[Devel] Re: [patch 08/10] unprivileged mounts: make fuse safe

[Serge E. Hallyn]

Quoting Miklos Szeredi (miklos at szeredi.hu):
> From: Miklos Szeredi <mszeredi at suse.cz>
> 
> Don't require the "user_id=" and "group_id=" options for unprivileged mounts,
> but if they are present, verify them for sanity.
> 
> Disallow the "allow_other" option for unprivileged mounts.
> 
> FUSE was designed from the beginning to be safe for unprivileged
> users.  This has also been verified in practice over many years, with
> some distributions enabling unprivileged FUSE mounts by default.
> 
> However there are some properties of FUSE, that could make it unsafe
> for certain situations (e.g. multiuser system with untrusted users):
> 
>  - It is not always possible to use kill(2) (not even with SIGKILL) to
>    terminate a process using a FUSE filesystem.  However it is
>    possible to use any of the following instead:
>      o kill the filesystem daemon
>      o use forced umounting
>      o use the "fusectl" control filesystem
> 
>  - As a special case of the above, killing a self-deadlocked FUSE
>    process is not possible, and even killall5 will not terminate it.
> 
>  - Due to the design of the process freezer, a hanging (due to network
>    problems, etc) or malicious filesystem may prevent suspending to
>    ram or hibernation to succeed.  This is not actually unique to
>    FUSE, as any hanging network filesystem will have the same affect.
> 
> If the above could pose a threat to the system, it is recommended,
> that the '/proc/sys/fs/types/fuse/safe' sysctl tunable is not turned
> on, and/or '/dev/fuse' is not made world-readable and writable.
> 
> Signed-off-by: Miklos Szeredi <mszeredi at suse.cz>

I was going to say "this should of course be acked by a fuse
maintainer", then I look at MAINTAINERS :)  So never mind.

Acked-by: Serge Hallyn <serue at us.ibm.com>

> ---
> 
> Index: linux/fs/fuse/inode.c
> ===================================================================
> --- linux.orig/fs/fuse/inode.c	2008-01-16 13:24:52.000000000 +0100
> +++ linux/fs/fuse/inode.c	2008-01-16 13:25:10.000000000 +0100
> @@ -357,6 +357,19 @@ static int parse_fuse_opt(char *opt, str
>  	d->max_read = ~0;
>  	d->blksize = 512;
> 
> +	/*
> +	 * For unprivileged mounts use current uid/gid.  Still allow
> +	 * "user_id" and "group_id" options for compatibility, but
> +	 * only if they match these values.
> +	 */
> +	if (!capable(CAP_SYS_ADMIN)) {
> +		d->user_id = current->uid;
> +		d->user_id_present = 1;
> +		d->group_id = current->gid;
> +		d->group_id_present = 1;
> +
> +	}
> +
>  	while ((p = strsep(&opt, ",")) != NULL) {
>  		int token;
>  		int value;
> @@ -385,6 +398,8 @@ static int parse_fuse_opt(char *opt, str
>  		case OPT_USER_ID:
>  			if (match_int(&args[0], &value))
>  				return 0;
> +			if (d->user_id_present && d->user_id != value)
> +				return 0;
>  			d->user_id = value;
>  			d->user_id_present = 1;
>  			break;
> @@ -392,6 +407,8 @@ static int parse_fuse_opt(char *opt, str
>  		case OPT_GROUP_ID:
>  			if (match_int(&args[0], &value))
>  				return 0;
> +			if (d->group_id_present && d->group_id != value)
> +				return 0;
>  			d->group_id = value;
>  			d->group_id_present = 1;
>  			break;
> @@ -596,6 +613,10 @@ static int fuse_fill_super(struct super_
>  	if (!parse_fuse_opt((char *) data, &d, is_bdev))
>  		return -EINVAL;
> 
> +	/* This is a privileged option */
> +	if ((d.flags & FUSE_ALLOW_OTHER) && !capable(CAP_SYS_ADMIN))
> +		return -EPERM;
> +
>  	if (is_bdev) {
>  #ifdef CONFIG_BLOCK
>  		if (!sb_set_blocksize(sb, d.blksize))
> 
> --
> -
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