Re: [PATCH v6 2/6] arm64: ptrace: allow tracer to skip a system call

On 08/22/2014 02:08 AM, Kees Cook wrote:

On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro takahiro.akashi@linaro.org wrote:

...

If tracer specifies -1 as a syscall number, this traced system call should be skipped with a value in x0 used as a return value. This patch enables this semantics, but there is a restriction here:

when syscall(-1) is issued by user, tracer cannot skip this system call
and modify a return value at syscall entry.

In order to ease this flavor, we need to treat whatever value in x0 as a return value, but this might result in a bogus value being returned, especially when tracer doesn't do anything at this syscall. So we always return ENOSYS instead, while we have another chance to change a return value at syscall exit.

Please also note:

• syscall entry tracing and syscall exit tracing (ftrace tracepoint and audit) are always executed, if enabled, even when skipping a system call (that is, -1). In this way, we can avoid a potential bug where audit_syscall_entry() might be called without audit_syscall_exit() at the previous system call being called, that would cause OOPs in audit_syscall_entry().

• syscallno may also be set to -1 if a fatal signal (SIGKILL) is detected in tracehook_report_syscall_entry(), but since a value set to x0 (ENOSYS) is not used in this case, we may neglect the case.

Signed-off-by: AKASHI Takahiro takahiro.akashi@linaro.org

arch/arm64/include/asm/ptrace.h | 8 ++++++++ arch/arm64/kernel/entry.S | 4 ++++ arch/arm64/kernel/ptrace.c | 20 ++++++++++++++++++++ 3 files changed, 32 insertions(+)

diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h index 501000f..a58cf62 100644 --- a/arch/arm64/include/asm/ptrace.h +++ b/arch/arm64/include/asm/ptrace.h @@ -65,6 +65,14 @@ #define COMPAT_PT_TEXT_ADDR 0x10000 #define COMPAT_PT_DATA_ADDR 0x10004 #define COMPAT_PT_TEXT_END_ADDR 0x10008

+/*

• • used to skip a system call when tracer changes its number to -1
• • with ptrace(PTRACE_SET_SYSCALL)
• */

+#define RET_SKIP_SYSCALL -1 +#define IS_SKIP_SYSCALL(no) ((int)(no & 0xffffffff) == -1)

• #ifndef __ASSEMBLY__

  /* sizeof(struct user) for AArch32 */

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S index f0b5e51..fdd6eae 100644 --- a/arch/arm64/kernel/entry.S +++ b/arch/arm64/kernel/entry.S @@ -25,6 +25,7 @@ #include <asm/asm-offsets.h> #include <asm/errno.h> #include <asm/esr.h> +#include <asm/ptrace.h> #include <asm/thread_info.h> #include <asm/unistd.h>

@@ -671,6 +672,8 @@ ENDPROC(el0_svc) __sys_trace: mov x0, sp bl syscall_trace_enter

•   cmp     w0, #RET_SKIP_SYSCALL           // skip syscall?
  

•   b.eq    __sys_trace_return_skipped
     adr     lr, __sys_trace_return          // return address
     uxtw    scno, w0                        // syscall number (possibly new)
     mov     x1, sp                          // pointer to regs
  

@@ -685,6 +688,7 @@ __sys_trace:

__sys_trace_return: str x0, [sp] // save returned x0 +__sys_trace_return_skipped: // x0 already in regs[0] mov x0, sp bl syscall_trace_exit b ret_to_user diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c index 8876049..c54dbcc 100644 --- a/arch/arm64/kernel/ptrace.c +++ b/arch/arm64/kernel/ptrace.c @@ -1121,9 +1121,29 @@ static void tracehook_report_syscall(struct pt_regs *regs,

asmlinkage int syscall_trace_enter(struct pt_regs *regs) {

•   unsigned int saved_syscallno = regs->syscallno;
  

•    if (test_thread_flag(TIF_SYSCALL_TRACE))
             tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
  
  

•   if (IS_SKIP_SYSCALL(regs->syscallno)) {
  

•           /*
  

•            * RESTRICTION: we can't modify a return value of user
  

•            * issued syscall(-1) here. In order to ease this flavor,
  

•            * we need to treat whatever value in x0 as a return value,
  

•            * but this might result in a bogus value being returned.
  

•            */
  

•           /*
  

•            * NOTE: syscallno may also be set to -1 if fatal signal is
  

•            * detected in tracehook_report_syscall_entry(), but since
  

•            * a value set to x0 here is not used in this case, we may
  

•            * neglect the case.
  

•            */
  

•           if (!test_thread_flag(TIF_SYSCALL_TRACE) ||
  

•                           (IS_SKIP_SYSCALL(saved_syscallno)))
  

•                   regs->regs[0] = -ENOSYS;
  

•   }
  

I don't have a runtime environment yet for arm64, so I can't test this directly myself, so I'm just trying to eyeball this. :)

Once the seccomp logic is added here, I don't think using -2 as a special value will work. Doesn't this mean the Oops is possible by the user issuing a "-2" syscall? As in, if TIF_SYSCALL_WORK is set, and the user passed -2 as the syscall, audit will be called only on entry, and then skipped on exit?

Oops, you're absolutely right. I didn't think of this case. syscall_trace_enter() should not return a syscallno directly, but always return -1 if syscallno < 0. (except when secure_computing() returns with -1) This also implies that tracehook_report_syscall() should also have a return value.

Will, is this fine with you?

-Takahiro AKASHI

-Kees

...

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

-- 1.7.9.5