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

[Serge E. Hallyn]

Quoting Mimi Zohar (zohar at linux.vnet.ibm.com):
> 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>

Acked-by: Serge Hallyn <serge at hallyn.com>

Of course when IMA namespacing hits, you'll want to compare the
sb->s_user_ns to the (~handwaving~) user_ns owning the ima ns
right?

> ---
>  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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