Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model

Josh Poimboeuf
2016-06-06T14:29:27Z

On Mon, Jun 06, 2016 at 03:54:41PM +0200, Petr Mladek wrote:
> On Thu 2016-04-28 15:44:48, Josh Poimboeuf wrote:
> > Change livepatch to use a basic per-task consistency model. This is the
> > foundation which will eventually enable us to patch those ~10% of
> > security patches which change function or data semantics. This is the
> > biggest remaining piece needed to make livepatch more generally useful.
>
> > diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> > new file mode 100644
> > index 0000000..92819bb
> > --- /dev/null
> > +++ b/kernel/livepatch/transition.c
> > +/*
> > + * Try to safely switch a task to the target patch state. If it's currently
> > + * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or
> > + * if the stack is unreliable, return false.
> > + */
> > +static bool klp_try_switch_task(struct task_struct *task)
> > +{
> > + struct rq *rq;
> > + unsigned long flags;
>
> This should be of type "struct rq_flags". Otherwise, I get compilation
> warnings:
>
> kernel/livepatch/transition.c: In function ‘klp_try_switch_task’:
> kernel/livepatch/transition.c:349:2: warning: passing argument 2 of ‘task_rq_lock’ from incompatible pointer type [enabled by default]
> rq = task_rq_lock(task, &flags);
> ^
> In file included from kernel/livepatch/transition.c:24:0:
> kernel/livepatch/../sched/sched.h:1468:12: note: expected ‘struct rq_flags *’ but argument is of type ‘long unsigned int *’
> struct rq *task_rq_lock(struct task_struct *p, struct rq_flags *rf)
> ^
> kernel/livepatch/transition.c:367:2: warning: passing argument 3 of ‘task_rq_unlock’ from incompatible pointer type [enabled by default]
> task_rq_unlock(rq, task, &flags);
> ^
> In file included from kernel/livepatch/transition.c:24:0:
> kernel/livepatch/../sched/sched.h:1480:1: note: expected ‘struct rq_flags *’ but argument is of type ‘long unsigned int *’
> task_rq_unlock(struct rq *rq, struct task_struct *p, struct rq_flags *rf)
>
>
> And even runtime warnings from lockdep:
>
> [ 212.847548] WARNING: CPU: 1 PID: 3847 at kernel/locking/lockdep.c:3532 lock_release+0x431/0x480
> [ 212.847549] releasing a pinned lock
> [ 212.847550] Modules linked in: livepatch_sample(E+)
> [ 212.847555] CPU: 1 PID: 3847 Comm: modprobe Tainted: G E K 4.7.0-rc1-next-20160602-4-default+ #336
> [ 212.847556] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> [ 212.847558] 0000000000000000 ffff880139823aa0 ffffffff814388dc ffff880139823af0
> [ 212.847562] 0000000000000000 ffff880139823ae0 ffffffff8106fad1 00000dcc82b11390
> [ 212.847565] ffff88013fc978d8 ffffffff810eea1e ffff8800ba0ed6d0 0000000000000003
> [ 212.847569] Call Trace:
> [ 212.847572] [<ffffffff814388dc>] dump_stack+0x85/0xc9
> [ 212.847575] [<ffffffff8106fad1>] __warn+0xd1/0xf0
> [ 212.847578] [<ffffffff810eea1e>] ? klp_try_switch_task.part.3+0x5e/0x2b0
> [ 212.847580] [<ffffffff8106fb3f>] warn_slowpath_fmt+0x4f/0x60
> [ 212.847582] [<ffffffff810cc151>] lock_release+0x431/0x480
> [ 212.847585] [<ffffffff8101e258>] ? dump_trace+0x118/0x310
> [ 212.847588] [<ffffffff8195d07c>] ? entry_SYSCALL_64_fastpath+0x1f/0xbd
> [ 212.847590] [<ffffffff8195c8bf>] _raw_spin_unlock+0x1f/0x30
> [ 212.847600] [<ffffffff810eea1e>] klp_try_switch_task.part.3+0x5e/0x2b0
> [ 212.847603] [<ffffffff810ef0e4>] klp_try_complete_transition+0x84/0x190
> [ 212.847605] [<ffffffff810ed370>] __klp_enable_patch+0xb0/0x130
> [ 212.847607] [<ffffffff810ed445>] klp_enable_patch+0x55/0x80
> [ 212.847610] [<ffffffffa0000030>] ? livepatch_cmdline_proc_show+0x30/0x30 [livepatch_sample]
> [ 212.847613] [<ffffffffa0000061>] livepatch_init+0x31/0x70 [livepatch_sample]
> [ 212.847615] [<ffffffffa0000030>] ? livepatch_cmdline_proc_show+0x30/0x30 [livepatch_sample]
> [ 212.847617] [<ffffffff8100041d>] do_one_initcall+0x3d/0x160
> [ 212.847629] [<ffffffff81196c9b>] ? do_init_module+0x27/0x1e4
> [ 212.847632] [<ffffffff810e7172>] ? rcu_read_lock_sched_held+0x62/0x70
> [ 212.847634] [<ffffffff811fdea2>] ? kmem_cache_alloc_trace+0x282/0x340
> [ 212.847636] [<ffffffff81196cd4>] do_init_module+0x60/0x1e4
> [ 212.847638] [<ffffffff81111fd2>] load_module+0x1482/0x1d40
> [ 212.847640] [<ffffffff8110ea10>] ? __symbol_put+0x40/0x40
> [ 212.847643] [<ffffffff81112aa9>] SYSC_finit_module+0xa9/0xd0
> [ 212.847645] [<ffffffff81112aee>] SyS_finit_module+0xe/0x10
> [ 212.847647] [<ffffffff8195d07c>] entry_SYSCALL_64_fastpath+0x1f/0xbd
> [ 212.847649] ---[ end trace e4e9f09d45443049 ]---

Thanks, I also saw this when rebasing onto a newer linux-next.

> > + int ret;
> > + bool success = false;
> > +
> > + /* check if this task has already switched over */
> > + if (task->patch_state == klp_target_state)
> > + return true;
> > +
> > + /*
> > + * For arches which don't have reliable stack traces, we have to rely
> > + * on other methods (e.g., switching tasks at the syscall barrier).
> > + */
> > + if (!IS_ENABLED(CONFIG_RELIABLE_STACKTRACE))
> > + return false;
> > +
> > + /*
> > + * Now try to check the stack for any to-be-patched or to-be-unpatched
> > + * functions. If all goes well, switch the task to the target patch
> > + * state.
> > + */
> > + rq = task_rq_lock(task, &flags);
> > +
> > + if (task_running(rq, task) && task != current) {
> > + pr_debug("%s: pid %d (%s) is running\n", __func__, task->pid,
> > + task->comm);
>
> Also I think about using printk_deferred() inside the rq_lock but
> it is not strictly needed. Also we use only pr_debug() here which
> is a NOP when not enabled.

Good catch. It's probably best to avoid it anyway. klp_check_stack()
also has some pr_debug() calls. I may restructure the code a bit to
release the lock before doing any of the pr_debug()'s.

--
Josh