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

Petr Mladek
2016-06-06T13:54:48Z

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 ]---

> + 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.

Best Regards,
Petr