a6f76f23d2
Make execve() take advantage of copy-on-write credentials, allowing it to set
up the credentials in advance, and then commit the whole lot after the point
of no return.
This patch and the preceding patches have been tested with the LTP SELinux
testsuite.
This patch makes several logical sets of alteration:
(1) execve().
The credential bits from struct linux_binprm are, for the most part,
replaced with a single credentials pointer (bprm->cred). This means that
all the creds can be calculated in advance and then applied at the point
of no return with no possibility of failure.
I would like to replace bprm->cap_effective with:
cap_isclear(bprm->cap_effective)
but this seems impossible due to special behaviour for processes of pid 1
(they always retain their parent's capability masks where normally they'd
be changed - see cap_bprm_set_creds()).
The following sequence of events now happens:
(a) At the start of do_execve, the current task's cred_exec_mutex is
locked to prevent PTRACE_ATTACH from obsoleting the calculation of
creds that we make.
(a) prepare_exec_creds() is then called to make a copy of the current
task's credentials and prepare it. This copy is then assigned to
bprm->cred.
This renders security_bprm_alloc() and security_bprm_free()
unnecessary, and so they've been removed.
(b) The determination of unsafe execution is now performed immediately
after (a) rather than later on in the code. The result is stored in
bprm->unsafe for future reference.
(c) prepare_binprm() is called, possibly multiple times.
(i) This applies the result of set[ug]id binaries to the new creds
attached to bprm->cred. Personality bit clearance is recorded,
but now deferred on the basis that the exec procedure may yet
fail.
(ii) This then calls the new security_bprm_set_creds(). This should
calculate the new LSM and capability credentials into *bprm->cred.
This folds together security_bprm_set() and parts of
security_bprm_apply_creds() (these two have been removed).
Anything that might fail must be done at this point.
(iii) bprm->cred_prepared is set to 1.
bprm->cred_prepared is 0 on the first pass of the security
calculations, and 1 on all subsequent passes. This allows SELinux
in (ii) to base its calculations only on the initial script and
not on the interpreter.
(d) flush_old_exec() is called to commit the task to execution. This
performs the following steps with regard to credentials:
(i) Clear pdeath_signal and set dumpable on certain circumstances that
may not be covered by commit_creds().
(ii) Clear any bits in current->personality that were deferred from
(c.i).
(e) install_exec_creds() [compute_creds() as was] is called to install the
new credentials. This performs the following steps with regard to
credentials:
(i) Calls security_bprm_committing_creds() to apply any security
requirements, such as flushing unauthorised files in SELinux, that
must be done before the credentials are changed.
This is made up of bits of security_bprm_apply_creds() and
security_bprm_post_apply_creds(), both of which have been removed.
This function is not allowed to fail; anything that might fail
must have been done in (c.ii).
(ii) Calls commit_creds() to apply the new credentials in a single
assignment (more or less). Possibly pdeath_signal and dumpable
should be part of struct creds.
(iii) Unlocks the task's cred_replace_mutex, thus allowing
PTRACE_ATTACH to take place.
(iv) Clears The bprm->cred pointer as the credentials it was holding
are now immutable.
(v) Calls security_bprm_committed_creds() to apply any security
alterations that must be done after the creds have been changed.
SELinux uses this to flush signals and signal handlers.
(f) If an error occurs before (d.i), bprm_free() will call abort_creds()
to destroy the proposed new credentials and will then unlock
cred_replace_mutex. No changes to the credentials will have been
made.
(2) LSM interface.
A number of functions have been changed, added or removed:
(*) security_bprm_alloc(), ->bprm_alloc_security()
(*) security_bprm_free(), ->bprm_free_security()
Removed in favour of preparing new credentials and modifying those.
(*) security_bprm_apply_creds(), ->bprm_apply_creds()
(*) security_bprm_post_apply_creds(), ->bprm_post_apply_creds()
Removed; split between security_bprm_set_creds(),
security_bprm_committing_creds() and security_bprm_committed_creds().
(*) security_bprm_set(), ->bprm_set_security()
Removed; folded into security_bprm_set_creds().
