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

[Kees Cook]

On Tue, Apr 8, 2014 at 12:48 PM, Rabin Vincent <rabin@xxxxxx> wrote:
> On Tue, Apr 08, 2014 at 09:59:07AM -0700, Kees Cook wrote:
>> On Tue, Apr 8, 2014 at 9:12 AM, Jon Medhurst (Tixy) <tixy@xxxxxxxxxx> wrote:
>> > 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.
>>
>> As I understood it, this would be true for small PTEs, but sections
>> are fully duplicated on each CPU so we don't run that risk. This was
>> the whole source of my problem with this patch series: even a full
>> all-CPU TLB flush wasn't working -- the section permissions were
>> unique to the CPU since the entries were duplicated.
>
> The PGD is per-mm_struct.  mm_structs can be shared between processes.
> So the PGD is not per CPU.
>
> This set_kernel_text_rw() is called from ftrace in stop_machine() on one
> CPU.  All other CPUs will be spinning in kernel threads inside the loop
> in multi_cpu_stop(), with interrupts disabled.  Since kernel threads use
> the last process' mm, it is possible for the other CPU(s) to be
> currently using the same mm as the modifying CPU.
>
> For any other CPU to pull in the writable entry it would have to get a
> TLB miss inside the loop in multi_cpu_stop(), after the state transition
> to MULTI_STOP_RUN and before the state transition to MULTI_STOP_EXIT.
> This is unlikely, but theoretically possible, for example if
> multi_cpu_stop() straddles sections.

Ah! Now I understand. Thanks for the clarification.

> To prevent any stale entries being used indefinitely, perhaps the all
> CPU TLB flush can be inserted into
> ftrace_arch_code_modify_post_process(), which is called after the
> stop_machine() and which is where x86 for example makes the entries
> read-only again.

Do you mean something like this?

diff --git a/arch/arm/kernel/ftrace.c b/arch/arm/kernel/ftrace.c
index ea446ae09c89..b8c75e45a950 100644
--- a/arch/arm/kernel/ftrace.c
+++ b/arch/arm/kernel/ftrace.c
@@ -90,6 +90,8 @@ int ftrace_arch_code_modify_prepare(void)
 int ftrace_arch_code_modify_post_process(void)
 {
        set_all_modules_text_ro();
+       /* Make sure any TLB misses during machine stop are cleared. */
+       flush_tlb_all();
        return 0;
 }

Thanks!

-Kees

-- 
Kees Cook
Chrome OS Security
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