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 11:13:48 -0700 |
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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 think this would be a nice alternative to another new syscall.
If this looks good to everyone, I can send out new patches as response to my current version, and then after all issues fixed, send v12.
Thanks, Yu-cheng
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