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    SubjectRe: [RFC PATCH] exec: Avoid recursive modprobe for binary format handlers
    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_);
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