perf/core: Fix mlock accounting in perf_mmap()

commit 003461559ef7a9bd0239bae35a22ad8924d6e9ad upstream.

Decreasing sysctl_perf_event_mlock between two consecutive perf_mmap()s of
a perf ring buffer may lead to an integer underflow in locked memory
accounting. This may lead to the undesired behaviors, such as failures in
BPF map creation.

Address this by adjusting the accounting logic to take into account the
possibility that the amount of already locked memory may exceed the
current limit.

Fixes: c4b75479741c ("perf/core: Make the mlock accounting simple again")
Suggested-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: <stable@vger.kernel.org>
Acked-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Link: https://lkml.kernel.org/r/20200123181146.2238074-1-songliubraving@fb.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 64ace5e9af2a01dec821665f94085d814c3b487d..97b90faceb97fd2e004415a8429d556045d35b7d 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5303,7 +5303,15 @@ accounting:
         */
        user_lock_limit *= num_online_cpus();
 
-       user_locked = atomic_long_read(&user->locked_vm) + user_extra;
+       user_locked = atomic_long_read(&user->locked_vm);
+
+       /*
+        * sysctl_perf_event_mlock may have changed, so that
+        *     user->locked_vm > user_lock_limit
+        */
+       if (user_locked > user_lock_limit)
+               user_locked = user_lock_limit;
+       user_locked += user_extra;
 
        if (user_locked > user_lock_limit)
                extra = user_locked - user_lock_limit;