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Subject[PATCH] 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 always use get_files_struct() to obtain struct_files
so that the refcount on the files_struct is used to prevent
a premature free. proc->files is removed since we get it every
time.

Signed-off-by: Todd Kjos <tkjos@google.com>
---
drivers/android/binder.c | 63 +++++++++++++++++++++++-------------------------
1 file changed, 30 insertions(+), 33 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index fddf76ef5bd6..794e58c14b15 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -458,9 +458,8 @@ struct binder_ref {
};

enum binder_deferred_state {
- BINDER_DEFERRED_PUT_FILES = 0x01,
- BINDER_DEFERRED_FLUSH = 0x02,
- BINDER_DEFERRED_RELEASE = 0x04,
+ BINDER_DEFERRED_FLUSH = 0x01,
+ BINDER_DEFERRED_RELEASE = 0x02,
};

/**
@@ -481,8 +480,6 @@ enum binder_deferred_state {
* (invariant after initialized)
* @tsk task_struct for group_leader of process
* (invariant after initialized)
- * @files files_struct for process
- * (invariant after initialized)
* @deferred_work_node: element for binder_deferred_list
* (protected by binder_deferred_lock)
* @deferred_work: bitmap of deferred work to perform
@@ -529,7 +526,6 @@ struct binder_proc {
struct list_head waiting_threads;
int pid;
struct task_struct *tsk;
- struct files_struct *files;
struct hlist_node deferred_work_node;
int deferred_work;
bool is_dead;
@@ -875,22 +871,34 @@ static void binder_free_thread(struct binder_thread *thread);
static void binder_free_proc(struct binder_proc *proc);
static void binder_inc_node_tmpref_ilocked(struct binder_node *node);

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

+ files = binder_get_files_struct(proc);
if (files == NULL)
return -ESRCH;

- if (!lock_task_sighand(proc->tsk, &irqs))
- return -EMFILE;
+ 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(files, 0, rlim_cur, flags);
+err:
+ put_files_struct(files);
+ return ret;
}

/*
@@ -899,8 +907,12 @@ 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)
{
- if (proc->files)
- __fd_install(proc->files, fd, file);
+ struct files_struct *files = binder_get_files_struct(proc);
+
+ if (files) {
+ __fd_install(files, fd, file);
+ put_files_struct(files);
+ }
}

/*
@@ -908,18 +920,20 @@ static void task_fd_install(
*/
static long task_close_fd(struct binder_proc *proc, unsigned int fd)
{
+ struct files_struct *files = binder_get_files_struct(proc);
int retval;

- if (proc->files == NULL)
+ if (files == NULL)
return -ESRCH;

- retval = __close_fd(proc->files, fd);
+ retval = __close_fd(files, fd);
/* can't restart close syscall because file table entry was cleared */
if (unlikely(retval == -ERESTARTSYS ||
retval == -ERESTARTNOINTR ||
retval == -ERESTARTNOHAND ||
retval == -ERESTART_RESTARTBLOCK))
retval = -EINTR;
+ put_files_struct(files);

return retval;
}
@@ -4561,7 +4575,6 @@ static void binder_vma_close(struct vm_area_struct *vma)
(vma->vm_end - vma->vm_start) / SZ_1K, vma->vm_flags,
(unsigned long)pgprot_val(vma->vm_page_prot));
binder_alloc_vma_close(&proc->alloc);
- binder_defer_work(proc, BINDER_DEFERRED_PUT_FILES);
}

static int binder_vm_fault(struct vm_fault *vmf)
@@ -4603,10 +4616,8 @@ static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
vma->vm_private_data = proc;

ret = binder_alloc_mmap_handler(&proc->alloc, vma);
- if (ret)
- return ret;
- proc->files = get_files_struct(current);
- return 0;
+
+ return ret;

err_bad_arg:
pr_err("binder_mmap: %d %lx-%lx %s failed %d\n",
@@ -4778,8 +4789,6 @@ static void binder_deferred_release(struct binder_proc *proc)
struct rb_node *n;
int threads, nodes, incoming_refs, outgoing_refs, active_transactions;

- BUG_ON(proc->files);
-
mutex_lock(&binder_procs_lock);
hlist_del(&proc->proc_node);
mutex_unlock(&binder_procs_lock);
@@ -4861,8 +4870,6 @@ static void binder_deferred_release(struct binder_proc *proc)
static void binder_deferred_func(struct work_struct *work)
{
struct binder_proc *proc;
- struct files_struct *files;
-
int defer;

do {
@@ -4879,21 +4886,11 @@ static void binder_deferred_func(struct work_struct *work)
}
mutex_unlock(&binder_deferred_lock);

- files = NULL;
- if (defer & BINDER_DEFERRED_PUT_FILES) {
- files = proc->files;
- if (files)
- proc->files = NULL;
- }
-
if (defer & BINDER_DEFERRED_FLUSH)
binder_deferred_flush(proc);

if (defer & BINDER_DEFERRED_RELEASE)
binder_deferred_release(proc); /* frees proc */
-
- if (files)
- put_files_struct(files);
} while (proc);
}
static DECLARE_WORK(binder_deferred_work, binder_deferred_func);
--
2.15.0.448.gf294e3d99a-goog
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