On Tue, Nov 15, 2022 at 01:26:23PM +0100, Peter Zijlstra wrote: > On Fri, Nov 04, 2022 at 03:35:51PM -0700, Rick Edgecombe wrote: > > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> > > > > Add three new arch_prctl() handles: > > > > - ARCH_CET_ENABLE/DISABLE enables or disables the specified > > feature. Returns 0 on success or an error. > > > > - ARCH_CET_LOCK prevents future disabling or enabling of the > > specified feature. Returns 0 on success or an error > > > > The features are handled per-thread and inherited over fork(2)/clone(2), > > but reset on exec(). > > > > This is preparation patch. It does not implement any features. > > Urgh... so much for sharing with other architectures I suppose :/ > > The ARM64 BTI thing is very similar to IBT (except I think their > approach to the legacy bitmap is much saner). > > Given that IBT isn't supported and needs the whole legacy bitmap mess, > do we really want to call this CET ? Why not just make a Shadow Stack > API and tackle IBT independently. On that; ARM64 exposes PROT_BTI (to be used by mprotect()) and have an ELF_ARM64_BTI note for the loader to bootstrap things. We could co-opt that same interface and instead of flipping actual PTE bits, have this thing manage the legacy bitmap -- basically have the legacy bitmap function as an external PTE bit array (in inverse). Basically, have every page mapped PROT_EXEC set the bit in the legacy bitmap while every page mapped PROT_EXEC|PROT_BTI will have the legacy bitmap bit to 0. And as long as there is a single 0 in the bitmap, the feature is enabled. (obviously we can delay allocating the bitmap until the first PROT_EXEC mapping that lacks PROT_BTI)
