Anonymous avatar Anonymous committed 446c9cf

ashmem: avoid deadlock between read and mmap calls

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.

Bug: 9261835
Change-Id: If1322e981d29c889a56cdc9dfcbc6df2729a45e9
Signed-off-by: Todd Poynor <toddpoynor@google.com>;

Comments (0)

Files changed (1)

drivers/staging/android/ashmem.c

 
 	/* 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;
 }
 
 static int set_name(struct ashmem_area *asma, void __user *name)
 {
+	char lname[ASHMEM_NAME_LEN];
+	int len;
 	int ret = 0;
 
+	len = strncpy_from_user(lname, name, ASHMEM_NAME_LEN);
+	if (len < 0)
+		return len;
+	if (len == ASHMEM_NAME_LEN)
+		lname[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;
-	}
+	else
+		strcpy(asma->name + ASHMEM_NAME_PREFIX_LEN, lname);
 
-	if (unlikely(copy_from_user(asma->name + ASHMEM_NAME_PREFIX_LEN,
-				    name, ASHMEM_NAME_LEN)))
-		ret = -EFAULT;
-	asma->name[ASHMEM_FULL_NAME_LEN-1] = '\0';
-
-out:
 	mutex_unlock(&ashmem_mutex);
-
 	return ret;
 }
 
 static int get_name(struct ashmem_area *asma, void __user *name)
 {
 	int ret = 0;
+	char lname[ASHMEM_NAME_LEN];
+	size_t len;
 
 	mutex_lock(&ashmem_mutex);
 	if (asma->name[ASHMEM_NAME_PREFIX_LEN] != '\0') {
-		size_t len;
-
 		/*
 		 * Copying only `len', instead of ASHMEM_NAME_LEN, bytes
 		 * prevents us from revealing one user's stack to another.
 		 */
 		len = strlen(asma->name + ASHMEM_NAME_PREFIX_LEN) + 1;
-		if (unlikely(copy_to_user(name,
-				asma->name + ASHMEM_NAME_PREFIX_LEN, len)))
-			ret = -EFAULT;
+		memcpy(lname, asma->name + ASHMEM_NAME_PREFIX_LEN, len);
 	} else {
-		if (unlikely(copy_to_user(name, ASHMEM_NAME_DEF,
-					  sizeof(ASHMEM_NAME_DEF))))
-			ret = -EFAULT;
+		len = strlen(ASHMEM_NAME_DEF) + 1;
+		memcpy(lname, ASHMEM_NAME_DEF, len);
 	}
 	mutex_unlock(&ashmem_mutex);
-
+	if (unlikely(copy_to_user(name, lname, len)))
+		ret = -EFAULT;
 	return ret;
 }
 
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