Messages in this thread | | | Subject | Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack | From | "Yu, Yu-cheng" <> | Date | Thu, 27 Aug 2020 12:33:56 -0700 |
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On 8/27/2020 11:56 AM, Andy Lutomirski wrote: > > >> On Aug 27, 2020, at 11:13 AM, Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote: >> >> On 8/27/2020 6:36 AM, Florian Weimer wrote: >>> * H. J. Lu: >>>>> On Thu, Aug 27, 2020 at 6:19 AM Florian Weimer <fweimer@redhat.com> wrote: >>>>>> >>>>>> * Dave Martin: >>>>>> >>>>>>> You're right that this has implications: for i386, libc probably pulls >>>>>>> more arguments off the stack than are really there in some situations. >>>>>>> This isn't a new problem though. There are already generic prctls with >>>>>>> fewer than 4 args that are used on x86. >>>>>> >>>>>> As originally posted, glibc prctl would have to know that it has to pull >>>>>> an u64 argument off the argument list for ARCH_X86_CET_DISABLE. But >>>>>> then the u64 argument is a problem for arch_prctl as well. >>>>>> >>>> >>>> Argument of ARCH_X86_CET_DISABLE is int and passed in register. >>> The commit message and the C source say otherwise, I think (not sure >>> about the C source, not a kernel hacker). >> >> H.J. Lu suggested that we fix x86 arch_prctl() to take four arguments, and then keep MMAP_SHSTK as an arch_prctl(). Because now the map flags and size are all in registers, this also solves problems being pointed out earlier. Without a wrapper, the shadow stack mmap call (from user space) will be: >> >> syscall(_NR_arch_prctl, ARCH_X86_CET_MMAP_SHSTK, size, MAP_32BIT). > > I admit I don’t see a show stopping technical reason we can’t add arguments to an existing syscall, but I’m pretty sure it’s unprecedented, and it doesn’t seem like a good idea. >
There are nine existing arch_prctl calls now. If the concern is the extra new arguments getting misused, we can mask them out for the existing calls. Otherwise, I have not seen anything that can break.
Yu-cheng
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