Re: [PATCH v2 1/3] util, conf: Add and use ns_is_init() helper

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

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On Mon, May 22, 2023 at 10:52:03AM +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, and check for the final newline too,
> because uid_map is organised in lines.
> 
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  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..3c5d51f 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\n";
> +	char buf[sizeof(root_uid_map)];
> +	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) - 1 ||
> +	    strncmp(buf, root_uid_map, sizeof(root_uid_map)))

Personally I'd use memcmp() when the size is known like this, but this
strncmp() should do the same thing.

> +		ret = false;
> +
> +	close(fd);
> +	return ret;
> +}
> +
>  /**
>   * pid_file() - Write PID to file, if requested to do so, and close it
>   * @fd:		Open PID file descriptor, closed on exit, -1 to skip writing it
> diff --git a/util.h b/util.h
> index ba3e3da..26892aa 100644
> --- a/util.h
> +++ b/util.h
> @@ -8,6 +8,7 @@
>  
>  #include <stdlib.h>
>  #include <stdarg.h>
> +#include <stdbool.h>
>  
>  #include "log.h"
>  
> @@ -216,6 +217,7 @@ char *line_read(char *buf, size_t len, int fd);
>  void procfs_scan_listen(struct ctx *c, uint8_t proto, int ip_version, int ns,
>  			uint8_t *map, uint8_t *exclude);
>  int ns_enter(const struct ctx *c);
> +bool ns_is_init(void);
>  void write_pidfile(int fd, pid_t pid);
>  int __daemon(int pidfile_fd, int devnull_fd);
>  int fls(unsigned long x);

-- 
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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