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SubjectRe: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations
On 07/22/2014 02:14 AM, AKASHI Takahiro wrote:
> Arm64 holds a syscall number in w8(x8) register. Ptrace tracer may change
> its value either to:
> * any valid syscall number to alter a system call, or
> * -1 to skip a system call
>
> This patch implements this behavior by reloading that value into syscallno
> in struct pt_regs after tracehook_report_syscall_entry() or
> secure_computing(). In case of '-1', a return value of system call can also
> be changed by the tracer setting the value to x0 register, and so
> sys_ni_nosyscall() should not be called.
>
> See also:
> 42309ab4, ARM: 8087/1: ptrace: reload syscall number after
> secure_computing() check
>
> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
> ---
> arch/arm64/kernel/entry.S | 2 ++
> arch/arm64/kernel/ptrace.c | 13 +++++++++++++
> 2 files changed, 15 insertions(+)
>
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 5141e79..de8bdbc 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -628,6 +628,8 @@ ENDPROC(el0_svc)
> __sys_trace:
> mov x0, sp
> bl syscall_trace_enter
> + cmp w0, #-1 // skip syscall?
> + b.eq ret_to_user

Does this mean that skipped syscalls will cause exit tracing to be
skipped? If so, then you risk (at least) introducing a nice
user-triggerable OOPS if audit is enabled. This bug existed for *years*
on x86_32, and it amazes me that no one ever triggered it by accident.
(Grr, audit.)

--Andy


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