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    SubjectRe: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations
    On 07/23/2014 05:15 AM, Kees Cook wrote:
    > 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.

    Yeah, I know this issue, but was not sure that setting a return value
    is mandatory. (x86 seems to return -ENOSYS by default if not explicitly
    specified.)
    Is "fake a system call" a more appropriate word than "skip"?

    I will defer to Will.

    Thanks,
    -Takahiro AKASHI

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


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