Re: [PATCH] mm: mmap_lock: fix use-after-free race and css ref leak in tracepoints

[Axel Rasmussen <axelrasmussen@google.com>]

On Mon, Nov 30, 2020 at 5:34 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Mon, Nov 30, 2020 at 3:43 PM Axel Rasmussen <axelrasmussen@google.com> wrote:
> >
> > syzbot reported[1] a use-after-free introduced in 0f818c4bc1f3. The bug
> > is that an ongoing trace event might race with the tracepoint being
> > disabled (and therefore the _unreg() callback being called). Consider
> > this ordering:
> >
> > T1: trace event fires, get_mm_memcg_path() is called
> > T1: get_memcg_path_buf() returns a buffer pointer
> > T2: trace_mmap_lock_unreg() is called, buffers are freed
> > T1: cgroup_path() is called with the now-freed buffer
>
> Any reason to use the cgroup_path instead of the cgroup_ino? There are
> other examples of trace points using cgroup_ino and no need to
> allocate buffers. Also cgroup namespace might complicate the path
> usage.

Hmm, so in general I would love to use a numeric identifier instead of a string.

I did some reading, and it looks like the cgroup_ino() mainly has to
do with writeback, instead of being just a general identifier?
https://www.kernel.org/doc/Documentation/cgroup-v2.txt

There is cgroup_id() which I think is almost what I'd want, but there
are a couple problems with it:

- I don't know of a way for userspace to translate IDs -> paths, to
make them human readable?
- Also I think the ID implementation we use for this is "dense",
meaning if a cgroup is removed, its ID is likely to be quickly reused.

>
> >
> > The solution in this commit is to modify trace_mmap_lock_unreg() to
> > first stop new buffers from being handed out, and then to wait (spin)
> > until any existing buffer references are dropped (i.e., those trace
> > events complete).
> >
> > I have a simple reproducer program which spins up two pools of threads,
> > doing the following in a tight loop:
> >
> >   Pool 1:
> >   mmap(NULL, 4096, PROT_READ | PROT_WRITE,
> >        MAP_PRIVATE | MAP_ANONYMOUS, -1, 0)
> >   munmap()
> >
> >   Pool 2:
> >   echo 1 > /sys/kernel/debug/tracing/events/mmap_lock/enable
> >   echo 0 > /sys/kernel/debug/tracing/events/mmap_lock/enable
> >
> > This triggers the use-after-free very quickly. With this patch, I let it
> > run for an hour without any BUGs.
> >
> > While fixing this, I also noticed and fixed a css ref leak. Previously
> > we called get_mem_cgroup_from_mm(), but we never called css_put() to
> > release that reference. get_mm_memcg_path() now does this properly.
> >
> > [1]: https://syzkaller.appspot.com/bug?extid=19e6dd9943972fa1c58a
> >
> > Fixes: 0f818c4bc1f3 ("mm: mmap_lock: add tracepoints around lock acquisition")
>
> The original patch is in mm tree, so the SHA1 is not stabilized.
> Usually Andrew squash the fixes into the original patches.

Ah, I added this because it also shows up in linux-next, under the
next-20201130 tag. I'll remove it in v2, squashing is fine. :)

