[PATCH v3 4/4] fuse: define the filesystem as untrusted

[Mimi Zohar]

Files on FUSE can change at any point in time without IMA being able
to detect it.  The file data read for the file signature verification
could be totally different from what is subsequently read, making the
signature verification useless.

FUSE can be mounted by unprivileged users either today with fusermount
installed with setuid, or soon with the upcoming patches to allow FUSE
mounts in a non-init user namespace.

This patch sets the SB_I_IMA_UNVERIFIABLE_SIGNATURE flag and when
appropriate sets the SB_I_UNTRUSTED_MOUNTER flag.

Signed-off-by: Mimi Zohar <zohar at linux.vnet.ibm.com>
Cc: Miklos Szeredi <miklos at szeredi.hu>
Cc: Seth Forshee <seth.forshee at canonical.com>
Cc: Eric W. Biederman <ebiederm at xmission.com>
Cc: Dongsu Park <dongsu at kinvolk.io>
Cc: Alban Crequy <alban at kinvolk.io>
Cc: "Serge E. Hallyn" <serge at hallyn.com>
---
 fs/fuse/inode.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 624f18bbfd2b..ef309958e060 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1080,6 +1080,9 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
 	sb->s_maxbytes = MAX_LFS_FILESIZE;
 	sb->s_time_gran = 1;
 	sb->s_export_op = &fuse_export_operations;
+	sb->s_iflags |= SB_I_IMA_UNVERIFIABLE_SIGNATURE;
+	if (sb->s_user_ns != &init_user_ns)
+		sb->s_iflags |= SB_I_UNTRUSTED_MOUNTER;
 
 	file = fget(d.fd);
 	err = -EINVAL;
-- 
2.7.5

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