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perf_counter: Close race in perf_lock_task_context() 

perf_lock_task_context() is buggy because it can return a dead
context.

the RCU read lock in perf_lock_task_context() only guarantees
the memory won't get freed, it doesn't guarantee the object is
valid (in our case refcount > 0).

Therefore we can return a locked object that can get freed the
moment we release the rcu read lock.

perf_pin_task_context() then increases the refcount and does an
unlock on freed memory.

That increased refcount will cause a double free, in case it
started out with 0.

Ammend this by including the get_ctx() functionality in
perf_lock_task_context() (all users already did this later
anyway), and return a NULL context when the found one is
already dead.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Peter Zijlstra 2009-06-19 17:39:33 +02:00 committed by Ingo Molnar
parent 0c87197142
commit b49a9e7e72
1 changed files with 5 additions and 6 deletions
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11
kernel/perf_counter.c
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@ -175,6 +175,11 @@ perf_lock_task_context(struct task_struct *task, unsigned long *flags)
spin_unlock_irqrestore(&ctx->lock, *flags);
goto retry;
}
if (!atomic_inc_not_zero(&ctx->refcount)) {
spin_unlock_irqrestore(&ctx->lock, *flags);
ctx = NULL;
}
}
rcu_read_unlock();
return ctx;
@ -193,7 +198,6 @@ static struct perf_counter_context *perf_pin_task_context(struct task_struct *ta
ctx = perf_lock_task_context(task, &flags);
if (ctx) {
++ctx->pin_count;
get_ctx(ctx);
spin_unlock_irqrestore(&ctx->lock, flags);
}
return ctx;
@ -1459,11 +1463,6 @@ static struct perf_counter_context *find_get_context(pid_t pid, int cpu)
put_ctx(parent_ctx);
ctx->parent_ctx = NULL; /* no longer a clone */
}
/*
* Get an extra reference before dropping the lock so that
* this context won't get freed if the task exits.
*/
get_ctx(ctx);
spin_unlock_irqrestore(&ctx->lock, flags);
}