binder: prevent UAF read in print_binder_transaction_log_entry()

When a binder transaction is initiated on a binder device coming from a
binderfs instance, a pointer to the name of the binder device is stashed
in the binder_transaction_log_entry's context_name member. Later on it
is used to print the name in print_binder_transaction_log_entry(). By
the time print_binder_transaction_log_entry() accesses context_name
binderfs_evict_inode() might have already freed the associated memory
thereby causing a UAF. Do the simple thing and prevent this by copying
the name of the binder device instead of stashing a pointer to it.

Reported-by: Jann Horn <jannh@google.com>
Fixes: 03e2e07e38 ("binder: Make transaction_log available in binderfs")
Link: https://lore.kernel.org/r/CAG48ez14Q0-F8LqsvcNbyR2o6gPW8SHXsm4u5jmD9MpsteM2Tw@mail.gmail.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: Todd Kjos <tkjos@google.com>
Reviewed-by: Hridya Valsaraju <hridya@google.com>
Link: https://lore.kernel.org/r/20191008130159.10161-1-christian.brauner@ubuntu.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder.c

@@ -57,6 +57,7 @@
 #include <linux/sched/signal.h>
 #include <linux/sched/mm.h>
 #include <linux/seq_file.h>
+#include <linux/string.h>
 #include <linux/uaccess.h>
 #include <linux/pid_namespace.h>
 #include <linux/security.h>
@@ -66,6 +67,7 @@
 #include <linux/task_work.h>
 
 #include <uapi/linux/android/binder.h>
+#include <uapi/linux/android/binderfs.h>
 
 #include <asm/cacheflush.h>
 
@@ -2876,7 +2878,7 @@ static void binder_transaction(struct binder_proc *proc,
 	e->target_handle = tr->target.handle;
 	e->data_size = tr->data_size;
 	e->offsets_size = tr->offsets_size;
-	e->context_name = proc->context->name;
+	strscpy(e->context_name, proc->context->name, BINDERFS_MAX_NAME);
 
 	if (reply) {
 		binder_inner_proc_lock(proc);

drivers/android/binder_internal.h

@@ -130,7 +130,7 @@ struct binder_transaction_log_entry {
 	int return_error_line;
 	uint32_t return_error;
 	uint32_t return_error_param;
-	const char *context_name;
+	char context_name[BINDERFS_MAX_NAME + 1];
 };
 
 struct binder_transaction_log {