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

[Serge Hallyn <serge.hallyn@ubuntu.com>]

Quoting Seth Forshee (seth.forshee@canonical.com):
> On Mon, Oct 06, 2014 at 04:00:06PM +0000, Serge Hallyn wrote:
> > 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.
> 
> cap_inode_setxattr() already requires CAP_SETFCAP in the host to do
> this, which I'd think root in in an unpriv container wouldn't have, so
> aren't you prevented from doing so already? I suppose the LSM could
> override this restriction though.

It's true we'd have to complicated that path.  And really I was being silly.
It's not safe (at least not trivially).

> > > 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;
> > >  		}
> > > 
> > --
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