Re: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations

From: AKASHI Takahiro
Date: Fri Jul 25 2014 - 06:37:07 EST

On 07/25/2014 12:01 AM, Andy Lutomirski wrote:
On Jul 23, 2014 10:57 PM, "AKASHI Takahiro" <takahiro.akashi@xxxxxxxxxx> 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.akashi@xxxxxxxxxx>
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)
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.

-Takahiro AKASHI

The current approach seems a bit scary.


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