Re: [PATCH v2 03/10] Consolidate determination of UID/GID to run as

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Sep 09, 2022 at 04:33:47PM +0200, Stefano Brivio wrote:
> Also mere nitpicking on this one:
> 
> On Thu,  8 Sep 2022 13:59:00 +1000
> David Gibson <david(a)gibson.dropbear.id.au> wrote:
> 
> > Currently the logic to work out what UID and GID we will run as is spread
> > across conf().  If --runas is specified it's handled in conf_runas(),
> > otherwise it's handled by check_root(), which depends on initialization of
> > the uid and gid variables by either conf() itself or conf_runas().
> > 
> > Make this clearer by putting all the UID and GID logic into a single
> > conf_ugid() function.
> > 
> > Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au>
> > ---
> >  conf.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++++------
> >  util.c | 50 ------------------------------------
> >  util.h |  1 -
> >  3 files changed, 73 insertions(+), 59 deletions(-)
> > 
> > diff --git a/conf.c b/conf.c
> > index 545f61d..5c293b5 100644
> > --- a/conf.c
> > +++ b/conf.c
> > @@ -1021,6 +1021,70 @@ static int conf_runas(const char *opt, unsigned int *uid, unsigned int *gid)
> >  #endif /* !GLIBC_NO_STATIC_NSS */
> >  }
> >  
> > +/**
> > + * conf_ugid() - Determine UID and GID to run as
> > + * @runas:	--runas option, may be NULL
> > + * @uid:	User ID, set on success
> > + * @gid:	Group ID, set on success
> > + *
> > + * Return: 0 on success, negative error code on failure
> > + */
> > +static int conf_ugid(const char *runas, uid_t *uid, gid_t *gid)
> > +{
> > +	const char root_uid_map[] = "         0          0 4294967295";
> > +	struct passwd *pw;
> > +	char buf[BUFSIZ];
> > +	int ret;
> > +	int fd;
> > +
> > +	/* If user has specified --runas, that takes precedence */
> > +	if (runas) {
> > +		ret = conf_runas(runas, uid, gid);
> > +		if (ret)
> > +			err("Invalid --runas option: %s", runas);
> > +		return ret;
> > +	}
> > +
> > +	/* Otherwise default to current user and group.. */
> > +	*uid = geteuid();
> > +	*gid = getegid();
> > +
> > +	/* ..as long as it's not root.. */
> > +	if (*uid)
> > +		return 0;
> > +
> > +	/* ..or at least not root in the init namespace.. */
> > +	if ((fd = open("/proc/self/uid_map", O_RDONLY | O_CLOEXEC)) < 0)
> > +		return 0;
> > +
> > +	if (read(fd, buf, BUFSIZ) != sizeof(root_uid_map) ||
> > +	    strncmp(buf, root_uid_map, sizeof(root_uid_map) - 1)) {
> > +		close(fd);
> > +		return 0;
> > +	}
> > +
> > +	close(fd);
> > +
> > +	/* ..otherwise use nobody:nobody */
> 
> I'd change all those comment to use ellipses (...) instead of "..".

Ok, nit picked.

> I think your comments here help a lot, by the way (and I couldn't find
> a better way to check for UID 0 in non-init other than that hack).

Right.  The extra complexity - both of code and of mental model - in
going from "not UID 0" to "not UID 0 in the init namespace" is what
makes me really wonder if this check is worth having.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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