Re: [Qemu-devel] [PATCH v2 2/4] Add access control support to qemu bridge helper

[Blue Swirl]

On Fri, Oct 21, 2011 at 15:07, Corey Bryant <address@hidden> wrote:
> We go to great lengths to restrict ourselves to just cap_net_admin as an OS
> enforced security mechanism.  However, we further restrict what we allow users
> to do to simply adding a tap device to a bridge interface by virtue of the 
> fact
> that this is the only functionality we expose.
>
> This is not good enough though.  An administrator is likely to want to 
> restrict
> the bridges that an unprivileged user can access, in particular, to restrict
> an unprivileged user from putting a guest on what should be isolated networks.
>
> This patch implements an ACL mechanism that is enforced by qemu-bridge-helper.
> The ACLs are fairly simple whitelist/blacklist mechanisms with a wildcard of
> 'all'.  All users are blacklisted by default, and deny takes precedence over
> allow.
>
> An interesting feature of this ACL mechanism is that you can include external
> ACL files.  The main reason to support this is so that you can set different
> file system permissions on those external ACL files.  This allows an
> administrator to implement rather sophisicated ACL policies based on 
> user/group

sophisticated

> policies via the file system.
>
> As an example:
>
> /etc/qemu/bridge.conf root:qemu 0640
>
>  allow br0
>  include /etc/qemu/alice.conf
>  include /etc/qemu/bob.conf
>  include /etc/qemu/charlie.conf
>
> /etc/qemu/alice.conf root:alice 0640
>  allow br1
>
> /etc/qemu/bob.conf root:bob 0640
>  allow br2
>
> /etc/qemu/charlie.conf root:charlie 0640
>  deny all

I think syntax 'include /etc/qemu/user.d/*.conf' or 'includedir
/etc/qemu/user.d' could be also useful.

> This ACL pattern allows any user in the qemu group to get a tap device
> connected to br0 (which is bridged to the physical network).
>
> Users in the alice group can additionally get a tap device connected to br1.
> This allows br1 to act as a private bridge for the alice group.
>
> Users in the bob group can additionally get a tap device connected to br2.
> This allows br2 to act as a private bridge for the bob group.
>
> Users in the charlie group cannot get a tap device connected to any bridge.
>
> Under no circumstance can the bob group get access to br1 or can the alice
> group get access to br2.  And under no cicumstance can the charlie group
> get access to any bridge.
>
> Signed-off-by: Anthony Liguori <address@hidden>
> Signed-off-by: Richa Marwaha <address@hidden>
> Signed-off-by: Corey Bryant <address@hidden>
> ---
>  qemu-bridge-helper.c |  141 
> ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 141 insertions(+), 0 deletions(-)
>
> diff --git a/qemu-bridge-helper.c b/qemu-bridge-helper.c
> index 2ce82fb..db257d5 100644
> --- a/qemu-bridge-helper.c
> +++ b/qemu-bridge-helper.c
> @@ -33,6 +33,105 @@
>
>  #include "net/tap-linux.h"
>
> +#define MAX_ACLS (128)

If all users (or groups) in the system have an ACL, this number could
be way too low. Please use a list instead.

> +#define DEFAULT_ACL_FILE CONFIG_QEMU_CONFDIR "/bridge.conf"
> +
> +enum {
> +    ACL_ALLOW = 0,
> +    ACL_ALLOW_ALL,
> +    ACL_DENY,
> +    ACL_DENY_ALL,
> +};
> +
> +typedef struct ACLRule {
> +    int type;
> +    char iface[IFNAMSIZ];
> +} ACLRule;
> +
> +static int parse_acl_file(const char *filename, ACLRule *acls, int 
> *pacl_count)
> +{
> +    int acl_count = *pacl_count;
> +    FILE *f;
> +    char line[4096];
> +
> +    f = fopen(filename, "r");
> +    if (f == NULL) {
> +        return -1;
> +    }
> +
> +    while (acl_count != MAX_ACLS &&
> +            fgets(line, sizeof(line), f) != NULL) {
> +        char *ptr = line;
> +        char *cmd, *arg, *argend;
> +
> +        while (isspace(*ptr)) {
> +            ptr++;
> +        }
> +
> +        /* skip comments and empty lines */
> +        if (*ptr == '#' || *ptr == 0) {
> +            continue;
> +        }
> +
> +        cmd = ptr;
> +        arg = strchr(cmd, ' ');
> +        if (arg == NULL) {
> +            arg = strchr(cmd, '\t');
> +        }
> +
> +        if (arg == NULL) {
> +            fprintf(stderr, "Invalid config line:\n  %s\n", line);
> +            fclose(f);
> +            errno = EINVAL;
> +            return -1;
> +        }
> +
> +        *arg = 0;
> +        arg++;
> +        while (isspace(*arg)) {
> +            arg++;
> +        }
> +
> +        argend = arg + strlen(arg);
> +        while (arg != argend && isspace(*(argend - 1))) {
> +            argend--;
> +        }

