procfs provides access to other processes memory spaces. This is
intended to be used in debugging and has many safeguards built into it
to prevent abuse.
II. Problem Description
In January 1997 a fatal flaw in *BSD procfs code (leading to a local
root compromise) was discussed on various security forums. The exploit
code dealt with /proc/pid/mem interface. Since then *BSD kernels
contained a simple fix which was meant to close this hole.
Unfortunately, throughout these three years it was still possible to
abuse /proc/pid/mem in a similar, though more complicated fashion,
which could lead to local root compromise.
III. Impact
Local users can gain root access.
IV. Workaround
You can unmount /proc. In both 3.x-stable and 4.0-current this will
break truss and gcore. In 3.x-stable systems only it will reduce the
amount of information ps reports.
/*
+ * For setuid/setgid programs we don't want to people to use that setuidness
+ * to generate error messages which write to a file which otherwise would
+ * otherwise be off limits to the proces.
+ *
+ * This is a gross hack to plug the hole. A better solution would involve
+ * a special vop or other form of generalized access control mechanism. We
+ * go ahead and just reject all procfs file systems accesses as dangerous.
+ *
+ * Since setugidsafety calls this only for fd 0, 1 and 2, this check is
+ * sufficient. We also don't for setugidness since we know we are.
+ */
+static int
+is_unsafe(struct file *fp)
+{
+ if (fp->f_type == DTYPE_VNODE &&
+ ((struct vnode *)(fp->f_data))->v_tag == VT_PROCFS)
+ return (1);
+ return (0);
+}
+
+/*
+ * Make this setguid thing safe, if at all possible.
+ */
+void
+setugidsafety(p)
+ struct proc *p;
+{
+ struct filedesc *fdp = p->p_fd;
+ struct file **fpp;
+ char *fdfp;
+ register int i;
+
+ /* Certain daemons might not have file descriptors. */
+ if (fdp == NULL)
+ return;
+
+ fpp = fdp->fd_ofiles;
+ fdfp = fdp->fd_ofileflags;
+ for (i = 0; i <= fdp->fd_lastfile; i++, fpp++, fdfp++) {
+ if (i > 2)
+ break;
+ if (*fpp != NULL && is_unsafe(*fpp)) {
+ if (*fdfp & UF_MAPPED)
+ (void) munmapfd(p, i);
+ (void) closef(*fpp, p);
+ *fpp = NULL;
+ *fdfp = 0;
+ if (i < fdp->fd_freefile)
+ fdp->fd_freefile = i;
+ }
+ }
+ while (fdp->fd_lastfile > 0 && fdp->fd_ofiles[fdp->fd_lastfile] == NULL)
+ fdp->fd_lastfile--;
+}
+
+/*
* Close any files on exec?
*/
void
Index: kern/kern_exec.c
===================================================================
RCS file: /base/FreeBSD-CVS/src/sys/kern/kern_exec.c,v
retrieving revision 1.93.2.3
diff -u -r1.93.2.3 kern_exec.c
--- kern_exec.c 1999/08/29 16:25:58 1.93.2.3
+++ kern_exec.c 2000/01/20 21:39:29
@@ -281,6 +281,7 @@
if (attr.va_mode & VSGID)
p->p_ucred->cr_gid = attr.va_gid;
setsugid(p);
+ setugidsafety(p);
} else {
if (p->p_ucred->cr_uid == p->p_cred->p_ruid &&
p->p_ucred->cr_gid == p->p_cred->p_rgid)
VI. Credits
We are republishing a heavily edited FEAR security advisory (number 1)
entitled "*BSD procfs vulnerability". More information about FEAR can
be found at http://www.fear.pl. We would like to thank
nergal@idea.avet.com.pl for sending a preliminary version of the
advisory to us in time to correct the problem.
=============================================================================
FreeBSD, Inc.
Web Site: http://www.freebsd.org/ Confidential contacts: security-officer@freebsd.org
Security notifications: security-notifications@freebsd.org
Security public discussion: freebsd-security@freebsd.org
PGP Key: ftp://ftp.freebsd.org/pub/FreeBSD/CERT/public_key.asc
Notice: Any patches in this document may not apply cleanly due to
modifications caused by digital signature or mailer software.
Please reference the URL listed at the top of this document
for original copies of all patches if necessary.
=============================================================================
