Re: [PATCH 1/3] util, conf: Add and use ns_is_init() helper

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 22 May 2023 15:41:17 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Mon, May 22, 2023 at 01:41:56AM +0200, Stefano Brivio wrote:
> > We'll need this in isolate_initial(). While at it, don't rely on
> > BUFSIZ: the earlier issue we had with musl reminded me it's not a
> > magic "everything will fit" value. Size the read buffer to what we
> > actually need from uid_map.
> > 
> > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>  
> 
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> 
> Although...
> 
> > ---
> >  conf.c | 16 +---------------
> >  util.c | 25 +++++++++++++++++++++++++
> >  util.h |  2 ++
> >  3 files changed, 28 insertions(+), 15 deletions(-)
> > 
> > diff --git a/conf.c b/conf.c
> > index 447b000..984c3ce 100644
> > --- a/conf.c
> > +++ b/conf.c
> > @@ -1096,10 +1096,6 @@ static int conf_runas(char *opt, unsigned int *uid, unsigned int *gid)
> >   */
> >  static void conf_ugid(char *runas, uid_t *uid, gid_t *gid)
> >  {
> > -	const char root_uid_map[] = "         0          0 4294967295";
> > -	char buf[BUFSIZ];
> > -	int fd;
> > -
> >  	/* If user has specified --runas, that takes precedence... */
> >  	if (runas) {
> >  		if (conf_runas(runas, uid, gid))
> > @@ -1116,18 +1112,8 @@ static void conf_ugid(char *runas, uid_t *uid, gid_t *gid)
> >  		return;
> >  
> >  	/* ...or at least not root in the init namespace... */
> > -	if ((fd = open("/proc/self/uid_map", O_RDONLY | O_CLOEXEC)) < 0) {
> > -		die("Can't determine if we're in init namespace: %s",
> > -		    strerror(errno));
> > -	}
> > -
> > -	if (read(fd, buf, BUFSIZ) != sizeof(root_uid_map) ||
> > -	    strncmp(buf, root_uid_map, sizeof(root_uid_map) - 1)) {
> > -		close(fd);
> > +	if (!ns_is_init())
> >  		return;
> > -	}
> > -
> > -	close(fd);
> >  
> >  	/* ...otherwise use nobody:nobody */
> >  	warn("Don't run as root. Changing to nobody...");
> > diff --git a/util.c b/util.c
> > index c3e3471..5ec8a6c 100644
> > --- a/util.c
> > +++ b/util.c
> > @@ -390,6 +390,31 @@ int ns_enter(const struct ctx *c)
> >  	return 0;
> >  }
> >  
> > +/**
> > + * ns_is_init() - Is the caller running in the "init" user namespace?
> > + *
> > + * Return: true if caller is in init, false otherwise, won't return on failure
> > + */
> > +bool ns_is_init(void)
> > +{
> > +	const char root_uid_map[] = "         0          0 4294967295";
> > +	char buf[sizeof(root_uid_map) + 1];
> > +	bool ret = true;
> > +	int fd;
> > +
> > +	if ((fd = open("/proc/self/uid_map", O_RDONLY | O_CLOEXEC)) < 0) {
> > +		die("Can't determine if we're in init namespace: %s",
> > +		    strerror(errno));
> > +	}
> > +
> > +	if (read(fd, buf, sizeof(root_uid_map)) != sizeof(root_uid_map) ||  
> 
> I don't think it can go bad in practice, but I think you want to pass
> a slightly larger buffer than root_uid_map[], otherwise this test will
> succeed if the uid_map contains the expected thing for init, followed
> by something else.

Ah, yes, I thought of doing that, then I also thought that with
4294967295 as range size in the first line, there can't be other lines.
Then I (clumsily) went back to the original idea, because... go figure,
let's say it gets extended with something one day...

But now it's inconsistent (and probably confusing): the buffer is long
enough, but I don't use it:

	const char root_uid_map[] = "         0          0 4294967295";
32 bytes string length, sizeof() is 33

	char buf[sizeof(root_uid_map) + 1];
34 bytes (could be 33)

	read(fd, buf, sizeof(root_uid_map))
read at most 33 bytes

	!= sizeof(root_uid_map) ||  
isn't 33 bytes (should be 32)

	strncmp(buf, root_uid_map, sizeof(root_uid_map) - 1))
or the first 32 bytes, up to "5", differ (we should compare one more)

and, by the way, uid_map has _lines_:

$ hexdump -C /proc/self/uid_map 
00000000  20 20 20 20 20 20 20 20  20 30 20 20 20 20 20 20  |         0      |
00000010  20 20 20 20 30 20 34 32  39 34 39 36 37 32 39 35  |    0 4294967295|
00000020  0a                                                |.|

so it kind of works, but not for the reason I intended. Respinning
now...

-- 
Stefano