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.fors...@canonical.com>
Acked-by: Serge Hallyn <serge.hal...@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;

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