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? 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(); } /* _
