Re: [PATCH v2 0/3] fuse: Add support for mounts from pid/user namespaces

Serge Hallyn
2014-10-06T16:00:31Z

Quoting Seth Forshee (seth.forshee@canonical.com):
...
> After digging into this some more I think I agree with you. At minimum
> letting users insert arbitrary xattrs via fuse bypasses the usual
> restrictions on setting xattrs. This is probably mitigated by the
> limited visibility of the fuse mount in the usual case for unprivileged
> users, but it does seem like a bad idea fundamentally.
>
> So I was thinking of something like the following (untested) to let root
> in the host support privileged xattrs while limiting unprivileged users
> to user.*. Miklos, does this look acceptable or would you prefer
> something different?

So it won't be possible to set capabilities in a fuse fs? This may
be necessary, but it will prevent i.e. live-iso builders from writing
for instance a CAP_NET_RAW=pe (instead of setuid-root) /bin/ping in the
iso.

> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> index e3123bfbc711..1a3ee5663dea 100644
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -1882,6 +1882,10 @@ static int fuse_setxattr(struct dentry *entry, const char *name,
> if (fc->no_setxattr)
> return -EOPNOTSUPP;
>
> + if (!(fc->flags & FUSE_PRIV_XATTRS) &&
> + strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN) != 0)
> + return -EOPNOTSUPP;
> +
> req = fuse_get_req_nopages(fc);
> if (IS_ERR(req))
> return PTR_ERR(req);
> @@ -1925,6 +1929,10 @@ static ssize_t fuse_getxattr(struct dentry *entry, const char *name,
> if (fc->no_getxattr)
> return -EOPNOTSUPP;
>
> + if (!(fc->flags & FUSE_PRIV_XATTRS) &&
> + strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN) != 0)
> + return -EOPNOTSUPP;
> +
> req = fuse_get_req_nopages(fc);
> if (IS_ERR(req))
> return PTR_ERR(req);
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 81187ba04e4a..bc0fd14b962a 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -46,6 +46,11 @@
> doing the mount will be allowed to access the filesystem */
> #define FUSE_ALLOW_OTHER (1 << 1)
>
> +/** If the FUSE_PRIV_XATTRS flag is given, then xattrs outside the
> + user.* namespace are allowed. This option is only allowed for
> + system root. */
> +#define FUSE_PRIV_XATTRS (1 << 2)
> +
> /** Number of page pointers embedded in fuse_req */
> #define FUSE_REQ_INLINE_PAGES 1
>
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index b88b5a780228..6716b56d43a1 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -493,6 +493,7 @@ enum {
> OPT_ALLOW_OTHER,
> OPT_MAX_READ,
> OPT_BLKSIZE,
> + OPT_PRIV_XATTRS,
> OPT_ERR
> };
>
> @@ -505,6 +506,7 @@ static const match_table_t tokens = {
> {OPT_ALLOW_OTHER, "allow_other"},
> {OPT_MAX_READ, "max_read=%u"},
> {OPT_BLKSIZE, "blksize=%u"},
> + {OPT_PRIV_XATTRS, "priv_xattr"},
> {OPT_ERR, NULL}
> };
>
> @@ -592,6 +594,12 @@ static int parse_fuse_opt(char *opt, struct fuse_mount_data *d, int is_bdev)
> d->blksize = value;
> break;
>
> + case OPT_PRIV_XATTRS:
> + if (!capable(CAP_SYS_ADMIN))
> + return 0;
> + d->flags |= FUSE_PRIV_XATTRS;
> + break;
> +
> default:
> return 0;
> }
>