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

Will Deacon
2015-08-26T10:36:57Z

Hi Chris,

On Tue, Aug 25, 2015 at 08:55:52PM +0100, Chris Metcalf wrote:
> With task_isolation mode, the task is in principle guaranteed not to
> be interrupted by the kernel, but only if it behaves. In particular,
> if it enters the kernel via system call, page fault, or any of a
> number of other synchronous traps, it may be unexpectedly exposed
> to long latencies. Add a simple flag that puts the process into
> a state where any such kernel entry is fatal.
>
> To allow the state to be entered and exited, we ignore the prctl()
> syscall so that we can clear the bit again later, and we ignore
> exit/exit_group to allow exiting the task without a pointless signal
> killing you as you try to do so.
>
> This change adds the syscall-detection hooks only for x86, arm64,
> and tile.
>
> The signature of context_tracking_exit() changes to report whether
> we, in fact, are exiting back to user space, so that we can track
> user exceptions properly separately from other kernel entries.
>
> Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
> ---
> arch/arm64/kernel/ptrace.c | 5 +++++
> arch/tile/kernel/ptrace.c | 5 ++++-
> arch/x86/kernel/ptrace.c | 2 ++
> include/linux/context_tracking.h | 11 ++++++++---
> include/linux/isolation.h | 16 ++++++++++++++++
> include/uapi/linux/prctl.h | 1 +
> kernel/context_tracking.c | 9 ++++++---
> kernel/isolation.c | 38 ++++++++++++++++++++++++++++++++++++++
> 8 files changed, 80 insertions(+), 7 deletions(-)
>
> 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.

> + 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.

Cheers,

Will