On Fri, Dec 04, 2015 at 09:04:35PM +0100, Dmitry Vyukov wrote:
> Hello,
>
> While running syzkaller fuzzer I am seeing lots of the following
> use-after-free reports. Unfortunately all my numerous attempts to
> reproduce them in a controlled environment failed. They pop up during
> fuzzing periodically (once in several hours in a single VM), but
> whenever I try to stress-replay what happened in the VM before the
> report, the use-after-free does not reproduce.
Typical that :/
> Can somebody knowledgeable in perf subsystem look at the report? Maybe
> it is possible to figure out what happened based purely on the report.
> I can pretty reliably test any proposed fixes.
So I'm still going over the code, but meanwhile I tried reproducing this
using the perf_fuzzer and some debug code, but no luck with that.
Since you seem to be able to reproduce, could you do a run with the
below patch in to see if it tickles something?
---
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 36babfd..58e504c 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -872,13 +872,22 @@ static void free_ctx(struct rcu_head *head)
kfree(ctx);
}
+#define for_each_task_context_nr(ctxn) \
+ for ((ctxn) = 0; (ctxn) < perf_nr_task_contexts; (ctxn)++)
+
static void put_ctx(struct perf_event_context *ctx)
{
if (atomic_dec_and_test(&ctx->refcount)) {
if (ctx->parent_ctx)
put_ctx(ctx->parent_ctx);
- if (ctx->task)
+ if (ctx->task) {
+ int ctxn;
+
+ for_each_task_context_nr(ctxn)
+ WARN_ON(ctx->task->perf_event_ctxp[ctxn] ==
ctx);
+
put_task_struct(ctx->task);
+ }
call_rcu(&ctx->rcu_head, free_ctx);
}
}
@@ -2649,9 +2658,6 @@ static void perf_pmu_sched_task(struct task_struct *prev,
static void perf_event_switch(struct task_struct *task,
struct task_struct *next_prev, bool sched_in);
-#define for_each_task_context_nr(ctxn) \
- for ((ctxn) = 0; (ctxn) < perf_nr_task_contexts; (ctxn)++)
-
/*
* Called from scheduler to remove the events of the current task,
* with interrupts disabled.
@@ -8931,6 +8937,8 @@ again:
mutex_unlock(&ctx->mutex);
+ task->perf_event_ctxp[ctxn] = NULL;
+
put_ctx(ctx);
}
}
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