Messages in this thread | | | Date | Wed, 23 Jul 2014 16:03:47 +0900 | From | AKASHI Takahiro <> | Subject | Re: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations |
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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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