Re: [PATCH v21 4/8] fork: Add shadow stack support to clone3()

[Will Deacon <will@kernel.org>]

On Thu, Sep 18, 2025 at 01:38:53PM +0100, Will Deacon wrote:
> On Tue, Sep 16, 2025 at 12:12:09AM +0100, Mark Brown wrote:
> > Unlike with the normal stack there is no API for configuring the shadow
> > stack for a new thread, instead the kernel will dynamically allocate a
> > new shadow stack with the same size as the normal stack. This appears to
> > be due to the shadow stack series having been in development since
> > before the more extensible clone3() was added rather than anything more
> > deliberate.
> > 
> > Add a parameter to clone3() specifying a shadow stack pointer to use
> > for the new thread, this is inconsistent with the way we specify the
> > normal stack but during review concerns were expressed about having to
> > identify where the shadow stack pointer should be placed especially in
> > cases where the shadow stack has been previously active.  If no shadow
> > stack is specified then the existing implicit allocation behaviour is
> > maintained.
> > 
> > If a shadow stack pointer is specified then it is required to have an
> > architecture defined token placed on the stack, this will be consumed by
> > the new task, the shadow stack is specified by pointing to this token.  If
> > no valid token is present then this will be reported with -EINVAL.  This
> > token prevents new threads being created pointing at the shadow stack of
> > an existing running thread.  On architectures with support for userspace
> > pivoting of shadow stacks it is expected that the same format and placement
> > of tokens will be used, this is the case for arm64 and x86.
> > 
> > If the architecture does not support shadow stacks the shadow stack
> > pointer must be not be specified, architectures that do support the
> > feature are expected to enforce the same requirement on individual
> > systems that lack shadow stack support.
> > 
> > Update the existing arm64 and x86 implementations to pay attention to
> > the newly added arguments, in order to maintain compatibility we use the
> > existing behaviour if no shadow stack is specified. Since we are now
> > using more fields from the kernel_clone_args we pass that into the
> > shadow stack code rather than individual fields.
> > 
> > Portions of the x86 architecture code were written by Rick Edgecombe.
> > 
> > Acked-by: Yury Khrustalev <yury.khrustalev@arm.com>
> > Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> > Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> > Signed-off-by: Mark Brown <broonie@kernel.org>
> > ---
> >  arch/arm64/mm/gcs.c              | 47 +++++++++++++++++++-
> >  arch/x86/include/asm/shstk.h     | 11 +++--
> >  arch/x86/kernel/process.c        |  2 +-
> >  arch/x86/kernel/shstk.c          | 53 ++++++++++++++++++++---
> >  include/asm-generic/cacheflush.h | 11 +++++
> >  include/linux/sched/task.h       | 17 ++++++++
> >  include/uapi/linux/sched.h       |  9 ++--
> >  kernel/fork.c                    | 93 ++++++++++++++++++++++++++++++++++------
> >  8 files changed, 217 insertions(+), 26 deletions(-)
> 
> It would be great if Christian could give this the thumbs up, given that
> it changes clone3(). I think the architecture parts are all ready at this
> point.

ah, I may have spoken too soon :/

Catalin pointed me at this glibc thread:

https://marc.info/?l=glibc-alpha&m=175811917427562

which sounds like they're not entirely on board with the new ABI.

Will