New Security Features, Please Comment

From: Geoffrey McRae
Date: Tue Dec 02 2008 - 18:55:59 EST

This is my first patch to the linux kernel, so please forgive me if I
have broken any posting rules. Also, I am not on the mailing list, so
please CC me on any replies to this.

A side note before I start, the links to the linux howtos on the mailing
list info page are broken, someone might want to fix that.

I posted a message in linux-api yesterday regarding this, but had no
responses. At this point I am looking for comments regarding these new
syscalls and the likelyhood of having this patch applied.

As the engineer for a web hosting company I have seen the need for
grater control over a processes uid/gid. Currently with shared virtual
hosting solutions, if for security we wish to run a shared CGI language
(such as PHP) as the user that owns the website we are forced to fork a
new process per request, then call setuid/gid and then launch the script
language. This ofcource is resource intensive, but at present there is
no other solution.

My patch adds four new syscalls to the kernel that allows this issue to
be overcome, and I am sure that it could find application in many other
programs. The syscalls are:

* long enable_setpresuid(uid_t start, uid_t end);

This function enables the use of the setpresuid call for the calling
process, the calling process has to have CAP_SETUID permission to call
this function. The start and end parameters specify the uid range that
the setpresuid must be called with.

* long enable_setpresgid(gid_t start, gid_t end);

This function is the same as above, except it enables the setpresgid
call for the specified gid range.

* long setpresuid(pid_t pid, uid_t ruid, uid_t euid, uid_t suid);

This function changes the specified IMMEDIATE child pid's real, saved
and effective uid to the specified uid. Returns -EPERM if the
enable_setpresuid call has not been made first.

* long setpresgid(pid_t pid, gid_t rgid, gid_t egid, gid_t sgid);

This function does the same as above, except it applies to the group of
the IMMEDIATE child.

The reason for the enable functions is so that the calling app can drop
root privlages after enabling the setpresuid/setpresgid for a certian
range of uid/gid numbers. This allows the parent process to change its
child processes (forked) uid/gid at any time without needing root access
while being secure so that it can not set its child processes to users
such as root.

An example of its usage follows...

int main()
pid_t child;

/* enable the setpres* functions for ids 1000-9999 */
enable_setpresuid(1000, 9999);
enable_setpresgid(1000, 9999);

/* drop to an un-privlaged user */
setresuid(65534, 65534, 65534);

/* fork a useless child for example sake */
child = fork();
if (child == 0) {
return 0;

/* change the childs uid/gid to 1000 */
setpresuid(child, 1000, 1000, 1000);
setpresgid(child, 1000, 1000, 1000);

/* wait for the child to terminate */
return 0;

For the patch and a bit more information please goto:

Even though not stated on the website, this code is released under
whatever licence is needed to get it into the kernel.

-Geoffrey McRae

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