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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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