[tip:perfcounters/core] perf_counter: Close race in perf_lock_task_context()

From: tip-bot for Peter Zijlstra
Date: Fri Jun 19 2009 - 12:28:39 EST

Commit-ID: b49a9e7e72103ea91946453c19703a4dfa1994fe
Gitweb: http://git.kernel.org/tip/b49a9e7e72103ea91946453c19703a4dfa1994fe
Author: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
AuthorDate: Fri, 19 Jun 2009 17:39:33 +0200
Committer: Ingo Molnar <mingo@xxxxxxx>
CommitDate: Fri, 19 Jun 2009 17:57:36 +0200

perf_counter: Close race in perf_lock_task_context()

perf_lock_task_context() is buggy because it can return a dead

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@xxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>

kernel/perf_counter.c | 11 +++++------
1 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index 8d4f0dd..adb6ae5 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -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;
+ }
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) {
- 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)
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);

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