Re: [PATCH v6 3/6] task_isolation: support PR_TASK_ISOLATION_STRICT mode

Will Deacon
2015-09-02T10:13:47Z

On Wed, Aug 26, 2015 at 04:10:34PM +0100, Chris Metcalf wrote:
> On 08/26/2015 06:36 AM, Will Deacon wrote:
> > On Tue, Aug 25, 2015 at 08:55:52PM +0100, Chris Metcalf wrote:
> >> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> >> index d882b833dbdb..e3d83a12f3cf 100644
> >> --- a/arch/arm64/kernel/ptrace.c
> >> +++ b/arch/arm64/kernel/ptrace.c
> >> @@ -37,6 +37,7 @@
> >> #include <linux/regset.h>
> >> #include <linux/tracehook.h>
> >> #include <linux/elf.h>
> >> +#include <linux/isolation.h>
> >>
> >> #include <asm/compat.h>
> >> #include <asm/debug-monitors.h>
> >> @@ -1150,6 +1151,10 @@ static void tracehook_report_syscall(struct pt_regs *regs,
> >>
> >> asmlinkage int syscall_trace_enter(struct pt_regs *regs)
> >> {
> >> + /* Ensure we report task_isolation violations in all circumstances. */
> >> + if (test_thread_flag(TIF_NOHZ) && task_isolation_strict())
> > This is going to force us to check TIF_NOHZ on the syscall slowpath even
> > when CONFIG_TASK_ISOLATION=n.
>
> Yes, good catch. I was thinking the "&& false" would suppress the TIF
> test but I forgot that test_bit() takes a volatile argument, so it gets
> evaluated even though the result isn't actually used.
>
> But I don't want to just reorder the two tests, because when isolation
> is enabled, testing TIF_NOHZ first is better. I think probably the right
> solution is just to put an #ifdef CONFIG_TASK_ISOLATION around that
> test, even though that is a little crufty. The alternative is to provide
> a task_isolation_configured() macro that just returns true or false, and
> make it a three-part "&&" test with that new macro first, but
> that seems a little crufty as well. Do you have a preference?

Maybe use IS_ENABLED(CONFIG_TASK_ISOLATION) ?

> >> + task_isolation_syscall(regs->syscallno);
> >> +
> >> /* Do the secure computing check first; failures should be fast. */
> > Here we have the usual priority problems with all the subsystems that
> > hook into the syscall path. If a prctl is later rewritten to a different
> > syscall, do you care about catching it? Either way, the comment about
> > doing secure computing "first" needs fixing.
>
> I admit I am unclear on the utility of rewriting prctl. My instinct is that
> we are trying to catch userspace invocations of prctl and allow them,
> and fail most everything else, so doing it pre-rewrite seems OK.
>
> I'm not sure if it makes sense to catch it before or after the
> secure computing check, though. On reflection maybe doing it
> afterwards makes more sense - what do you think?

I don't have a strong preference (I really hate all these hooks we have
on the syscall entry/exit path), but we do need to make sure that the
behaviour is consistent across architectures.

Will