[PATCH 02/11] mtd: Check permissions towards mtd block device inode when mounting

Dongsu Park dongsu at kinvolk.io
Fri Dec 22 14:32:26 UTC 2017

From: Seth Forshee <seth.forshee at canonical.com>

Unprivileged users should not be able to mount mtd block devices
when they lack sufficient privileges towards the block device
inode.  Update mount_mtd() to validate that the user has the
required access to the inode at the specified path. The check
will be skipped for CAP_SYS_ADMIN, so privileged mounts will
continue working as before.

Patch v3 is available: https://patchwork.kernel.org/patch/7640011/

Cc: linux-mtd at lists.infradead.org
Cc: linux-kernel at vger.kernel.org
Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
Signed-off-by: Dongsu Park <dongsu at kinvolk.io>
 drivers/mtd/mtdsuper.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/mtd/mtdsuper.c b/drivers/mtd/mtdsuper.c
index 4a4d40c0..3c8734f3 100644
--- a/drivers/mtd/mtdsuper.c
+++ b/drivers/mtd/mtdsuper.c
@@ -129,6 +129,7 @@ struct dentry *mount_mtd(struct file_system_type *fs_type, int flags,
 	struct block_device *bdev;
 	int ret, major;
+	int perm;
 	int mtdnr;
@@ -180,7 +181,10 @@ struct dentry *mount_mtd(struct file_system_type *fs_type, int flags,
 	/* try the old way - the hack where we allowed users to mount
 	 * /dev/mtdblock$(n) but didn't actually _use_ the blockdev
-	bdev = lookup_bdev(dev_name, 0);
+	perm = MAY_READ;
+	if (!(flags & MS_RDONLY))
+		perm |= MAY_WRITE;
+	bdev = lookup_bdev(dev_name, perm);
 	if (IS_ERR(bdev)) {
 		ret = PTR_ERR(bdev);
 		pr_debug("MTDSB: lookup_bdev() returned %d\n", ret);

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