>
> > Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
> > ---
> >  mm/mmap_lock.c | 100 +++++++++++++++++++++++++++++++++++++++++--------
> >  1 file changed, 85 insertions(+), 15 deletions(-)
> >
> > diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
> > index 12af8f1b8a14..be38dc58278b 100644
> > --- a/mm/mmap_lock.c
> > +++ b/mm/mmap_lock.c
> > @@ -3,6 +3,7 @@
> >  #include <trace/events/mmap_lock.h>
> >
> >  #include <linux/mm.h>
> > +#include <linux/atomic.h>
> >  #include <linux/cgroup.h>
> >  #include <linux/memcontrol.h>
> >  #include <linux/mmap_lock.h>
> > @@ -18,13 +19,28 @@ EXPORT_TRACEPOINT_SYMBOL(mmap_lock_released);
> >  #ifdef CONFIG_MEMCG
> >
> >  /*
> > - * Our various events all share the same buffer (because we don't want or need
> > - * to allocate a set of buffers *per event type*), so we need to protect against
> > - * concurrent _reg() and _unreg() calls, and count how many _reg() calls have
> > - * been made.
> > + * This is unfortunately complicated... _reg() and _unreg() may be called
> > + * in parallel, separately for each of our three event types. To save memory,
> > + * all of the event types share the same buffers. Furthermore, trace events
> > + * might happen in parallel with _unreg(); we need to ensure we don't free the
> > + * buffers before all inflights have finished. Because these events happen
> > + * "frequently", we also want to prevent new inflights from starting once the
> > + * _unreg() process begins. And, for performance reasons, we want to avoid any
> > + * locking in the trace event path.
> > + *
> > + * So:
> > + *
> > + * - Use a spinlock to serialize _reg() and _unreg() calls.
> > + * - Keep track of nested _reg() calls with a lock-protected counter.
> > + * - Define a flag indicating whether or not unregistration has begun (and
> > + *   therefore that there should be no new buffer uses going forward).
> > + * - Keep track of inflight buffer users with a reference count.
> >   */
> >  static DEFINE_SPINLOCK(reg_lock);
> > -static int reg_refcount;
> > +static int reg_types_rc; /* Protected by reg_lock. */
> > +static bool unreg_started; /* Doesn't need synchronization. */
> > +/* atomic_t instead of refcount_t, as we want ordered inc without locks. */
> > +static atomic_t inflight_rc = ATOMIC_INIT(0);
> >
> >  /*
> >   * Size of the buffer for memcg path names. Ignoring stack trace support,
> > @@ -46,9 +62,14 @@ int trace_mmap_lock_reg(void)
> >         unsigned long flags;
> >         int cpu;
> >
> > +       /*
> > +        * Serialize _reg() and _unreg(). Without this, e.g. _unreg() might
> > +        * start cleaning up while _reg() is only partially completed.
> > +        */
> >         spin_lock_irqsave(&reg_lock, flags);
> >
> > -       if (reg_refcount++)
> > +       /* If the refcount is going 0->1, proceed with allocating buffers. */
> > +       if (reg_types_rc++)
> >                 goto out;
> >
> >         for_each_possible_cpu(cpu) {
> > @@ -62,6 +83,11 @@ int trace_mmap_lock_reg(void)
> >                 per_cpu(memcg_path_buf_idx, cpu) = 0;
> >         }
> >
> > +       /* Reset unreg_started flag, allowing new trace events. */
> > +       WRITE_ONCE(unreg_started, false);
> > +       /* Add the registration +1 to the inflight refcount. */
> > +       atomic_inc(&inflight_rc);
> > +
> >  out:
> >         spin_unlock_irqrestore(&reg_lock, flags);
> >         return 0;
> > @@ -74,7 +100,8 @@ int trace_mmap_lock_reg(void)
> >                         break;
> >         }
> >
> > -       --reg_refcount;
> > +       /* Since we failed, undo the earlier increment. */
> > +       --reg_types_rc;
> >
> >         spin_unlock_irqrestore(&reg_lock, flags);
> >         return -ENOMEM;
> > @@ -87,9 +114,23 @@ void trace_mmap_lock_unreg(void)
> >
> >         spin_lock_irqsave(&reg_lock, flags);
> >
> > -       if (--reg_refcount)
> > +       /* If the refcount is going 1->0, proceed with freeing buffers. */
> > +       if (--reg_types_rc)
> >                 goto out;
> >
> > +       /* This was the last registration; start preventing new events... */
> > +       WRITE_ONCE(unreg_started, true);
> > +       /* Remove the registration +1 from the inflight refcount. */
> > +       atomic_dec(&inflight_rc);
> > +       /*
> > +        * Wait for inflight refcount to be zero (all inflights stopped). Since
> > +        * we have a spinlock we can't sleep, so just spin. Because trace events
> > +        * are "fast", and because we stop new inflights from starting at this
> > +        * point with unreg_started, this should be a short spin.
> > +        */
> > +       while (atomic_read(&inflight_rc))
> > +               barrier();
> > +
> >         for_each_possible_cpu(cpu) {
> >                 kfree(per_cpu(memcg_path_buf, cpu));
> >         }
> > @@ -102,6 +143,20 @@ static inline char *get_memcg_path_buf(void)
> >  {
> >         int idx;
> >
> > +       /*
> > +        * If unregistration is happening, stop. Yes, this check is racy;
> > +        * that's fine. It just means _unreg() might spin waiting for an extra
> > +        * event or two. Use-after-free is actually prevented by the refcount.
> > +        */
> > +       if (READ_ONCE(unreg_started))
> > +               return NULL;
> > +       /*
> > +        * Take a reference, unless the registration +1 has been released
> > +        * and there aren't already existing inflights (refcount is zero).
> > +        */
> > +       if (!atomic_inc_not_zero(&inflight_rc))
> > +               return NULL;
> > +
> >         idx = this_cpu_add_return(memcg_path_buf_idx, MEMCG_PATH_BUF_SIZE) -
> >               MEMCG_PATH_BUF_SIZE;
> >         return &this_cpu_read(memcg_path_buf)[idx];
> > @@ -110,27 +165,42 @@ static inline char *get_memcg_path_buf(void)
> >  static inline void put_memcg_path_buf(void)
> >  {
> >         this_cpu_sub(memcg_path_buf_idx, MEMCG_PATH_BUF_SIZE);
> > +       /* We're done with this buffer; drop the reference. */
> > +       atomic_dec(&inflight_rc);
> >  }
> >
> >  /*
> >   * Write the given mm_struct's memcg path to a percpu buffer, and return a
> > - * pointer to it. If the path cannot be determined, NULL is returned.
> > + * pointer to it. If the path cannot be determined, or no buffer was available
> > + * (because the trace event is being unregistered), NULL is returned.
> >   *
> >   * Note: buffers are allocated per-cpu to avoid locking, so preemption must be
> >   * disabled by the caller before calling us, and re-enabled only after the
> >   * caller is done with the pointer.
> > + *
> > + * The caller must call put_memcg_path_buf() once the buffer is no longer
> > + * needed. This must be done while preemption is still disabled.
> >   */
> >  static const char *get_mm_memcg_path(struct mm_struct *mm)
> >  {
> > +       char *buf = NULL;
> >         struct mem_cgroup *memcg = get_mem_cgroup_from_mm(mm);
> >
> > -       if (memcg != NULL && likely(memcg->css.cgroup != NULL)) {
> > -               char *buf = get_memcg_path_buf();
> > +       if (memcg == NULL)
> > +               goto out;
> > +       if (unlikely(memcg->css.cgroup == NULL))
> > +               goto out_put;
> >
> > -               cgroup_path(memcg->css.cgroup, buf, MEMCG_PATH_BUF_SIZE);
> > -               return buf;
> > -       }
> > -       return NULL;
> > +       buf = get_memcg_path_buf();
> > +       if (buf == NULL)
> > +               goto out_put;
> > +
> > +       cgroup_path(memcg->css.cgroup, buf, MEMCG_PATH_BUF_SIZE);
> > +
> > +out_put:
> > +       css_put(&memcg->css);
> > +out:
> > +       return buf;
> >  }
> >
> >  #define TRACE_MMAP_LOCK_EVENT(type, mm, ...)                                   \
> > --
> > 2.29.2.454.gaff20da3a2-goog
> >