On 4/10/18 1:54 AM, Daniel Borkmann wrote:
On 04/10/2018 09:21 AM, Yonghong Song wrote:
syzbot reported a possible deadlock in perf_event_detach_bpf_prog.
The error details:
   WARNING: possible circular locking dependency detected
   4.16.0-rc7+ #3 Not tainted
   syz-executor7/24531 is trying to acquire lock:
    (bpf_event_mutex){+.+.}, at: [<000000008a849b07>] 
perf_event_detach_bpf_prog+0x92/0x3d0 kernel/trace/bpf_trace.c:854

   but task is already holding lock:
    (&mm->mmap_sem){++++}, at: [<0000000038768f87>] vm_mmap_pgoff+0x198/0x280 

   which lock already depends on the new lock.

   the existing dependency chain (in reverse order) is:

   -> #1 (&mm->mmap_sem){++++}:
        __might_fault+0x13a/0x1d0 mm/memory.c:4571
        _copy_to_user+0x2c/0xc0 lib/usercopy.c:25
        copy_to_user include/linux/uaccess.h:155 [inline]
        bpf_prog_array_copy_info+0xf2/0x1c0 kernel/bpf/core.c:1694
        perf_event_query_prog_array+0x1c7/0x2c0 kernel/trace/bpf_trace.c:891
        _perf_ioctl kernel/events/core.c:4750 [inline]
        perf_ioctl+0x3e1/0x1480 kernel/events/core.c:4770
        vfs_ioctl fs/ioctl.c:46 [inline]
        do_vfs_ioctl+0x1b1/0x1520 fs/ioctl.c:686
        SYSC_ioctl fs/ioctl.c:701 [inline]
        SyS_ioctl+0x8f/0xc0 fs/ioctl.c:692
        do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287

   -> #0 (bpf_event_mutex){+.+.}:
        lock_acquire+0x1d5/0x580 kernel/locking/lockdep.c:3920
        __mutex_lock_common kernel/locking/mutex.c:756 [inline]
        __mutex_lock+0x16f/0x1a80 kernel/locking/mutex.c:893
        mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:908
        perf_event_detach_bpf_prog+0x92/0x3d0 kernel/trace/bpf_trace.c:854
        perf_event_free_bpf_prog kernel/events/core.c:8147 [inline]
        _free_event+0xbdb/0x10f0 kernel/events/core.c:4116
        put_event+0x24/0x30 kernel/events/core.c:4204
        perf_mmap_close+0x60d/0x1010 kernel/events/core.c:5172
        remove_vma+0xb4/0x1b0 mm/mmap.c:172
        remove_vma_list mm/mmap.c:2490 [inline]
        do_munmap+0x82a/0xdf0 mm/mmap.c:2731
        mmap_region+0x59e/0x15a0 mm/mmap.c:1646
        do_mmap+0x6c0/0xe00 mm/mmap.c:1483
        do_mmap_pgoff include/linux/mm.h:2223 [inline]
        vm_mmap_pgoff+0x1de/0x280 mm/util.c:355
        SYSC_mmap_pgoff mm/mmap.c:1533 [inline]
        SyS_mmap_pgoff+0x462/0x5f0 mm/mmap.c:1491
        SYSC_mmap arch/x86/kernel/sys_x86_64.c:100 [inline]
        SyS_mmap+0x16/0x20 arch/x86/kernel/sys_x86_64.c:91
        do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287

   other info that might help us debug this:

    Possible unsafe locking scenario:

          CPU0                    CPU1
          ----                    ----

    *** DEADLOCK ***

The bug is introduced by Commit f371b304f12e ("bpf/tracing: allow
user space to query prog array on the same tp") where copy_to_user,
which requires mm->mmap_sem, is called inside bpf_event_mutex lock.
At the same time, during perf_event file descriptor close,
mm->mmap_sem is held first and then subsequent
perf_event_detach_bpf_prog needs bpf_event_mutex lock.
Such a senario caused a deadlock.

As suggested by Danial, moving copy_to_user out of the

Nit: typo :)
Oh, my bad. will correct.

bpf_event_mutex lock should fix the problem.

Fixes: f371b304f12e ("bpf/tracing: allow user space to query prog array on the same 
Reported-by: syzbot+dc5ca0e4c9bfafaf2...@syzkaller.appspotmail.com
Signed-off-by: Yonghong Song <y...@fb.com>
  include/linux/bpf.h      |  4 ++--
  kernel/bpf/core.c        | 45 +++++++++++++++++++++++++++++----------------
  kernel/trace/bpf_trace.c | 19 +++++++++++++++----
  3 files changed, 46 insertions(+), 22 deletions(-)

   v2 -> v3:
     . Remove the redundant label in function perf_event_query_prog_array.
   v1 -> v2:
     . Use the common core function for prog_id copying for two
       different prog_array copy routines, suggested by Alexei.

  static void bpf_prog_free_deferred(struct work_struct *work)
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index d88e96d..f505d43 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -977,6 +977,7 @@ int perf_event_query_prog_array(struct perf_event *event, 
void __user *info)
        struct perf_event_query_bpf __user *uquery = info;
        struct perf_event_query_bpf query = {};
+       u32 *ids, prog_cnt, ids_len;
        int ret;
if (!capable(CAP_SYS_ADMIN))
@@ -985,16 +986,26 @@ int perf_event_query_prog_array(struct perf_event *event, 
void __user *info)
                return -EINVAL;
        if (copy_from_user(&query, uquery, sizeof(query)))
                return -EFAULT;
-       if (query.ids_len > BPF_TRACE_MAX_PROGS)
+       ids_len = query.ids_len;
+       if (ids_len > BPF_TRACE_MAX_PROGS)
                return -E2BIG;
+       ids = kcalloc(ids_len, sizeof(u32), GFP_USER | __GFP_NOWARN);
+       if (!ids)
+               return -ENOMEM;

Fix looks good to me, but could you still add a comment stating that we don't
need to check for ZERO_SIZE_PTR above since we handle this gracefully in the
bpf_prog_array_copy_info() plus it's also required for the case where the user
only wants to check for the uquery->prog_cnt, but nothing else.

Will add the comment and send another version.


        ret = bpf_prog_array_copy_info(event->tp_event->prog_array,
-                                      uquery->ids,
-                                      query.ids_len,
-                                      &uquery->prog_cnt);
+                                      ids,
+                                      ids_len,
+                                      &prog_cnt);
+ if (copy_to_user(&uquery->prog_cnt, &prog_cnt, sizeof(prog_cnt)) ||
+           copy_to_user(uquery->ids, ids, ids_len * sizeof(u32)))
+               ret = -EFAULT;
+       kfree(ids);
        return ret;


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