On Mon, Dec 7, 2020 at 5:27 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
On Mon, 7 Dec 2020 13:33:58 -0800 Axel Rasmussen <axelrasmussen@xxxxxxxxxx> 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
>
> The solution in this commit is to switch to mutex + RCU. With the RCU
> API we can first stop new buffers from being handed out, then wait for
> existing users to finish, and *then* free the buffers.
>
> 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
>
So... how does this fix differ from the previous version of this fix?
Apologies, I'll include a cover letter with this information next time. The diff below from your mail includes both of these steps (v1 -> v3).
Changes from v2 -> v3:
Split up the free loop, so now we do it in three steps: 1) loop through setting the buffers to NULL, 2) synchronize_rcu() *once*, 3) loop through freeing each of the buffers. This requires allocating some memory to hold the not-yet-freed buffer pointers, but it means much less waiting as doing synchronize_rcu() in a loop is expensive. Again, per Steven's suggestion.
Changes from v1 -> v2:
Rewrote the fix to use mutex + RCU instead of doing some hand-rolled reference count thing, as per Steven's suggestion.
The difference is quite large:
--- a/mm/mmap_lock.c~mmap_lock-add-tracepoints-around-lock-acquisition-fix-fix
+++ a/mm/mmap_lock.c
@@ -3,13 +3,13 @@
#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>
+#include <linux/mutex.h>
#include <linux/percpu.h>
+#include <linux/rcupdate.h>
#include <linux/smp.h>
-#include <linux/spinlock.h>
#include <linux/trace_events.h>
EXPORT_TRACEPOINT_SYMBOL(mmap_lock_start_locking);
@@ -19,28 +19,13 @@ EXPORT_TRACEPOINT_SYMBOL(mmap_lock_relea
#ifdef CONFIG_MEMCG
/*
- * 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.
+ * 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.
*/
-static DEFINE_SPINLOCK(reg_lock);
-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);
+static DEFINE_MUTEX(reg_lock);
+static int reg_refcount; /* Protected by reg_lock. */
/*
* Size of the buffer for memcg path names. Ignoring stack trace support,
@@ -54,119 +39,107 @@ static atomic_t inflight_rc = ATOMIC_INI
*/
#define CONTEXT_COUNT 4
-DEFINE_PER_CPU(char *, memcg_path_buf);
-DEFINE_PER_CPU(int, memcg_path_buf_idx);
+static DEFINE_PER_CPU(char __rcu *, memcg_path_buf);
+static char **tmp_bufs;
+static DEFINE_PER_CPU(int, memcg_path_buf_idx);
+
+/* Called with reg_lock held. */
+static void free_memcg_path_bufs(void)
+{
+ int cpu;
+ char **old = tmp_bufs;
+
+ for_each_possible_cpu(cpu) {
+ *(old++) = rcu_dereference_protected(
+ per_cpu(memcg_path_buf, cpu),
+ lockdep_is_held(®_lock));
+ rcu_assign_pointer(per_cpu(memcg_path_buf, cpu), NULL);
+ }
+
+ /* Wait for inflight memcg_path_buf users to finish. */
+ synchronize_rcu();
+
+ old = tmp_bufs;
+ for_each_possible_cpu(cpu) {
+ kfree(*(old++));
+ }
+
+ kfree(tmp_bufs);
+ tmp_bufs = NULL;
+}
int trace_mmap_lock_reg(void)
{
- unsigned long flags;
int cpu;
+ char *new;
- /*
- * Serialize _reg() and _unreg(). Without this, e.g. _unreg() might
- * start cleaning up while _reg() is only partially completed.
- */
- spin_lock_irqsave(®_lock, flags);
+ mutex_lock(®_lock);
/* If the refcount is going 0->1, proceed with allocating buffers. */
- if (reg_types_rc++)
+ if (reg_refcount++)
goto out;
+ tmp_bufs = kmalloc_array(num_possible_cpus(), sizeof(*tmp_bufs),
+ GFP_KERNEL);
+ if (tmp_bufs == NULL)
+ goto out_fail;
+
for_each_possible_cpu(cpu) {
- per_cpu(memcg_path_buf, cpu) = NULL;
- }
- for_each_possible_cpu(cpu) {
- per_cpu(memcg_path_buf, cpu) = kmalloc(
- MEMCG_PATH_BUF_SIZE * CONTEXT_COUNT, GFP_NOWAIT);
- if (per_cpu(memcg_path_buf, cpu) == NULL)
- goto out_fail;
- per_cpu(memcg_path_buf_idx, cpu) = 0;
+ new = kmalloc(MEMCG_PATH_BUF_SIZE * CONTEXT_COUNT, GFP_KERNEL);
+ if (new == NULL)
+ goto out_fail_free;
+ rcu_assign_pointer(per_cpu(memcg_path_buf, cpu), new);
+ /* Don't need to wait for inflights, they'd have gotten NULL. */
}
- /* 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(®_lock, flags);
+ mutex_unlock(®_lock);
return 0;
+out_fail_free:
+ free_memcg_path_bufs();
out_fail:
- for_each_possible_cpu(cpu) {
- if (per_cpu(memcg_path_buf, cpu) != NULL)
- kfree(per_cpu(memcg_path_buf, cpu));
- else
- break;
- }
+ /* Since we failed, undo the earlier ref increment. */
+ --reg_refcount;
- /* Since we failed, undo the earlier increment. */
- --reg_types_rc;
-
- spin_unlock_irqrestore(®_lock, flags);
+ mutex_unlock(®_lock);
return -ENOMEM;
}
void trace_mmap_lock_unreg(void)
{
- unsigned long flags;
- int cpu;
-
- spin_lock_irqsave(®_lock, flags);
+ mutex_lock(®_lock);
/* If the refcount is going 1->0, proceed with freeing buffers. */
- if (--reg_types_rc)
+ if (--reg_refcount)
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));
- }
+ free_memcg_path_bufs();
out:
- spin_unlock_irqrestore(®_lock, flags);
+ mutex_unlock(®_lock);
}
static inline char *get_memcg_path_buf(void)
{
+ char *buf;
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))
+ rcu_read_lock();
+ buf = rcu_dereference(*this_cpu_ptr(&memcg_path_buf));
+ if (buf == NULL) {
+ rcu_read_unlock();
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];
+ return &buf[idx];
}
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);
+ rcu_read_unlock();
}
/*
_
