kcov: detect double association with a single task

Currently KCOV_ENABLE does not check if the current task is already
associated with another kcov descriptor.  As the result it is possible
to associate a single task with more than one kcov descriptor, which
later leads to a memory leak of the old descriptor.  This relation is
really meant to be one-to-one (task has only one back link).

Extend validation to detect such misuse.

Link: http://lkml.kernel.org/r/20180122082520.15716-1-dvyukov@google.com
Fixes: 5c9a8750a640 ("kernel: add kcov code coverage")
Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Reported-by: Shankara Pailoor <sp3485@columbia.edu>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/kernel/kcov.c b/kernel/kcov.c
index 7594c033d98a..2c16f1ab5e10 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -358,7 +358,8 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
                  */
                 if (kcov->mode != KCOV_MODE_INIT || !kcov->area)
                         return -EINVAL;
-                if (kcov->t != NULL)
+                t = current;
+                if (kcov->t != NULL || t->kcov != NULL)
                         return -EBUSY;
                 if (arg == KCOV_TRACE_PC)
                         kcov->mode = KCOV_MODE_TRACE_PC;
@@ -370,7 +371,6 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
 #endif
                 else
                         return -EINVAL;
-                t = current;
                 /* Cache in task struct for performance. */
                 t->kcov_size = kcov->size;
                 t->kcov_area = kcov->area;