Add nid_namelen bounds check to nfssvc system call

[emaste]

Summary

This is only allowed by root and only used by the nfs daemon, which should not provide an incorrect value. However, it's still good practice to avoid integer overflow on data provided by userland.

PR:         206626
Reported by:       CTurt <[e-mail]>
MFC After:  1 month

[emaste]

sys/fs/nfs/nfs_commonsubs.c

3177 ↗

(On Diff #15889)

I think MAXHOSTNAMELEN is a reasonable upper bound, rather than bare 128 in the submitted patch.

[rmacklem]

nid_namelen is set to the length of one of three strings:

• the fully qualified domain name
• a group name returned by getgrent()
• a user name returned by getpwent()

I'd say 0 isn't a valid value and I have no idea what the upper bound
should be, but I'd say at least 1024.
There is a definition of MAXNAME in nfsuserd.c which is set to 1024.

[rmacklem]

For this specific case, MAXHOSTNAMELEN (which I think is 1024)
would be correct, since I think that is the limit for fully qualified
domain name?

[emaste]

In D6201#132163, @rmacklem wrote:

For this specific case, MAXHOSTNAMELEN (which I think is 1024)
would be correct, since I think that is the limit for fully qualified
domain name?

In sys/param.h:

#define MAXHOSTNAMELEN      256

but I think this is still a valid bound for the length of a domain name, and user and group names have a smaller limit.

[rmacklem]

Given that the patch only checks for the domain name case, I think this is fine.
(I might suggest "if (nid->nid_namelen <= 0 ...", but since I didn't feel the
check was needed in the first place, I don't think making it "<=" is a big deal.