Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the fast path

From	"H. Peter Anvin" <hpa@zytor.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the fast path
Date	2016-04-18 08:10 +0200
Message-ID	<rpaxH-Jy-5@gated-at.bofh.it> (permalink)
References	(3 earlier) <rp9Lk-8w6-7@gated-at.bofh.it> <rp9UZ-94-3@gated-at.bofh.it> <rpa4G-f9-3@gated-at.bofh.it> <rpaem-jl-9@gated-at.bofh.it> <rpaem-jl-7@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Show all headers | View raw

On 04/17/16 22:48, Andy Lutomirski wrote:
> 
> I think I prefer the "reject weird input" behavior over the "accept
> and normalize weird input" if we can get away with it, and I'm fairly
> confident that we can get away with "reject weird input" given that
> distro kernels do exactly that already.
> 

It's not "weird", it is the ABI as defined.  We have to do this for all
the system call arguments, too; you just don't notice it because the
compiler does it for us.  Some other architectures, e.g. s390, has the
opposite convention where the caller is responsible for normalizing the
result; in that case we have to do it *again* in the kernel, which is
one of the major reasons for the SYSCALL_*() macros.

So I'm not sure this is a valid consideration.  The reason it generally
works is because the natural way for the user space code to work is to
load a value into %eax which will naturally zero-extend to %rax, but it
isn't inherently required to work that way.

	-hpa

Thread

Re: System call number masking Ben Hutchings <ben@decadent.org.uk> - 2016-04-18 02:50 +0200
  [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path Ben Hutchings <ben@decadent.org.uk> - 2016-04-18 02:50 +0200
    Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path "H. Peter Anvin" <hpa@zytor.com> - 2016-04-18 07:00 +0200
      Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path Andy Lutomirski <luto@amacapital.net> - 2016-04-18 07:20 +0200
        Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path "H. Peter Anvin" <hpa@zytor.com> - 2016-04-18 07:30 +0200
        Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path "H. Peter Anvin" <hpa@zytor.com> - 2016-04-18 07:30 +0200
          Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path Andy Lutomirski <luto@amacapital.net> - 2016-04-18 07:40 +0200
            Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path "H. Peter Anvin" <hpa@zytor.com> - 2016-04-18 07:50 +0200
              Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path Andy Lutomirski <luto@amacapital.net> - 2016-04-18 07:50 +0200
                Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path "H. Peter Anvin" <hpa@zytor.com> - 2016-04-18 08:10 +0200
                Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path "H. Peter Anvin" <hpa@zytor.com> - 2016-04-18 08:20 +0200
                Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the  fast path Andy Lutomirski <luto@amacapital.net> - 2016-04-18 08:20 +0200

csiph-web