Re: [PATCH v3 3/3] MIPS: set stack/data protection as non-executable

[Kees Cook]

On Fri, Dec 5, 2014 at 11:06 AM, David Daney <ddaney.cavm@xxxxxxxxx> wrote:
> On 12/05/2014 10:51 AM, Kees Cook wrote:
>>
>> On Fri, Dec 5, 2014 at 9:28 AM, David Daney <ddaney.cavm@xxxxxxxxx> wrote:
>>>
>>> On 12/02/2014 05:58 PM, Leonid Yegoshin wrote:
>>>>
>>>>
>>>> This is a last step of 3 patches which shift FPU emulation out of
>>>> stack into protected area. So, it disables a default executable stack.
>>>>
>>>> Additionally, it sets a default data area non-executable protection.
>>>>
>>>> Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@xxxxxxxxxx>
>>>
>>>
>>>
>>> NAK!
>>>
>>> Some programs require an executable stack, this patch will break them.
>>
>> Have you tested this?
>
> Do you require empirical evidence that the patch is incorrect, or is it
> enough to just to trust me when I say that it is incorrect?  Typically the
> burden of proof is with those proposing the patches.

My fault, I misunderstood. (See below.)

>>> You can only select a non-executable stack in response to PT_GNU_STACK
>>> program headers in the ELF file of the executable program.
>>
>>
>> This is already handled by fs/binfmt_elf.c. It does the parsing of the
>> PT_GNU_STACK needs, and sets up the stack flags appropriately. All the
>> change to VM_DATA_DEFAULT_FLAGS does is make sure that EXSTACK_DEFAULT
>> now means no VM_EXEC by default. If PT_GNU_STACK requires it, it gets
>> added back in.
>>
>
> The problem is not with "modern" executables that are properly annotated
> with PT_GNU_STACK.
>
> My objection is to the intentional breaking of old executables that have no
> PT_GNU_STACK annotation, but require an executable stack.  Since we usually
> try not to break userspace, we cannot merge a patch like this one.

Ah! Okay. If legacy executables expected an executable stack for more
reasons than FPU emulation, then yes, absolutely I agree with you.

-Kees

>
> David Daney.
>
>
>
>> -Kees
>>
>>>
>>> David Daney
>>>
>>>
>>>
>>>> ---
>>>>    arch/mips/include/asm/page.h |    2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/mips/include/asm/page.h b/arch/mips/include/asm/page.h
>>>> index 3be81803595d..d49ba81cb4ed 100644
>>>> --- a/arch/mips/include/asm/page.h
>>>> +++ b/arch/mips/include/asm/page.h
>>>> @@ -230,7 +230,7 @@ extern int __virt_addr_valid(const volatile void
>>>> *kaddr);
>>>>    #define virt_addr_valid(kaddr)
>>>> \
>>>>          __virt_addr_valid((const volatile void *) (kaddr))
>>>>
>>>> -#define VM_DATA_DEFAULT_FLAGS  (VM_READ | VM_WRITE | VM_EXEC | \
>>>> +#define VM_DATA_DEFAULT_FLAGS  (VM_READ | VM_WRITE | \
>>>>                                   VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC)
>>>>
>>>>    #define UNCAC_ADDR(addr)      ((addr) - PAGE_OFFSET + UNCAC_BASE)
>>>>
>>>>
>>>>
>>>>
>>>
>>
>>
>>
>



-- 
Kees Cook
Chrome OS Security
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