[PATCH] efi/capsule-loader: Add write mutex to prevent uaf

[Hyunwoo Kim <imv4bel@xxxxxxxxx>]

If the user calls close() in the middle of copy operation in
copy_from_user() of efi_capsule_write(),
the user buffer may be copied to the released page.

This is because ->flush is called unconditionally regardless
of f_count, unlike ->release.
This driver is not a security vulnerability, as only
root privileges can write to it.
However, it seems to be a good idea to add mutexes to 
efi_capsule_write() and efi_capsule_flush() as root can 
accidentally break the page while in use.

Signed-off-by: Hyunwoo Kim <imv4bel@xxxxxxxxx>
---
 drivers/firmware/efi/capsule-loader.c | 12 ++++++++++++
 include/linux/efi.h                   |  1 +
 2 files changed, 13 insertions(+)

diff --git a/drivers/firmware/efi/capsule-loader.c b/drivers/firmware/efi/capsule-loader.c
index 4dde8edd53b6..e50ede51ef38 100644
--- a/drivers/firmware/efi/capsule-loader.c
+++ b/drivers/firmware/efi/capsule-loader.c
@@ -177,6 +177,8 @@ static ssize_t efi_capsule_write(struct file *file, const char __user *buff,
 	if (count == 0)
 		return 0;
 
+	mutex_lock(&cap_info->write_lock);
+
 	/* Return error while NO_FURTHER_WRITE_ACTION is flagged */
 	if (cap_info->index < 0)
 		return -EIO;
@@ -233,12 +235,16 @@ static ssize_t efi_capsule_write(struct file *file, const char __user *buff,
 			goto failed;
 	}
 
+	mutex_unlock(&cap_info->write_lock);
+
 	return write_byte;
 
 fail_unmap:
 	kunmap(page);
 failed:
 	efi_free_all_buff_pages(cap_info);
+	mutex_unlock(&cap_info->write_lock);
+
 	return ret;
 }
 
@@ -256,12 +262,16 @@ static int efi_capsule_flush(struct file *file, fl_owner_t id)
 	int ret = 0;
 	struct capsule_info *cap_info = file->private_data;
 
+	mutex_lock(&cap_info->write_lock);
+
 	if (cap_info->index > 0) {
 		pr_err("capsule upload not complete\n");
 		efi_free_all_buff_pages(cap_info);
 		ret = -ECANCELED;
 	}
 
+	mutex_unlock(&cap_info->write_lock);
+
 	return ret;
 }
 
@@ -315,6 +325,8 @@ static int efi_capsule_open(struct inode *inode, struct file *file)
 		return -ENOMEM;
 	}
 
+	mutex_init(&cap_info->write_lock);
+
 	file->private_data = cap_info;
 
 	return 0;
diff --git a/include/linux/efi.h b/include/linux/efi.h
index 7d9b0bb47eb3..e274c4e8d7c6 100644
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -204,6 +204,7 @@ struct efi_image_auth {
 struct capsule_info {
 	efi_capsule_header_t	header;
 	efi_capsule_header_t	*capsule;
+	struct mutex		write_mutex;
 	int			reset_type;
 	long			index;
 	size_t			count;
-- 
2.25.1