linux-kernel - [PATCH v3] perf/core: fix mlock accounting in perf

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Date:   Thu, 23 Jan 2020 10:11:46 -0800
From:   Song Liu <songliubraving@...com>
To:     <linux-kernel@...r.kernel.org>
CC:     <kernel-team@...com>, Song Liu <songliubraving@...com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH v3] perf/core: fix mlock accounting in perf_mmap()

Eecreasing 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")
Signed-off-by: Song Liu <songliubraving@...com>
Suggested-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Jiri Olsa <jolsa@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
---
 kernel/events/core.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index e543bab787e5..fdb7f7ef380c 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5917,7 +5917,15 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 	 */
 	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) {
 		/*
-- 
2.17.1