These while loops to skip spaces are repeated, but the comment
skipping part is not, so it is not possible to have comments after
rules or split rules to several lines. I'd add a simple state variable
to track at which stage we are in parsing instead.

> +        *argend = 0;
> +
> +        if (strcmp(cmd, "deny") == 0) {
> +            if (strcmp(arg, "all") == 0) {
> +                acls[acl_count].type = ACL_DENY_ALL;
> +            } else {
> +                acls[acl_count].type = ACL_DENY;
> +                snprintf(acls[acl_count].iface, IFNAMSIZ, "%s", arg);
> +            }
> +            acl_count++;
> +        } else if (strcmp(cmd, "allow") == 0) {
> +            if (strcmp(arg, "all") == 0) {
> +                acls[acl_count].type = ACL_ALLOW_ALL;
> +            } else {
> +                acls[acl_count].type = ACL_ALLOW;
> +                snprintf(acls[acl_count].iface, IFNAMSIZ, "%s", arg);
> +            }
> +            acl_count++;
> +        } else if (strcmp(cmd, "include") == 0) {
> +            /* ignore errors */
> +            parse_acl_file(arg, acls, &acl_count);
> +        } else {
> +            fprintf(stderr, "Unknown command `%s'\n", cmd);
> +            fclose(f);
> +            errno = EINVAL;
> +            return -1;
> +        }
> +    }
> +
> +    *pacl_count = acl_count;
> +
> +    fclose(f);
> +
> +    return 0;
> +}
> +
>  static int has_vnet_hdr(int fd)
>  {
>     unsigned int features = 0;
> @@ -95,6 +194,9 @@ int main(int argc, char **argv)
>     const char *bridge;
>     char iface[IFNAMSIZ];
>     int index;
> +    ACLRule acls[MAX_ACLS];
> +    int acl_count = 0;
> +    int i, access_allowed, access_denied;
>
>     /* parse arguments */
>     if (argc < 3 || argc > 4) {
> @@ -115,6 +217,45 @@ int main(int argc, char **argv)
>     bridge = argv[index++];
>     unixfd = atoi(argv[index++]);
>
> +    /* parse default acl file */
> +    if (parse_acl_file(DEFAULT_ACL_FILE, acls, &acl_count) == -1) {
> +        fprintf(stderr, "failed to parse default acl file `%s'\n",
> +                DEFAULT_ACL_FILE);
> +        return -errno;
> +    }
> +
> +    /* validate bridge against acl -- default policy is to deny
> +     * according acl policy if we have a deny and allow both
> +     * then deny should always win over allow
> +     */
> +    access_allowed = 0;
> +    access_denied = 0;
> +    for (i = 0; i < acl_count; i++) {
> +        switch (acls[i].type) {
> +        case ACL_ALLOW_ALL:
> +            access_allowed = 1;
> +            break;
> +        case ACL_ALLOW:
> +            if (strcmp(bridge, acls[i].iface) == 0) {
> +                access_allowed = 1;
> +            }
> +            break;
> +        case ACL_DENY_ALL:
> +            access_denied = 1;
> +            break;
> +        case ACL_DENY:
> +            if (strcmp(bridge, acls[i].iface) == 0) {
> +                access_denied = 1;
> +            }
> +            break;
> +        }
> +    }
> +
> +    if ((access_allowed == 0) || (access_denied == 1)) {
> +        fprintf(stderr, "access denied by acl file\n");
> +        return -EPERM;
> +    }
> +
>     /* open a socket to use to control the network interfaces */
>     ctlfd = socket(AF_INET, SOCK_STREAM, 0);
>     if (ctlfd == -1) {
> --
> 1.7.3.4
>
>
>