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    SubjectRe: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations
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    On Tue, Jul 22, 2014 at 2:14 AM, AKASHI Takahiro
    <takahiro.akashi@linaro.org> 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
    > adr lr, __sys_trace_return // return address
    > uxtw scno, w0 // syscall number (possibly new)
    > mov x1, sp // pointer to regs
    > diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
    > index 70526cf..100d7d1 100644
    > --- a/arch/arm64/kernel/ptrace.c
    > +++ b/arch/arm64/kernel/ptrace.c
    > @@ -21,6 +21,7 @@
    >
    > #include <linux/audit.h>
    > #include <linux/compat.h>
    > +#include <linux/errno.h>
    > #include <linux/kernel.h>
    > #include <linux/sched.h>
    > #include <linux/mm.h>
    > @@ -1109,9 +1110,21 @@ static void tracehook_report_syscall(struct pt_regs *regs,
    >
    > asmlinkage int syscall_trace_enter(struct pt_regs *regs)
    > {
    > + unsigned long saved_x0, saved_x8;
    > +
    > + saved_x0 = regs->regs[0];
    > + saved_x8 = regs->regs[8];
    > +
    > if (test_thread_flag(TIF_SYSCALL_TRACE))
    > tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
    >
    > + regs->syscallno = regs->regs[8];
    > + if ((long)regs->syscallno == ~0UL) { /* skip this syscall */
    > + regs->regs[8] = saved_x8;
    > + if (regs->regs[0] == saved_x0) /* not changed by user */
    > + regs->regs[0] = -ENOSYS;

    I'm not sure this is right compared to other architectures. Generally
    when a tracer performs a syscall skip, it's up to them to also adjust
    the return value. They may want to be faking a syscall, and what if
    the value they want to return happens to be what x0 was going into the
    tracer? It would have no way to avoid this -ENOSYS case. I think
    things are fine without this test.

    -Kees

    > + }
    > +
    > if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
    > trace_sys_enter(regs, regs->syscallno);
    >
    > --
    > 1.7.9.5
    >



    --
    Kees Cook
    Chrome OS Security


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