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[PATCH 02/19] block_dev: Check permissions towards block device inode when mounting

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Unprivileged users should not be able to mount block devices when
they lack sufficient privileges towards the block device inode.
Update blkdev_get_by_path() 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.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
---
 fs/block_dev.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 3ebbde85d898..4fdb6ab59816 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1424,9 +1424,14 @@ struct block_device *blkdev_get_by_path(const char *path, fmode_t mode,
 					void *holder)
 {
 	struct block_device *bdev;
+	int perm = 0;
 	int err;
 
-	bdev = lookup_bdev(path, 0);
+	if (mode & FMODE_READ)
+		perm |= MAY_READ;
+	if (mode & FMODE_WRITE)
+		perm |= MAY_WRITE;
+	bdev = lookup_bdev(path, perm);
 	if (IS_ERR(bdev))
 		return bdev;
 
-- 
1.9.1

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[PATCH 00/19] Support fuse mounts in user namespaces Seth Forshee <seth.forshee@canonical.com> - 2015-12-02 16:50 +0100
  [PATCH 01/19] block_dev: Support checking inode permissions in lookup_bdev() Seth Forshee <seth.forshee@canonical.com> - 2015-12-02 17:00 +0100
    Re: [PATCH 01/19] block_dev: Support checking inode permissions in  lookup_bdev() "Serge E. Hallyn" <serge.hallyn@ubuntu.com> - 2015-12-04 17:30 +0100
  [PATCH 03/19] fs: Treat foreign mounts as nosuid Seth Forshee <seth.forshee@canonical.com> - 2015-12-02 17:00 +0100
    Re: [PATCH 03/19] fs: Treat foreign mounts as nosuid "Serge E. Hallyn" <serge.hallyn@ubuntu.com> - 2015-12-04 18:00 +0100
  [PATCH 02/19] block_dev: Check permissions towards block device inode when mounting Seth Forshee <seth.forshee@canonical.com> - 2015-12-02 17:00 +0100
    Re: [PATCH 02/19] block_dev: Check permissions towards block device  inode when mounting "Serge E. Hallyn" <serge.hallyn@ubuntu.com> - 2015-12-04 17:30 +0100

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