[PATCH v2] nvmem: core: Set no-read-write provider to avoid userspace read/write

[Gaurav Kohli <gkohli@codeaurora.org>]

Current nvmem framework allows user space to read all register space
populated by nvmem binary file, In case we don't want to expose value
of registers to userspace and only want kernel space to read cell
value from nvmem_cell_read_u32.

To protect the same, Add no-read-write property to prevent read
from userspace.

Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>
---
v2: Fix build error
v1: Fix no_read_write update condition

diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index f24008b..f1c44fc 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -27,6 +27,7 @@ struct nvmem_device {
 	struct kref		refcnt;
 	size_t			size;
 	bool			read_only;
+	bool			no_read_write;
 	int			flags;
 	enum nvmem_type		type;
 	struct bin_attribute	eeprom;
@@ -120,6 +121,10 @@ static ssize_t bin_attr_nvmem_read(struct file *filp, struct kobject *kobj,
 		dev = container_of(kobj, struct device, kobj);
 	nvmem = to_nvmem_device(dev);
 
+	/* if no-read-write, then stop from reading */
+	if (nvmem->no_read_write)
+		return -EPERM;
+
 	/* Stop the user from reading */
 	if (pos >= nvmem->size)
 		return 0;
@@ -154,6 +159,10 @@ static ssize_t bin_attr_nvmem_write(struct file *filp, struct kobject *kobj,
 		dev = container_of(kobj, struct device, kobj);
 	nvmem = to_nvmem_device(dev);
 
+	/* if no-read-write, then stop from writing */
+	if (nvmem->no_read_write)
+		return -EPERM;
+
 	/* Stop the user from writing */
 	if (pos >= nvmem->size)
 		return -EFBIG;
@@ -651,6 +660,9 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
 	nvmem->read_only = device_property_present(config->dev, "read-only") ||
 			   config->read_only || !nvmem->reg_write;
 
+	nvmem->no_read_write = device_property_present(config->dev,
+				"no-read-write");
+
 	if (config->root_only)
 		nvmem->dev.groups = nvmem->read_only ?
 			nvmem_ro_root_dev_groups :
-- 
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