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

From: AKASHI Takahiro
Date: Wed Aug 27 2014 - 01:55:58 EST

On 08/27/2014 02:51 AM, Will Deacon wrote:
On Fri, Aug 22, 2014 at 01:35:17AM +0100, AKASHI Takahiro wrote:
On 08/22/2014 02:08 AM, Kees Cook wrote:
On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro
<takahiro.akashi@xxxxxxxxxx> wrote:
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?

Well, the first thing that jumps out at me is why this is being done
completely differently for arm64 and arm. I thought adding the new ptrace
requests would reconcile the differences?

I'm not sure what portion of my code you mentioned as "completely different", but

setting x0 to -ENOSYS is necessary because, otherwise, user-issued syscall(-1) will
return a bogus value when audit tracing is on.

Please note that, on arm,
not traced traced
------ ------
syscall(-1) aborted OOPs(BUG_ON)
syscall(-3000) aborted aborted
syscall(1000) ENOSYS ENOSYS

So, anyhow, its a bit difficult and meaningless to mimic these invalid cases.

branching a new label, syscall_trace_return_skip (see entry.S), after syscall_trace_enter()
is necessary in order to avoid OOPS in audit_syscall_enter() as we discussed.

Did I make it clear?

-Takahiro AKASHI


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