(*) security_bprm_set_creds(), ->bprm_set_creds()
New. The new credentials in bprm->creds should be checked and set up
as appropriate. bprm->cred_prepared is 0 on the first call, 1 on the
second and subsequent calls.
(*) security_bprm_committing_creds(), ->bprm_committing_creds()
(*) security_bprm_committed_creds(), ->bprm_committed_creds()
New. Apply the security effects of the new credentials. This
includes closing unauthorised files in SELinux. This function may not
fail. When the former is called, the creds haven't yet been applied
to the process; when the latter is called, they have.
The former may access bprm->cred, the latter may not.
(3) SELinux.
SELinux has a number of changes, in addition to those to support the LSM
interface changes mentioned above:
(a) The bprm_security_struct struct has been removed in favour of using
the credentials-under-construction approach.
(c) flush_unauthorized_files() now takes a cred pointer and passes it on
to inode_has_perm(), file_has_perm() and dentry_open().
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: James Morris <jmorris@namei.org>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>
103 lines
2.7 KiB
C
103 lines
2.7 KiB
C
/*
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* Root Plug sample LSM module
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*
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* Originally written for a Linux Journal.
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*
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* Copyright (C) 2002 Greg Kroah-Hartman <greg@kroah.com>
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*
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* Prevents any programs running with egid == 0 if a specific USB device
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* is not present in the system. Yes, it can be gotten around, but is a
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* nice starting point for people to play with, and learn the LSM
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* interface.
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*
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* If you want to turn this into something with a semblance of security,
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* you need to hook the task_* functions also.
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*
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* See http://www.linuxjournal.com/article.php?sid=6279 for more information
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* about this code.
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License as
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* published by the Free Software Foundation, version 2 of the
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* License.
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*/
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#include <linux/kernel.h>
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#include <linux/init.h>
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#include <linux/security.h>
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#include <linux/usb.h>
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#include <linux/moduleparam.h>
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/* default is a generic type of usb to serial converter */
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static int vendor_id = 0x0557;
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static int product_id = 0x2008;
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module_param(vendor_id, uint, 0400);
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module_param(product_id, uint, 0400);
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/* should we print out debug messages */
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static int debug = 0;
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module_param(debug, bool, 0600);
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#define MY_NAME "root_plug"
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#define root_dbg(fmt, arg...) \
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do { \
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if (debug) \
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printk(KERN_DEBUG "%s: %s: " fmt , \
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MY_NAME , __func__ , \
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## arg); \
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} while (0)
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static int rootplug_bprm_check_security (struct linux_binprm *bprm)
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{
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struct usb_device *dev;
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root_dbg("file %s, e_uid = %d, e_gid = %d\n",
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bprm->filename, bprm->cred->euid, bprm->cred->egid);
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if (bprm->cred->egid == 0) {
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dev = usb_find_device(vendor_id, product_id);
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if (!dev) {
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root_dbg("e_gid = 0, and device not found, "
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"task not allowed to run...\n");
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return -EPERM;
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}
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usb_put_dev(dev);
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}
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return 0;
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}
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static struct security_operations rootplug_security_ops = {
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/* Use the capability functions for some of the hooks */
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.ptrace_may_access = cap_ptrace_may_access,
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.ptrace_traceme = cap_ptrace_traceme,
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.capget = cap_capget,
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.capset = cap_capset,
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.capable = cap_capable,
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.bprm_set_creds = cap_bprm_set_creds,
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.task_fix_setuid = cap_task_fix_setuid,
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.task_prctl = cap_task_prctl,
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.bprm_check_security = rootplug_bprm_check_security,
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};
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static int __init rootplug_init (void)
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{
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/* register ourselves with the security framework */
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if (register_security (&rootplug_security_ops)) {
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printk (KERN_INFO
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"Failure registering Root Plug module with the kernel\n");
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return -EINVAL;
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}
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printk (KERN_INFO "Root Plug module initialized, "
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"vendor_id = %4.4x, product id = %4.4x\n", vendor_id, product_id);
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return 0;
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}
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security_initcall (rootplug_init);
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