[kernel-hardening] Re: [PATCH v2 3/7] arm64: Introduce uaccess_{disable, enable} functionality based on TTBR0_EL1

[Ard Biesheuvel]

On 6 September 2016 at 11:45, Mark Rutland <mark.rutland at arm.com> wrote:
> On Tue, Sep 06, 2016 at 11:27:42AM +0100, Catalin Marinas wrote:
>> On Mon, Sep 05, 2016 at 06:20:38PM +0100, Mark Rutland wrote:
>> > On Fri, Sep 02, 2016 at 04:02:09PM +0100, Catalin Marinas wrote:
>> > > +static inline void uaccess_ttbr0_enable(void)
>> > > +{
>> > > + unsigned long flags;
>> > > +
>> > > + /*
>> > > +  * Disable interrupts to avoid preemption and potential saved
>> > > +  * TTBR0_EL1 updates between reading the variable and the MSR.
>> > > +  */
>> > > + local_irq_save(flags);
>> > > + write_sysreg(current_thread_info()->ttbr0, ttbr0_el1);
>> > > + isb();
>> > > + local_irq_restore(flags);
>> > > +}
>> >
>> > I don't follow what problem this actually protects us against. In the
>> > case of preemption everything should be saved+restored transparently, or
>> > things would go wrong as soon as we enable IRQs anyway.
>> >
>> > Is this a hold-over from a percpu approach rather than the
>> > current_thread_info() approach?
>>
>> If we get preempted between reading current_thread_info()->ttbr0 and
>> writing TTBR0_EL1, a series of context switches could lead to the update
>> of the ASID part of ttbr0. The actual MSR would store an old ASID in
>> TTBR0_EL1.
>
> Ah! Can you fold something about racing with an ASID update into the
> description?
>
>> > > +#else
>> > > +static inline void uaccess_ttbr0_disable(void)
>> > > +{
>> > > +}
>> > > +
>> > > +static inline void uaccess_ttbr0_enable(void)
>> > > +{
>> > > +}
>> > > +#endif
>> >
>> > I think that it's better to drop the ifdef and add:
>> >
>> >     if (!IS_ENABLED(CONFIG_ARM64_TTBR0_PAN))
>> >             return;
>> >
>> > ... at the start of each function. GCC should optimize the entire thing
>> > away when not used, but we'll get compiler coverage regardless, and
>> > therefore less breakage. All the symbols we required should exist
>> > regardless.
>>
>> The reason for this is that thread_info.ttbr0 is conditionally defined.
>> I don't think the compiler would ignore it.
>
> Good point; I missed that.
>
> [...]
>
>> > How about something like:
>> >
>> >     .macro alternative_endif_else_nop
>> >     alternative_else
>> >     .rept ((662b-661b) / 4)
>> >            nop
>> >     .endr
>> >     alternative_endif
>> >     .endm
>> >
>> > So for the above we could have:
>> >
>> >     alternative_if_not ARM64_HAS_PAN
>> >             save_and_disable_irq \tmp2
>> >             uaccess_ttbr0_enable \tmp1
>> >             restore_irq \tmp2
>> >     alternative_endif_else_nop
>> >
>> > I'll see about spinning a patch, or discovering why that happens to be
>> > broken.
>>
>> This looks better. Minor comment, I would actually name the ending
>> statement alternative_else_nop_endif to match the order in which you'd
>> normally write them.
>
> Completely agreed. I already made this change locally, immediately after
> sending the suggestion. :)
>
>> > >    * tables again to remove any speculatively loaded cache lines.
>> > >    */
>> > >   mov     x0, x25
>> > > - add     x1, x26, #SWAPPER_DIR_SIZE
>> > > + add     x1, x26, #SWAPPER_DIR_SIZE + RESERVED_TTBR0_SIZE
>> > >   dmb     sy
>> > >   bl      __inval_cache_range
>> > >
>> > > diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
>> > > index 659963d40bb4..fe393ccf9352 100644
>> > > --- a/arch/arm64/kernel/vmlinux.lds.S
>> > > +++ b/arch/arm64/kernel/vmlinux.lds.S
>> > > @@ -196,6 +196,11 @@ SECTIONS
>> > >   swapper_pg_dir = .;
>> > >   . += SWAPPER_DIR_SIZE;
>> > >
>> > > +#ifdef CONFIG_ARM64_TTBR0_PAN
>> > > + reserved_ttbr0 = .;
>> > > + . += PAGE_SIZE;
>> > > +#endif
>> >
>> > Surely RESERVED_TTBR0_SIZE, as elsewhere?
>>
>> I'll try to move it somewhere where it can be included in vmlinux.lds.S
>> (I can probably include cpufeature.h directly).
>

Do we really need another zero page? The ordinary zero page is already
statically allocated these days, so we could simply move it between
idmap_pg_dir[] and swapper_pg_dir[], and get all the changes in the
early boot code for free (given that it covers the range between the
start of idmap_pg_dir[] and the end of swapper_pg_dir[])

That way, we could refer to __pa(empty_zero_page) anywhere by reading
ttbr1_el1 and subtracting PAGE_SIZE