On 07/25/2014 12:01 AM, Andy Lutomirski wrote:
On Jul 23, 2014 10:57 PM, "AKASHI Takahiro" <takahiro.aka...@linaro.org> wrote:
On 07/24/2014 12:54 PM, Andy Lutomirski wrote:
On 07/22/2014 02:14 AM, AKASHI Takahiro 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.aka...@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
Does this mean that skipped syscalls will cause exit tracing to be skipped?
Yes. (and I guess yes on arm, too)
If so, then you risk (at least) introducing
a nice user-triggerable OOPS if audit is enabled.
Can you please elaborate this?
Since I didn't find any definition of audit's behavior when syscall is
rewritten to -1, I thought it is reasonable to skip "exit tracing" of
"skipped" syscall.
(otherwise, "fake" seems to be more appropriate :)
The audit entry hook will oops if you call it twice in a row without
calling the exit hook in between.
Thank you, I could reproduce this problem which hits BUG(in_syscall) in
audit_syscall_entry(). Really bad, and I fixed it in my next version and
now a "skipped" system call is also traced by audit.
I ran libseccomp test and Kees' test under auditd running with a rule,
auditctl -a exit,always -S all
and all the tests seemed to pass.
I can also imagine ptracers getting
confused if ptrace entry and exit don't line up.
FYI, on arm64, we can distinguish syscall enter/exit with x12 register.
What happens if user code directly issues syscall ~0? Does the return
value register get set? Is the behavior different between traced and
untraced syscalls?
Interesting cases. Let me think about it.
Thanks,
-Takahiro AKASHI
The current approach seems a bit scary.
--Andy
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
