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Date: Fri, 28 Oct 2016 06:59:09 -0400
From: David Windsor <dave@...gbits.org>
To: kernel-hardening@...ts.openwall.com
Cc: Colin Vidal <colin@...dal.org>, "Reshetova, Elena" <elena.reshetova@...el.com>,
AKASHI Takahiro <takahiro.akashi@...aro.org>, Kees Cook <keescook@...omium.org>,
Hans Liljestrand <ishkamiel@...il.com>
Subject: Re: [RFC 2/2] arm: implementation for HARDENED_ATOMIC
On Fri, Oct 28, 2016 at 6:20 AM, Mark Rutland <mark.rutland@....com> wrote:
> On Fri, Oct 28, 2016 at 10:33:15AM +0200, Colin Vidal wrote:
>> On Thu, 2016-10-27 at 14:24 +0100, Mark Rutland wrote:
>> > On Tue, Oct 18, 2016 at 04:59:21PM +0200, Colin Vidal wrote:
>> > > +#define HARDENED_ATOMIC_INSN "bkpt 0xf103"
>> >
>> > Please put the immediate in a #define somewhere.
>>
>> I an not sure to get what do you mean here. 0xf103 should be in a
>> #define? (as for the A1 encoded version, for sure).
>
> I mean have something like:
>
> #ifdef CONFIG_THUMB2_KERNEL
> #define ATOMIC_BKPT_IMM 0xf1
> #else
> #define ATOMIC_BKPT_IMM 0xf103
> #endif
>
> With code having something like:
>
> HARDENED_ATOMIC_INSN "bkpt" #ATOMIC_BKPT_IMM
>
> ... or passing that in via an %i template to the asm.
>
> That way, we can also build the encoding for the exception path from the
> same immediate. That will keep the two in sync, making the relationship
> between the two obvious. e.g. first add something like arm_get_bkpt(imm)
> to <asm/insn.h>.
>
>> > > +#define _ASM_EXTABLE(from, to) \
>> > > + ".pushsection __ex_table,\"a\"\n" \
>> > > + ".align 3\n" \
>> > > + ".long "#from","#to"\n" \
>> > > + ".popsection"
>> > > +#define __OVERFLOW_POST \
>> > > + "bvc 3f\n" \
>> > > + "2: "HARDENED_ATOMIC_INSN"\n" \
>> > > + "3:\n"
>> > > +#define __OVERFLOW_POST_RETURN \
>> > > + "bvc 3f\n" \
>> > > + "mov %0,%1\n" \
>> > > + "2: "HARDENED_ATOMIC_INSN"\n" \
>> > > + "3:\n"
>> > > +#define __OVERFLOW_EXTABLE \
>> > > + "4:\n" \
>> > > + _ASM_EXTABLE(2b, 4b)
>> > > +#else
>> > > +#define __OVERFLOW_POST
>> > > +#define __OVERFLOW_POST_RETURN
>> > > +#define __OVERFLOW_EXTABLE
>> > > +#endif
>> > > +
>> > All this should live close to the assembly using it, to make it possible
>> > to follow.
>> >
>> > This may also not be the best way of structuring this code. The
>> > additional indirection of passing this in at a high level makes it hard
>> > to read and potentially fragile. For single instructions it was ok, but
>> > I'm not so sure that it's ok for larger sequences like this.
>>
>> I agree that is quite difficult to follow, but as Takahiro said, the
>> current macro are already complex. Still, I will try to put those
>> definitions closer of the code using them (for instance, just before
>> the macro expansions?), but I am not sure that would change anything:
>> the code using it is broke up in different area of the file anyways.
>
> I'm not sure that's true. IIRC each of the cases above is only used by
> one template case, and we could instead fold that into the template.
> For example, if we had something like the ALTERNATIVE macros that worked
> on some boolean passed in, so you could have:
>
> #define __foo_asm_template(__bool, params ...) \
> asm( \
> "insn reg, whatever" \
> TEMPLATE(__bool, "code_if_bool", "code if_not_bool") \
> "more insns..." \
> clobbers_etc)
>
> That way all the code is in one place, and easier to follow, and we can
> generate both the instrumented and non-instrumented variants from the
> same template.
>
>> Besides, I really would avoid using an extable entry, since the fixup
>> address is never used, I am not sure it is mandatory (and it seems
>> Takahiro does not using it, too).
>
> From the grsecurity blog post [1], per the comments in the ARM detection
> logic, this is used to continue after warning (and not incrementing) in
> the overflow case.
>
> What do we do differently in the overflow case?
>
In the overflow case, exception/trap code is called, which invokes
hardened_atomic_report_overflow(), which then calls BUG(). At least
on x86.
>> > > diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
>> > > index 3a2e678..ce8ee00 100644
>> > > --- a/arch/arm/mm/fault.c
>> > > +++ b/arch/arm/mm/fault.c
>> > > @@ -580,6 +580,21 @@ do_PrefetchAbort(unsigned long addr, unsigned int ifsr, struct pt_regs *regs)
>> > > const struct fsr_info *inf = ifsr_info + fsr_fs(ifsr);
>> > > struct siginfo info;
>> > >
>> > > +#ifdef CONFIG_HARDENED_ATOMIC
>> > > + if (fsr_fs(ifsr) == FAULT_CODE_DEBUG) {
>> >
>> > You'll need to justify why this isn't in the ifsr_info table. It has the
>> > same basic shape as the usual set of handlers.
>> >
>> > I note that we don't seem to use SW breakpoints at all currently, and I
>> > suspect there's a reason for that which we need to consider.
>> >
>> > Also, if this *must* live here, please make it a static inline with an
>> > empty stub, rather than an ifdef'd block.
>> >
>> > >
>> > > + unsigned long pc = instruction_pointer(regs);
>> > > + unsigned int bkpt;
>> > > +
>> > > + if (!probe_kernel_address((const unsigned int *)pc, bkpt) &&
>> > > + cpu_to_le32(bkpt) == 0xe12f1073) {
>> >
>> > This appears to be the A1 encoding from the ARM ARM. What about the T1
>> > encoding, i.e. thumb?
>> >
>> > Regardless, *please* de-magic the number using a #define.
>> >
>> > Also, this should be le32_to_cpu -- in the end we're treating the
>> > coverted value as cpu-native. The variable itself should be a __le32.
>>
>> I must confess that I am still very confused about the code in fault.c
>> (and the ifsr_info table). I will take time to try to understand a
>> little bit more that code before submitting a new RFC. Still, it you
>> have some pointers/documentations about it, I would be grateful.
>
> Unfortuantely, I'm not aware of any documentation for this code. The
> ifsr_info table is just a set of handlers indexed by the ifsr value.
>
> Ignoring the conflict with the HW breakpoint handler, you'd be able to
> install this in the FAULT_CODE_DEBUG slot of ifsr_info.
>
> Thanks,
> Mark.
>
> [1] https://forums.grsecurity.net/viewtopic.php?f=7&t=4173
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