linux-hardening - Re: [PATCH] [RFC][PR102768] aarch64: Add compiler support for Shadow Call Stack

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Date:   Wed, 3 Nov 2021 12:00:47 +0000
From:   Szabolcs Nagy <szabolcs.nagy@....com>
To:     Dan Li <ashimida@...ux.alibaba.com>
Cc:     gcc-patches@....gnu.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] [RFC][PR102768] aarch64: Add compiler support for Shadow
 Call Stack

The 11/03/2021 00:24, Dan Li wrote:
> On 11/2/21 9:04 PM, Szabolcs Nagy wrote:
> > The 11/02/2021 00:06, Dan Li via Gcc-patches wrote:
> > > Shadow Call Stack can be used to protect the return address of a
> > > function at runtime, and clang already supports this feature[1].
> > > 
> > > To enable SCS in user mode, in addition to compiler, other support
> > > is also required (as described in [2]). This patch only adds basic
> > > support for SCS from the compiler side, and provides convenience
> > > for users to enable SCS.
> > > 
> > > For linux kernel, only the support of the compiler is required.
> > > 
> > > [1] https://clang.llvm.org/docs/ShadowCallStack.html
> > > [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102768
> > 
> > i'm not a gcc maintainer, but i prefer such feature
> > to be in upstream gcc instead of in a plugin.
> > 
> > it will require update to the documentation:
> > 
> > which should mention that it depends on -ffixed-x18
> > (probably that should be enforced too) which is an
> > important abi issue: functions following the normal
> > pcs can clobber x18 and break scs.
> > 
> Thanks Szabolcs, I will update the documentation in next version.
> 
> It sounds reasonable to enforced -ffixed-x18 with scs, but I see
> that clang doesn’t do that. Maybe it is better to be consistent
> with clang here?

i mean gcc can issue a diagnostic if -ffixed-x18 is not passed.
(it seems clang rejects scs too without -ffixed-x18)

> > and that there is no unwinder support.
> > 
> Ok, let me try to add a support for this.

i assume exception handling info has to change for scs to
work (to pop the shadow stack when transferring control),
so either scs must require -fno-exceptions or the eh info
changes must be implemented.

i think the kernel does not require exceptions and does
not depend on the unwinder runtime in libgcc, so this
is optional for the linux kernel use-case.