x86, PEBS/DS: fix code flow in ds_request()

this compiler warning:

  arch/x86/kernel/ds.c: In function 'ds_request':
  arch/x86/kernel/ds.c:368: warning: 'context' may be used uninitialized in this function

Shows that the code flow in ds_request() is buggy - it goes into
the unlock+release-context path even when the context is not allocated
yet.

First allocate the context, then do the other checks.

Also, take care with GFP allocations under the ds_lock spinlock.

Cc: <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 21 insertions, 5 deletions

diff --git a/arch/x86/kernel/ds.c b/arch/x86/kernel/ds.c
index ac1d5b0586b..d1a121443bd 100644
--- a/arch/x86/kernel/ds.c
+++ b/arch/x86/kernel/ds.c
@@ -236,17 +236,33 @@ static inline struct ds_context *ds_alloc_context(struct task_struct *task)
         struct ds_context *context = *p_context;
 
         if (!context) {
+                spin_unlock(&ds_lock);
+
                 context = kzalloc(sizeof(*context), GFP_KERNEL);
 
-                if (!context)
+                if (!context) {
+                        spin_lock(&ds_lock);
                         return NULL;
+                }
 
                 context->ds = kzalloc(ds_cfg.sizeof_ds, GFP_KERNEL);
                 if (!context->ds) {
                         kfree(context);
+                        spin_lock(&ds_lock);
                         return NULL;
                 }
 
+                spin_lock(&ds_lock);
+                /*
+                 * Check for race - another CPU could have allocated
+                 * it meanwhile:
+                 */
+                if (*p_context) {
+                        kfree(context->ds);
+                        kfree(context);
+                        return *p_context;
+                }
+
                 *p_context = context;
 
                 context->this = p_context;
@@ -384,15 +400,15 @@ static int ds_request(struct task_struct *task, void *base, size_t size,
 
         spin_lock(&ds_lock);
 
-        error = -EPERM;
-        if (!check_tracer(task))
-                goto out_unlock;
-
         error = -ENOMEM;
         context = ds_alloc_context(task);
         if (!context)
                 goto out_unlock;
 
+        error = -EPERM;
+        if (!check_tracer(task))
+                goto out_unlock;
+
         error = -EALREADY;
         if (context->owner[qual] == current)
                 goto out_unlock;