MIPS seccomp and changing syscalls

[Kees Cook]

Hi,

I recently fixed a bug in seccomp on ARM that I think may be present
in the MIPS implementation too. In arch/mips/kernel/ptrace.c
syscall_trace_enter, the syscall variable is used (and returned), but
the syscall may be changed by either secure_computing or
tracehook_report_syscall_entry (via ptracers which can block and
change the registers). (I would note that "ret" is also set but never
used, so tracehook_report_syscall_entry failures actually won't get
noticed.)

The discussion about this bug on ARM is here:
https://lkml.org/lkml/2014/6/20/439

I don't yet have a working MIPS environment to test this on, but it
feels like the same bug. (Though, for testing, what's the right way to
change syscall during PTRACE_SYSCALL? On x86 it's the orig_ax
register, on ARM it's a arch-specific ptrace function
(PTRACE_SET_SYSCALL).

Thanks!

-Kees

-- 
Kees Cook
Chrome OS Security
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