[PATCH 08/16] staging: ashmem: Avoid deadlock between read and mmap calls

John Stultz
2014-02-03T18:16:46Z

From: Todd Poynor <toddpoynor@google.com>

Avoid holding ashmem_mutex across code that can page fault. Page faults
grab the mmap_sem for the process, which are also held by mmap calls
prior to calling ashmem_mmap, which locks ashmem_mutex. The reversed
order of locking between the two can deadlock.

The calls that can page fault are read() and the ASHMEM_SET_NAME and
ASHMEM_GET_NAME ioctls. Move the code that accesses userspace pages
outside the ashmem_mutex.

Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Colin Cross <ccross@android.com>
Cc: Android Kernel Team <kernel-team@android.com>
Signed-off-by: Todd Poynor <toddpoynor@google.com>
[jstultz: minor commit message tweaks]
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
drivers/staging/android/ashmem.c | 45 +++++++++++++++++++++++-----------------
1 file changed, 26 insertions(+), 19 deletions(-)

diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
index 23948f1..713a972 100644
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -295,21 +295,29 @@ static ssize_t ashmem_read(struct file *file, char __user *buf,

/* If size is not set, or set to 0, always return EOF. */
if (asma->size == 0)
- goto out;
+ goto out_unlock;

if (!asma->file) {
ret = -EBADF;
- goto out;
+ goto out_unlock;
}

- ret = asma->file->f_op->read(asma->file, buf, len, pos);
- if (ret < 0)
- goto out;
+ mutex_unlock(&ashmem_mutex);

- /** Update backing file pos, since f_ops->read() doesn't */
- asma->file->f_pos = *pos;
+ /*
+ * asma and asma->file are used outside the lock here. We assume
+ * once asma->file is set it will never be changed, and will not
+ * be destroyed until all references to the file are dropped and
+ * ashmem_release is called.
+ */
+ ret = asma->file->f_op->read(asma->file, buf, len, pos);
+ if (ret >= 0) {
+ /** Update backing file pos, since f_ops->read() doesn't */
+ asma->file->f_pos = *pos;
+ }
+ return ret;

-out:
+out_unlock:
mutex_unlock(&ashmem_mutex);
return ret;
}
@@ -498,6 +506,7 @@ out:

static int set_name(struct ashmem_area *asma, void __user *name)
{
+ int len;
int ret = 0;
char local_name[ASHMEM_NAME_LEN];

@@ -510,21 +519,19 @@ static int set_name(struct ashmem_area *asma, void __user *name)
* variable that does not need protection and later copy the local
* variable to the structure member with lock held.
*/
- if (copy_from_user(local_name, name, ASHMEM_NAME_LEN))
- return -EFAULT;
-
+ len = strncpy_from_user(local_name, name, ASHMEM_NAME_LEN);
+ if (len < 0)
+ return len;
+ if (len == ASHMEM_NAME_LEN)
+ local_name[ASHMEM_NAME_LEN - 1] = '\0';
mutex_lock(&ashmem_mutex);
/* cannot change an existing mapping's name */
- if (unlikely(asma->file)) {
+ if (unlikely(asma->file))
ret = -EINVAL;
- goto out;
- }
- memcpy(asma->name + ASHMEM_NAME_PREFIX_LEN,
- local_name, ASHMEM_NAME_LEN);
- asma->name[ASHMEM_FULL_NAME_LEN-1] = '\0';
-out:
- mutex_unlock(&ashmem_mutex);
+ else
+ strcpy(asma->name + ASHMEM_NAME_PREFIX_LEN, local_name);

+ mutex_unlock(&ashmem_mutex);
return ret;
}

--
1.8.3.2