[PATCH v4 2/7] kcov: fix potential use-after-free in kcov_remote_start

[Andrey Konovalov <andreyknvl@xxxxxxxxxx>]

If vmalloc() fails in kcov_remote_start() we'll access remote->kcov
without holding kcov_remote_lock, so remote might potentially be freed
at that point. Cache kcov pointer in a local variable.

Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
---
 kernel/kcov.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/kernel/kcov.c b/kernel/kcov.c
index f6bd119c9419..cc5900ac2467 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -748,6 +748,7 @@ static const struct file_operations kcov_fops = {
 void kcov_remote_start(u64 handle)
 {
 	struct kcov_remote *remote;
+	struct kcov *kcov;
 	void *area;
 	struct task_struct *t;
 	unsigned int size;
@@ -774,16 +775,17 @@ void kcov_remote_start(u64 handle)
 		spin_unlock(&kcov_remote_lock);
 		return;
 	}
+	kcov = remote->kcov;
 	/* Put in kcov_remote_stop(). */
-	kcov_get(remote->kcov);
-	t->kcov = remote->kcov;
+	kcov_get(kcov);
+	t->kcov = kcov;
 	/*
 	 * Read kcov fields before unlock to prevent races with
 	 * KCOV_DISABLE / kcov_remote_reset().
 	 */
-	size = remote->kcov->remote_size;
-	mode = remote->kcov->mode;
-	sequence = remote->kcov->sequence;
+	size = kcov->remote_size;
+	mode = kcov->mode;
+	sequence = kcov->sequence;
 	area = kcov_remote_area_get(size);
 	spin_unlock(&kcov_remote_lock);
 
@@ -791,7 +793,7 @@ void kcov_remote_start(u64 handle)
 		area = vmalloc(size * sizeof(unsigned long));
 		if (!area) {
 			t->kcov = NULL;
-			kcov_put(remote->kcov);
+			kcov_put(kcov);
 			return;
 		}
 	}
-- 
2.26.0.rc2.310.g2932bb562d-goog