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

[David Daney]

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.

> > 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)

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