On Thu, Jan 19, 2023 at 01:23:15PM -0800, Rick Edgecombe wrote: > From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> > > Some applications (like GDB) would like to tweak shadow stack state via > ptrace. This allows for existing functionality to continue to work for > seized shadow stack applications. Provide an regset interface for > manipulating the shadow stack pointer (SSP). > > There is already ptrace functionality for accessing xstate, but this > does not include supervisor xfeatures. So there is not a completely > clear place for where to put the shadow stack state. Adding it to the > user xfeatures regset would complicate that code, as it currently shares > logic with signals which should not have supervisor features. > > Don't add a general supervisor xfeature regset like the user one, > because it is better to maintain flexibility for other supervisor > xfeatures to define their own interface. For example, an xfeature may > decide not to expose all of it's state to userspace, as is actually the > case for shadow stack ptrace functionality. A lot of enum values remain > to be used, so just put it in dedicated shadow stack regset. > > The only downside to not having a generic supervisor xfeature regset, > is that apps need to be enlightened of any new supervisor xfeature > exposed this way (i.e. they can't try to have generic save/restore > logic). But maybe that is a good thing, because they have to think > through each new xfeature instead of encountering issues when new a new > supervisor xfeature was added. > > By adding a shadow stack regset, it also has the effect of including the > shadow stack state in a core dump, which could be useful for debugging. > > The shadow stack specific xstate includes the SSP, and the shadow stack > and WRSS enablement status. Enabling shadow stack or wrss in the kernel > involves more than just flipping the bit. The kernel is made aware that > it has to do extra things when cloning or handling signals. That logic > is triggered off of separate feature enablement state kept in the task > struct. So the flipping on HW shadow stack enforcement without notifying > the kernel to change its behavior would severely limit what an application > could do without crashing, and the results would depend on kernel > internal implementation details. There is also no known use for controlling > this state via prtace today. So only expose the SSP, which is something > that userspace already has indirect control over. > > Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx> > Tested-by: John Allen <john.allen@xxxxxxx> > Co-developed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> -- Kees Cook
