Re: [RFC PATCH] exec: Avoid recursive modprobe for binary format handlers

["Luis R. Rodriguez" <mcgrof@kernel.org>]

On Tue, Aug 01, 2017 at 07:28:20PM -0700, Kees Cook wrote:
> On Tue, Aug 1, 2017 at 5:12 PM, Luis R. Rodriguez <mcgrof@kernel.org> wrote:
> > On Fri, Jul 21, 2017 at 03:05:20PM +0100, Matt Redfearn wrote:
> >> Commit 6d7964a722af ("kmod: throttle kmod thread limit") which was
> >> merged in v4.13-rc1 broke this behaviour since the recursive modprobe is
> >> no longer caught, it just ends up waiting indefinitely for the kmod_wq
> >> wait queue. Hence the kernel appears to hang silently when starting
> >> userspace.
> >
> > Indeed, the recursive issue were no longer expected to exist.
> 
> Errr, yeah, recursive binfmt loads can still happen.
> 
> > The *old* implementation would also prevent a set of binaries to daisy chain
> > a set of 50 different binaries which require different binfmt loaders. The
> > current implementation enables this and we'd just wait. There's a bound to
> > the number of binfmd loaders though, so this would be bounded. If however
> > a 2nd loader loaded the first binary we'd run into the same issue I think.
> >
> > If we can't think of a good way to resolve this we'll just have to revert
> > 6d7964a722af for now.
> 
> The weird but "normal" recursive case is usually a script calling a
> script calling a misc format. Getting a chain of modprobes running,
> though, seems unlikely. I *think* Matt's patch is okay, but I agree,
> it'd be better for the request_module() to fail.

In that case how about we just have each waiter only wait max X seconds,
if the number of concurrent ongoing modprobe calls hasn't reduced by
a single digit in X seconds we give up on request_module() for the
module and clearly indicate what happened.

Matt, can you test?

Note I've used wait_event_killable_timeout() to only accept SIGKILL
for now. I've seen issues wit SIGCHILD and at modprobe this could
even be a bigger issue, so this would restrict the signals received
*only* to SIGKILL.

It would be good to come up with a simple test case for this in
tools/testing/selftests/kmod/kmod.sh

  Luis

diff --git a/include/linux/wait.h b/include/linux/wait.h
index 5b74e36c0ca8..dc19880c02f5 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -757,6 +757,43 @@ extern int do_wait_intr_irq(wait_queue_head_t *, wait_queue_entry_t *);
 	__ret;									\
 })
 
+#define __wait_event_killable_timeout(wq_head, condition, timeout)		\
+	___wait_event(wq_head, ___wait_cond_timeout(condition),			\
+		      TASK_KILLABLE, 0, timeout,				\
+		      __ret = schedule_timeout(__ret))
+
+/**
+ * wait_event_killable_timeout - sleep until a condition gets true or a timeout elapses
+ * @wq_head: the waitqueue to wait on
+ * @condition: a C expression for the event to wait for
+ * @timeout: timeout, in jiffies
+ *
+ * The process is put to sleep (TASK_KILLABLE) until the
+ * @condition evaluates to true or a kill signal is received.
+ * The @condition is checked each time the waitqueue @wq_head is woken up.
+ *
+ * wake_up() has to be called after changing any variable that could
+ * change the result of the wait condition.
+ *
+ * Returns:
+ * 0 if the @condition evaluated to %false after the @timeout elapsed,
+ * 1 if the @condition evaluated to %true after the @timeout elapsed,
+ * the remaining jiffies (at least 1) if the @condition evaluated
+ * to %true before the @timeout elapsed, or -%ERESTARTSYS if it was
+ * interrupted by a kill signal.
+ *
+ * Only kill signals interrupt this process.
+ */
+#define wait_event_killable_timeout(wq_head, condition, timeout)		\
+({										\
+	long __ret = timeout;							\
+	might_sleep();								\
+	if (!___wait_cond_timeout(condition))					\
+		__ret = __wait_event_killable_timeout(wq_head,			\
+						condition, timeout);		\
+	__ret;									\
+})
+
 
 #define __wait_event_lock_irq(wq_head, condition, lock, cmd)			\
 	(void)___wait_event(wq_head, condition, TASK_UNINTERRUPTIBLE, 0, 0,	\
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 6d016c5d97c8..1b5f7bada8d2 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -71,6 +71,13 @@ static atomic_t kmod_concurrent_max = ATOMIC_INIT(MAX_KMOD_CONCURRENT);
 static DECLARE_WAIT_QUEUE_HEAD(kmod_wq);
 
 /*
+ * If modprobe can't be called after this time we assume its very likely
+ * your userspace has created a recursive dependency, and we'll have no
+ * option but to fail.
+ */
+#define MAX_KMOD_TIMEOUT 5
+
+/*
 	modprobe_path is set via /proc/sys.
 */
 char modprobe_path[KMOD_PATH_LEN] = "/sbin/modprobe";
@@ -167,8 +174,18 @@ int __request_module(bool wait, const char *fmt, ...)
 		pr_warn_ratelimited("request_module: kmod_concurrent_max (%u) close to 0 (max_modprobes: %u), for module %s, throttling...",
 				    atomic_read(&kmod_concurrent_max),
 				    MAX_KMOD_CONCURRENT, module_name);
-		wait_event_interruptible(kmod_wq,
-					 atomic_dec_if_positive(&kmod_concurrent_max) >= 0);
+		ret = wait_event_killable_timeout(kmod_wq,
+						  atomic_dec_if_positive(&kmod_concurrent_max) >= 0,
+						  MAX_KMOD_TIMEOUT * HZ);
+		if (!ret) {
+			pr_warn_ratelimited("request_module: modprobe %s cannot be processed, kmod busy with %d threads for more than %d seconds now",
+					    module_name, atomic_read(&kmod_concurrent_max), MAX_KMOD_TIMEOUT);
+			pr_warn_ratelimited("request_module: recursive modprobe call very likely!");
+			return -ETIME;
+		} else if (ret == -ERESTARTSYS) {
+			pr_warn_ratelimited("request_module: sigkill sent for modprobe %s, giving up", module_name);
+			return ret;
+		}
 	}
 
 	trace_module_request(module_name, wait, _RET_IP_);