On Mon, 2020-03-09 at 10:21 -0700, Dave Hansen wrote: > On 3/9/20 10:00 AM, Yu-cheng Yu wrote: > > On Wed, 2020-02-26 at 09:57 -0800, Dave Hansen wrote: > > > > index ade4e6ec23e0..8b69ebf0baed 100644 > > > > --- a/Documentation/admin-guide/kernel-parameters.txt > > > > +++ b/Documentation/admin-guide/kernel-parameters.txt > > > > @@ -3001,6 +3001,12 @@ > > > > noexec=on: enable non-executable mappings (default) > > > > noexec=off: disable non-executable mappings > > > > > > > > + no_cet_shstk [X86-64] Disable Shadow Stack for user-mode > > > > + applications > > > > > > If we ever add kernel support, "no_cet_shstk" will mean "no cet shstk > > > for userspace"? > > > > What about no_user_shstk, no_kernel_shstk? [...] > > > > +Note: > > > > + There is no CET-enabling arch_prctl function. By design, CET is > > > > + enabled automatically if the binary and the system can support it. > > > > > > This is kinda interesting. It means that a JIT couldn't choose to > > > protect the code it generates and have different rules from itself? > > > > JIT needs to be updated for CET first. Once that is done, it runs with CET > > enabled. It can use the NOTRACK prefix, for example. > > Am I missing something? > > What's the direct connection between shadow stacks and Indirect Branch > Tracking other than Intel marketing umbrellas? What I meant is that JIT code needs to be updated first; if it skips RETs, it needs to unwind the stack, and if it does indirect JMPs somewhere it needs to fix up the branch target or use NOTRACK. > > > > + The parameters passed are always unsigned 64-bit. When an IA32 > > > > + application passing pointers, it should only use the lower 32 bits. > > > > > > Won't a 32-bit app calling prctl() use the 32-bit ABI? How would it > > > even know it's running on a 64-bit kernel? > > > > The 32-bit app is passing only a pointer to an array of 64-bit numbers. > > Well, the documentation just talked about pointers and I naively assume > it means the "unsigned long *" you had in there. > > Rather than make suggestions, just say that the ABI is universally > 64-bit. Saying that the pointers must be valid is just kinda silly. > It's also not 100% clear what an "IA32 application" *MEANS* given fun > things like x32. Ok, I will update the text. > > Also, I went to go find this implementation in your series. I couldn't > find it. Did I miss a patch? Or are you documenting things you didn't > even implement? In patch #27: Add arch_prctl functions for Shadow Stack. Yu-cheng
