Re: [PATCH v3 3/3] arm64: audit: Add audit hook in ptrace/syscall_trace

[AKASHI Takahiro]

On 02/05/2014 02:31 AM, Will Deacon wrote:
> On Mon, Feb 03, 2014 at 06:56:30AM +0000, AKASHI Takahiro wrote:
> > This patch adds auditing functions on entry to or exit from
> > every system call invocation.
> >
> > Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
> > ---
> >   arch/arm64/include/asm/thread_info.h |    1 +
> >   arch/arm64/kernel/entry.S            |    3 +++
> >   arch/arm64/kernel/ptrace.c           |   10 ++++++++++
> >   3 files changed, 14 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
> > index 720e70b..7468388 100644
> > --- a/arch/arm64/include/asm/thread_info.h
> > +++ b/arch/arm64/include/asm/thread_info.h
> > @@ -101,6 +101,7 @@ static inline struct thread_info *current_thread_info(void)
> >   #define TIF_NEED_RESCHED	1
> >   #define TIF_NOTIFY_RESUME	2	/* callback before returning to user */
> >   #define TIF_SYSCALL_TRACE	8
> > +#define TIF_SYSCALL_AUDIT	9
> >   #define TIF_POLLING_NRFLAG	16
> >   #define TIF_MEMDIE		18	/* is terminating due to OOM killer */
> >   #define TIF_FREEZE		19
> > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> > index 827cbad..83c4b29 100644
> > --- a/arch/arm64/kernel/entry.S
> > +++ b/arch/arm64/kernel/entry.S
> > @@ -630,6 +630,9 @@ el0_svc_naked:					// compat entry point
> >   	get_thread_info tsk
> >   	ldr	x16, [tsk, #TI_FLAGS]		// check for syscall tracing
> >   	tbnz	x16, #TIF_SYSCALL_TRACE, __sys_trace // are we tracing syscalls?
> > +#ifdef CONFIG_AUDITSYSCALL
> > +	tbnz	x16, #TIF_SYSCALL_AUDIT, __sys_trace // auditing syscalls?
> > +#endif
>
> Could we avoid the back-to-back tbnz instructions with a single mask? It's
> not obvious that it will end up any better, but it would be good to know.

When first implementing ftrace support, TIF_SYSCALL_TRACEPOINT is defined as 10
and 'tst' instruction doesn't allow the following code:
    tst x16, #(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_TRACEPOINT)

That is why I've used "back-to-back" tbnz since then, but now that I'm going to
submit ftrace, audit and later seccomp, I will replace it with:
    #define TIF_SYSCALL_WORK (_TIF_SYSCALL_TRACE|TRACEPOINT|AUDIT|SECCOMP)

    tst x16, #TIF_SYSCALL_WORK
    b.ne __syscall_trace


> >   	adr	lr, ret_fast_syscall		// return address
> >   	cmp     scno, sc_nr                     // check upper syscall limit
> >   	b.hs	ni_sys
> > diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> > index 6777a21..75a3f23 100644
> > --- a/arch/arm64/kernel/ptrace.c
> > +++ b/arch/arm64/kernel/ptrace.c
> > @@ -19,6 +19,7 @@
> >    * along with this program.  If not, see <http://www.gnu.org/licenses/>.
> >    */
> >
> > +#include <linux/audit.h>
> >   #include <linux/kernel.h>
> >   #include <linux/sched.h>
> >   #include <linux/mm.h>
> > @@ -38,6 +39,7 @@
> >   #include <asm/compat.h>
> >   #include <asm/debug-monitors.h>
> >   #include <asm/pgtable.h>
> > +#include <asm/syscall.h>
> >   #include <asm/traps.h>
> >   #include <asm/system_misc.h>
> >
> > @@ -1064,6 +1066,14 @@ asmlinkage int syscall_trace(int dir, struct pt_regs *regs)
> >   {
> >   	unsigned long saved_reg;
> >
> > +	if (dir)
> > +		audit_syscall_exit(regs);
> > +	else
> > +		audit_syscall_entry(syscall_get_arch(current, regs),
> > +			(int)regs->syscallno,
> > +			regs->orig_x0, regs->regs[1],
> > +			regs->regs[2], regs->regs[3]);
> > +
>
> Do we really want to perform the audit checks before the tracehook calls?
> Remember that the latter can rewrite all of the registers.

OK. I will change the code to make calls in the following order:
On entry,
   *secure_computing
   *tracehook_report_syscall(ENTER)
   *trace_sys_enter
   *audit_syscall_entry
On exit,
   *audit_syscall_exit
   *trace_sys_exit
   *tracehook_report_syscall(EXIT)

The order here is the exact same as on x86, but such change might
decrease the readability in syscall_trace().

Thanks,
-Takahiro AKASHI

> Will
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