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SubjectRe: [patch 08/10] unprivileged mounts: make fuse safe
Quoting Miklos Szeredi (miklos@szeredi.hu):
> From: Miklos Szeredi <mszeredi@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@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@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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