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Date:	Thu, 17 Jul 2014 15:29:57 -0700
From:	Kees Cook <>
To:	Markos Chandras <>
Cc:	James Hogan <>,
	Paul Burton <>,
	Linux MIPS Mailing List <>,
	Ralf Baechle <>,
	LKML <>
Subject: MIPS seccomp and changing syscalls


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

The discussion about this bug on ARM is here:

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



Kees Cook
Chrome OS Security
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