[PATCH] [RFC]arm64:Mark __stack_chk_guard as __ro_after_init

[Mark Rutland]

On Wed, Sep 15, 2021 at 09:57:14AM +0800, ashimida wrote:
> Hi King, Rutland:
> 
> Thanks for the reply and let me understand the reason here.
> 
> Then may I first submit a patch to modify the attributes of
> __stack_chk_guard of the arm/aarch64 platform?

This patch looks fine as-is (hence the Acked-by). Doing the same for
arch/arm makes sense, but that should be a separate patch.

I was suggesting that in future we should probably do the same in more
places, not that you need to do so now.

Thanks,
Mark.

> 
> On 9/14/21 6:17 PM, Mark Rutland wrote:
> > On Tue, Sep 14, 2021 at 05:44:02PM +0800, Dan Li wrote:
> > > __stack_chk_guard is setup once while init stage and never changed
> > > after that.
> > > 
> > > Although the modification of this variable at runtime will usually
> > > cause the kernel to crash (so dose the attacker), it should be marked
> > > as _ro_after_init, and it should not affect performance if it is
> > > placed in the ro_after_init section.
> > > 
> > > This should also be the case on the ARM platform, or am I missing
> > > something?
> > > 
> > > Signed-off-by: Dan Li <ashimida at linux.alibaba.com>
> > 
> > FWIW, this makes sense to me:
> > 
> > Acked-by: Mark Rutland <mark.rutland at arm.com>
> > 
> > Looking at the history, this was added to arm64 in commit:
> > 
> >    c0c264ae5112d1cd ("arm64: Add CONFIG_CC_STACKPROTECTOR")
> > 
> > ... whereas __ro_after_init was introduced around 2 years later in
> > commit:
> > 
> >    c74ba8b3480da6dd ("arch: Introduce post-init read-only memory")
> > 
> > ... so we weren't deliberately avoiding __ro_after_init, and there are
> > probably a significant number of other variables we could apply it to.
> > 
> > Mark.
> > 
> > > ---
> > >   arch/arm64/kernel/process.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> > > index c8989b9..c858b85 100644
> > > --- a/arch/arm64/kernel/process.c
> > > +++ b/arch/arm64/kernel/process.c
> > > @@ -60,7 +60,7 @@
> > >   #if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK)
> > >   #include <linux/stackprotector.h>
> > > -unsigned long __stack_chk_guard __read_mostly;
> > > +unsigned long __stack_chk_guard __ro_after_init;
> > >   EXPORT_SYMBOL(__stack_chk_guard);
> > >   #endif
> > > -- 
> > > 2.7.4
> > >