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Subject[PATCH v3] binder: fix proc->files use-after-free
Date
proc->files cleanup is initiated by binder_vma_close. Therefore
a reference on the binder_proc is not enough to prevent the
files_struct from being released while the binder_proc still has
a reference. This can lead to an attempt to dereference the
stale pointer obtained from proc->files prior to proc->files
cleanup. This has been seen once in task_get_unused_fd_flags()
when __alloc_fd() is called with a stale "files".

The fix is to protect proc->files with a mutex to prevent cleanup
while in use.

Signed-off-by: Todd Kjos <tkjos@google.com>
---
v2: declare binder_get_files_struct as static
v3: rework to protect proc->files with a mutex instead of using get_files_struct

Also needed in 4.14

drivers/android/binder.c | 44 +++++++++++++++++++++++++++++++-------------
1 file changed, 31 insertions(+), 13 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index a73596a4f804..7c027ee61375 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -482,7 +482,8 @@ enum binder_deferred_state {
* @tsk task_struct for group_leader of process
* (invariant after initialized)
* @files files_struct for process
- * (invariant after initialized)
+ * (protected by @files_lock)
+ * @files_lock mutex to protect @files
* @deferred_work_node: element for binder_deferred_list
* (protected by binder_deferred_lock)
* @deferred_work: bitmap of deferred work to perform
@@ -530,6 +531,7 @@ struct binder_proc {
int pid;
struct task_struct *tsk;
struct files_struct *files;
+ struct mutex files_lock;
struct hlist_node deferred_work_node;
int deferred_work;
bool is_dead;
@@ -877,20 +879,26 @@ static void binder_inc_node_tmpref_ilocked(struct binder_node *node);

static int task_get_unused_fd_flags(struct binder_proc *proc, int flags)
{
- struct files_struct *files = proc->files;
unsigned long rlim_cur;
unsigned long irqs;
+ int ret;

- if (files == NULL)
- return -ESRCH;
-
- if (!lock_task_sighand(proc->tsk, &irqs))
- return -EMFILE;
-
+ mutex_lock(&proc->files_lock);
+ if (proc->files == NULL) {
+ ret = -ESRCH;
+ goto err;
+ }
+ if (!lock_task_sighand(proc->tsk, &irqs)) {
+ ret = -EMFILE;
+ goto err;
+ }
rlim_cur = task_rlimit(proc->tsk, RLIMIT_NOFILE);
unlock_task_sighand(proc->tsk, &irqs);

- return __alloc_fd(files, 0, rlim_cur, flags);
+ ret = __alloc_fd(proc->files, 0, rlim_cur, flags);
+err:
+ mutex_unlock(&proc->files_lock);
+ return ret;
}

/*
@@ -899,8 +907,10 @@ static int task_get_unused_fd_flags(struct binder_proc *proc, int flags)
static void task_fd_install(
struct binder_proc *proc, unsigned int fd, struct file *file)
{
+ mutex_lock(&proc->files_lock);
if (proc->files)
__fd_install(proc->files, fd, file);
+ mutex_unlock(&proc->files_lock);
}

/*
@@ -910,9 +920,11 @@ static long task_close_fd(struct binder_proc *proc, unsigned int fd)
{
int retval;

- if (proc->files == NULL)
- return -ESRCH;
-
+ mutex_lock(&proc->files_lock);
+ if (proc->files == NULL) {
+ retval = -ESRCH;
+ goto err;
+ }
retval = __close_fd(proc->files, fd);
/* can't restart close syscall because file table entry was cleared */
if (unlikely(retval == -ERESTARTSYS ||
@@ -920,7 +932,8 @@ static long task_close_fd(struct binder_proc *proc, unsigned int fd)
retval == -ERESTARTNOHAND ||
retval == -ERESTART_RESTARTBLOCK))
retval = -EINTR;
-
+err:
+ mutex_unlock(&proc->files_lock);
return retval;
}

@@ -4605,7 +4618,9 @@ static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
ret = binder_alloc_mmap_handler(&proc->alloc, vma);
if (ret)
return ret;
+ mutex_lock(&proc->files_lock);
proc->files = get_files_struct(current);
+ mutex_unlock(&proc->files_lock);
return 0;

err_bad_arg:
@@ -4629,6 +4644,7 @@ static int binder_open(struct inode *nodp, struct file *filp)
spin_lock_init(&proc->outer_lock);
get_task_struct(current->group_leader);
proc->tsk = current->group_leader;
+ mutex_init(&proc->files_lock);
INIT_LIST_HEAD(&proc->todo);
proc->default_priority = task_nice(current);
binder_dev = container_of(filp->private_data, struct binder_device,
@@ -4881,9 +4897,11 @@ static void binder_deferred_func(struct work_struct *work)

files = NULL;
if (defer & BINDER_DEFERRED_PUT_FILES) {
+ mutex_lock(&proc->files_lock);
files = proc->files;
if (files)
proc->files = NULL;
+ mutex_unlock(&proc->files_lock);
}

if (defer & BINDER_DEFERRED_FLUSH)
--
2.15.0.417.g466bffb3ac-goog
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