
>> On Tue, Sep 13, 2016 at 10:46 AM, Catalin Marinas
>> > +static inline bool system_uses_ttbr0_pan(void)
>> > +{
>> > +       return IS_ENABLED(CONFIG_ARM64_SW_TTBR0_PAN) &&
>> > +               !cpus_have_cap(ARM64_HAS_PAN);
>> > +}
>> > +
>
> [...]
>
>> >  #define __uaccess_enable(alt)                                          \
>> >  do {                                                                   \
>> > -       asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), alt,                  \
>> > -                       CONFIG_ARM64_PAN));                             \
>> > +       if (system_uses_ttbr0_pan())                                    \
>> > +               uaccess_ttbr0_enable();                                 \
>> > +       else                                                            \
>> > +               asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), alt,          \
>> > +                               CONFIG_ARM64_PAN));                     \
>> >  } while (0)
>>
>> Does this mean that with CONFIG_ARM64_SW_TTBR0_PAN, even with ARMv8.1,
>> a cpu capability bitmask check is done each time we go through
>> __uaccess_{en,dis}able?
>
> Catalin reworked cpus_have_cap() to use static keys [1], and that's
> queued in the arm64 for-next/core branch [2].

Oh awesome! Okay, thanks.

> So this should expand to a single branch or nop that we patch when we
> detect the presence/absence of PAN. There should be no bitmap check.

/me is looking forward to v4.9 :)

>
> Thanks,
> Mark.
>
> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-September/454025.html
> [2] https://git.kernel.org/cgit/linux/kernel/git/arm64/linux.git/log/?h=for-next/core

-Kees

-- 
Kees Cook
Nexus Security