[PATCH 2/2] ARM: mm: make text and rodata read-only

[tixy@linaro.org (Jon Medhurst (Tixy))]

On Tue, 2014-04-08 at 09:01 -0700, Kees Cook wrote:
> On Tue, Apr 8, 2014 at 5:41 AM, Jon Medhurst (Tixy) <tixy@linaro.org> wrote:
> > On Fri, 2014-04-04 at 17:07 -0700, Kees Cook wrote:
> >> On Fri, Apr 4, 2014 at 12:58 PM, Rabin Vincent <rabin@rab.in> wrote:
> > [...]
> >> > You need a TLB flush.  I had a flush_tlb_all() in my example patch,
> >> > http://lists.infradead.org/pipermail/linux-arm-kernel/2014-April/244335.html,
> >> > but the following is probably nicer (on top of this patch):
> >> >
> >> > diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> >> > index 9bea524..a92c45a 100644
> >> > --- a/arch/arm/mm/init.c
> >> > +++ b/arch/arm/mm/init.c
> >> > @@ -741,6 +741,8 @@ static inline bool arch_has_strict_perms(void)
> >> >                      addr += SECTION_SIZE)                              \
> >> >                         section_update(addr, perms[i].mask,             \
> >> >                                        perms[i].field);                 \
> >> > +                                                                       \
> >> > +               flush_tlb_kernel_range(perms[i].start, perms[i].end);   \
> >> >         }                                                               \
> >> >  }
> >> >
> >>
> >> When I do this, I hang the system, and get a WARN due to the tlb call
> >> attempting to flush on all CPUs, I think:
> >>
> >> [   34.246034] WARNING: at
> >> /mnt/host/source/src/third_party/kernel-next/kernel/smp.c:466
> >> smp_call_function_many+0xac/0x26c()
> >> ...
> >> [   34.246617] Backtrace:
> >> [   34.246697] [<c010d3b8>] (unwind_backtrace+0x0/0x118) from
> >> [<c060b9d8>] (dump_stack+0x28/0x30)
> >> [   34.246765] [<c060b9d8>] (dump_stack+0x28/0x30) from [<c0123044>]
> >> (warn_slowpath_null+0x44/0x5c)
> >> [   34.246824] [<c0123044>] (warn_slowpath_null+0x44/0x5c) from
> >> [<c017426c>] (smp_call_function_many+0xac/0x26c)
> >> [   34.246881] [<c017426c>] (smp_call_function_many+0xac/0x26c) from
> >> [<c0174468>] (smp_call_function+0x3c/0x48)
> >> [   34.246937] [<c0174468>] (smp_call_function+0x3c/0x48) from
> >> [<c010c0fc>] (broadcast_tlb_a15_erratum+0x40/0x4c)
> >> [   34.246994] [<c010c0fc>] (broadcast_tlb_a15_erratum+0x40/0x4c) from
> >> [<c010c590>] (flush_tlb_kernel_range+0x74/0xa0)
> >> [   34.247046] [<c010c590>] (flush_tlb_kernel_range+0x74/0xa0) from
> >> [<c011403c>] (set_kernel_text_rw+0xd8/0xec)
> >> [   34.247099] [<c011403c>] (set_kernel_text_rw+0xd8/0xec) from
> >> [<c010c878>] (__ftrace_modify_code+0x14/0x28)
> >> [   34.247156] [<c010c878>] (__ftrace_modify_code+0x14/0x28) from
> >> [<c0184318>] (stop_machine_cpu_stop+0xc0/0x114)
> >> [   34.247212] [<c0184318>] (stop_machine_cpu_stop+0xc0/0x114) from
> >> [<c01841cc>] (cpu_stopper_thread+0xd8/0x164)
> >> [   34.247266] [<c01841cc>] (cpu_stopper_thread+0xd8/0x164) from
> >> [<c0145c14>] (kthread+0xc8/0xd8)
> >> [   34.247323] [<c0145c14>] (kthread+0xc8/0xd8) from [<c0106118>]
> >> (ret_from_fork+0x14/0x20)
> >>
> >> Using local_flush_tlb_kernel_range() fixed it though.
> >
> > What about if another CPU had a TLB entry with the old permissions in?
> > Or do you consider that the likelihood and consequences of that aren't
> > significant?
> 
> The purpose of the function is to temporarily make text writable, do
> the write, and then restore read-only. Since only the writer needs to
> care about TLB state, this works fine. It's actually nice that only
> the current CPU can make text writes.

And is the page table being modified unique to the current CPU? I
thought a common set of page tables was shared across all of them. If
that is the case then one CPU can modify the PTE to be writeable,
another CPU take a TLB miss and pull in that writeable entry, which will
stay there until it drops out the TLB at some indefinite point in the
future. That's the scenario I was getting at with my previous comment.

-- 
Tixy