Messages in this thread | | | Date | Tue, 22 Jul 2014 13:15:01 -0700 | Subject | Re: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations | From | Kees Cook <> |
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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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