Date: Fri, 03 Sep 1999 23:29:36 -0600
From: [email protected]
To: [email protected]Subject: FreeBSD-SA-99:01: BSD File Flags and Programming Techniques
Cc: [email protected]
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FreeBSD-SA-99:01 Security Advisory
FreeBSD, Inc.
Topic: BSD File Flags and Programming Techniques
Category: core
Module: kernel
Announced: 1999-09-04
Affects: FreeBSD 3.2 (and earlier)
FreeBSD-current before the correction date.
Corrected: FreeBSD-3.3 RELEASE
FreeBSD-current as of 1999/08/02
FreeBSD-3.2-stable as of 1999/08/02
FreeBSD-2.2.8-stable as of 1999/08/04
FreeBSD only: NO
Patches: ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-99:01/
I. Background
BSD 4.4 added various flags to files in the file system. These flags
control various aspects of which operations are permitted on those
files. Historically, root has been been able to do all of these
operations so many programs that knew they were running as root didn't
check to make sure that these operations succeeded.
II. Problem Description
A user can set flags and mode on the device which they logged into.
Since a bug in login and other similar programs causes the normal
chown to fail, this first user will own the terminal of any login.
III. Impact
Local users can execute a man-in-the-middle attack against any other
user (including root) when the other users logs in. This give them
the ability to snoop and alter all text that the user writes. Results
of this include the ability to execute commands as the user, and
stealing the user's password (and anything else the users writes over
the connection, including passwords for other machines).
IV. Workaround
None.
V. Solution
FreeBSD-current
Index: kern/vfs_syscalls.c
RCS file: /home/imp/FreeBSD/CVS/src/sys/kern/vfs_syscalls.c,v
retrieving revision 1.125
retrieving revision 1.128
diff -u -r1.125 -r1.128
--- vfs_syscalls.c 1999/07/29 17:02:56 1.125
+ vfs_syscalls.c 1999/08/04 04:52:18 1.128
@@ -1892,13 +1892,23 @@
int error;
struct vattr vattr;
+ /*
+ * Prevent non-root users from setting flags on devices. When
+ * a device is reused, users can retain ownership of the device
+ * if they are allowed to set flags and programs assume that
+ * chown can't fail when done as root.
+ */
+ if ((vp->v_type == VCHR || vp->v_type == VBLK) &&
+ ((error = suser_xxx(p->p_ucred, p, PRISON_ROOT)) != 0))
+ return (error);
+
VOP_LEASE(vp, p, p->p_ucred, LEASE_WRITE);
vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, p);
VATTR_NULL(&vattr);
vattr.va_flags = flags;
error = VOP_SETATTR(vp, &vattr, p->p_ucred, p);
VOP_UNLOCK(vp, 0, p);
- return error;
+ return (error);
}
/*
FreeBSD-3.2-stable
Index: kern/vfs_syscalls.c
RCS file: /home/imp/FreeBSD/CVS/src/sys/kern/vfs_syscalls.c,v
retrieving revision 1.112.2.3
retrieving revision 1.112.2.5
diff -u -r1.112.2.3 -r1.112.2.5
--- vfs_syscalls.c 1999/07/30 01:07:23 1.112.2.3
+ vfs_syscalls.c 1999/08/11 21:39:50 1.112.2.5
@@ -1839,13 +1839,23 @@
int error;
struct vattr vattr;
+ /*
+ * Prevent non-root users from setting flags on devices. When
+ * a device is reused, users can retain ownership of the device
+ * if they are allowed to set flags and programs assume that
+ * chown can't fail when done as root.
+ */
+ if ((vp->v_type == VCHR || vp->v_type == VBLK) &&
+ ((error = suser(p->p_ucred, &p->p_acflag)) != 0))
+ return (error);
+
VOP_LEASE(vp, p, p->p_ucred, LEASE_WRITE);
vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, p);
VATTR_NULL(&vattr);
vattr.va_flags = flags;
error = VOP_SETATTR(vp, &vattr, p->p_ucred, p);
VOP_UNLOCK(vp, 0, p);
- return error;
+ return (error);
}
/*
FreeBSD 2.2.8-stable:
Index: kern/vfs_syscalls.c
RCS file: /home/imp/FreeBSD/CVS/src/sys/kern/vfs_syscalls.c,v
retrieving revision 1.51.2.7
retrieving revision 1.51.2.8
diff -u -r1.51.2.7 -r1.51.2.8
--- vfs_syscalls.c 1998/07/03 03:50:31 1.51.2.7
+ vfs_syscalls.c 1999/08/04 18:58:56 1.51.2.8
@@ -1439,6 +1439,17 @@
if (error)
return (error);
vp = nd.ni_vp;
+ if ((error = VOP_GETATTR(vp, &vattr, p->p_ucred, p)))
+ return (error);
+ /*
+ * Prevent non-root users from setting flags on devices. When
+ * a device is reused, users can retain ownership of the device
+ * if they are allowed to set flags and programs assume that
+ * chown can't fail when done as root.
+ */
+ if ((vp->v_type == VCHR || vp->v_type == VBLK) &&
+ ((error = suser(p->p_ucred, &p->p_acflag)) != 0))
+ return (error);
LEASE_CHECK(vp, p, p->p_ucred, LEASE_WRITE);
VOP_LOCK(vp);
VATTR_NULL(&vattr);
VI. Credits
Theo de Raadt came up with the firewalling solution presented here.
[email protected] brought this problem to light.
FreeBSD, Inc.
Web Site: http://www.freebsd.org/
Confidential contacts: [email protected]
Security notifications: [email protected]
Security public discussion: [email protected]
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.
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