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

[David Daney <ddaney.cavm@gmail.com>]

On 12/05/2014 10:51 AM, Kees Cook wrote:
> On Fri, Dec 5, 2014 at 9:28 AM, David Daney <ddaney.cavm@gmail.com> 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@imgtec.com>
>>
>>
>> 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.

>
>> 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.

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)
>>>
>>>
>>>
>>>
>>
>
>
>