From b49a9e7e72103ea91946453c19703a4dfa1994fe Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Fri, 19 Jun 2009 17:39:33 +0200 Subject: [PATCH] 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 Cc: Mike Galbraith Cc: Paul Mackerras Cc: Arnaldo Carvalho de Melo LKML-Reference: Signed-off-by: Ingo Molnar --- kernel/perf_counter.c | 11 +++++------ 1 file 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; + } } 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); } -- 2.7.4