kern/181127: [PATCH] set{domain, host}name doesn't permit NUL terminated strings that are MAXHOSTNAMELEN long

[Bruce Evans]

The following reply was made to PR kern/181127; it has been noted by GNATS.

From: Bruce Evans <brde at optusnet.com.au>
To: Garrett Cooper <yaneurabeya at gmail.com>
Cc: Bruce Evans <brde at optusnet.com.au>, freebsd-gnats-submit at freebsd.org, 
    freebsd-bugs at freebsd.org
Subject: Re: kern/181127: [PATCH] set{domain, host}name doesn't permit NUL
 terminated strings that are MAXHOSTNAMELEN long
Date: Fri, 9 Aug 2013 10:39:37 +1000 (EST)

 On Thu, 8 Aug 2013, Garrett Cooper wrote:
 
 > On Aug 8, 2013, at 4:35 AM, Bruce Evans wrote:
 >
 >> On Thu, 8 Aug 2013, Garrett Cooper wrote:
 >>
 >>>> Synopsis:       [PATCH] set{domain,host}name doesn't permit NUL terminated strings that are MAXHOSTNAMELEN long
 >>> ...
 >>>> Description:
 >>> The noted link/patch fixes POSIX and generic requirement compliance for set{domain,host}name per the manpages by accounting for the fact that the string
 >>> must be NUL terminated.
 >>
 >> The bugs seem to be mainly in the tests, so the proposed fix enlarges them.
 >> MAXHOSTNAMELEN is already 1 larger than the POSIX limit {HOST_NAME_MAX}
 >> (see the sysconf(3) sources).
 >
 > So the fix is bogus. Ok, missed that MAXHOSTNAMELEN was '\0' inclusive.
 >
 >>> Found with the NetBSD t_set{domain,host}name testcases:
 >>>
 >>> Before:
 >>>
 >>> $ pwd
 >>> /usr/tests/lib/libc/gen
 >>> $ sudo atf-run t_setdomainname | atf-report
 >>> t_setdomainname (1/1): 3 test cases
 >>>   setdomainname_basic: [0.019497s] Failed: /usr/src/lib/libc/tests/gen/t_setdomainname.c:66: setdomainname(domains[i],sizeof(domains[i])) == 0 not met
 >>>   setdomainname_limit: [0.004173s] Passed.
 >>>   setdomainname_perm: [0.005297s] Passed.
 >>> [0.029872s]
 >>
 >> I'm not sure what these do, but according to the Synopsis,
 >> set{domain,host}name correctly doesn't permit NUL terminated strings that
 >> are MAXHOSTNAMELEN long (not counting space for the NUL).  MAXHOSTNAMELEN
 >> counts space for the NUL and is 1 larger than {HOST_NAME_MAX}.
 >
 > Yes. It's kind of odd why NetBSD passes here, but this should work on FreeBSD as well as they aren't doing anything going out-of-bounds in the testcases (see https://github.com/yaneurabeya/freebsd/blob/master/lib/libc/tests/gen/t_setdomainname.c , https://github.com/yaneurabeya/freebsd/blob/master/lib/libc/tests/gen/t_sethostname.c if you're curious).
 
 It uses MAXHOSTNAMELEN + 1 in (only) 1 place, and then seems to check that
 setdomainname() on a null name with "length" MAXHOSTNAMELEN + 1 fails.
 It doesn't seem to test any strings of nearly length MAXHOSTNAMELEN.
 
 > ...
 >
 >>> @@ -314,11 +314,11 @@ sysctl_hostname(SYSCTL_HANDLER_ARGS)
 >>>
 >>> SYSCTL_PROC(_kern, KERN_HOSTNAME, hostname,
 >>>    CTLTYPE_STRING | CTLFLAG_RW | CTLFLAG_PRISON | CTLFLAG_MPSAFE,
 >>> -    (void *)(offsetof(struct prison, pr_hostname)), MAXHOSTNAMELEN,
 >>> +    (void *)(offsetof(struct prison, pr_hostname)), MAXHOSTNAMELEN+1,
 >>>    sysctl_hostname, "A", "Hostname");
 >>> SYSCTL_PROC(_kern, KERN_NISDOMAINNAME, domainname,
 >>>    CTLTYPE_STRING | CTLFLAG_RW | CTLFLAG_PRISON | CTLFLAG_MPSAFE,
 >>> -    (void *)(offsetof(struct prison, pr_domainname)), MAXHOSTNAMELEN,
 >>> +    (void *)(offsetof(struct prison, pr_domainname)), MAXHOSTNAMELEN+1,
 >>>    sysctl_hostname, "A", "Name of the current YP/NIS domain");
 >>> SYSCTL_PROC(_kern, KERN_HOSTUUID, hostuuid,
 >>>    CTLTYPE_STRING | CTLFLAG_RW | CTLFLAG_PRISON | CTLFLAG_MPSAFE,
 >>
 >> The sysctls were originally simple SYSCTL_STRING()s and I think they
 >> worked then.  Now they are quite complicated, to  support jails, etc.,
 >> but they still use sysctl_handle_string() so I think they handle
 >> (non)strings and (non)termination the same.  Note that
 >> sysctl_handle_string() doesn't actually return strings unless the
 >> buffer is large enough to hold the NUL terminator.  It just truncates.
 >> This is reflected in the gethostname(3) API.  The name length for
 >> gethostname() must be 1 larger than {HOST_NAME_MAX} to ensure
 >> getting a string.  OTOH, the name length for sethostname(3) should
 >> not include space for the NUL, so it must not be larger than
 >> {HOST_NAME_MAX}.  If it is larger than {HOST_NAME_MAX}, then the
 >> syscall will just fail.  If it is larger than the string length
 >> (to include the NUL and possibly more) but not larger than
 >> {HOST_NAME_MAX}, then the syscall will succeed and the string will
 >> just be terminated more than once.  (It would be safer to write NULs
 >> from the end of the string until the end of the buffer in all cases.)
 >
 > So translation is: is there's a bug in the sysctl handler after jail support was added and there's no reasonable way to fix it without reverting things back to their sane forms?
 
 No.  I suspected a bug in the jail support, but couldn't see any.  You will
 have to check with name and string lengths of nearly MAXHOSTNAMELEN + 1 on
 (or whatever the kernel buffer size is for plain SYSCTL_STRING()) to see if
 the jail support gives any differences.
 
 Bruce