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authorDavid Howells <dhowells@redhat.com>2008-11-13 18:39:24 -0500
committerJames Morris <jmorris@namei.org>2008-11-13 18:39:24 -0500
commita6f76f23d297f70e2a6b3ec607f7aeeea9e37e8d (patch)
tree8f95617996d0974507f176163459212a7def8b9a /security/keys/process_keys.c
parentd84f4f992cbd76e8f39c488cf0c5d123843923b1 (diff)
CRED: Make execve() take advantage of copy-on-write credentials
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>
Diffstat (limited to 'security/keys/process_keys.c')
-rw-r--r--security/keys/process_keys.c42
1 files changed, 0 insertions, 42 deletions
diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index df329f684a65..2f5d89e92b85 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -276,48 +276,6 @@ static int install_session_keyring(struct key *keyring)
276 276
277/*****************************************************************************/ 277/*****************************************************************************/
278/* 278/*
279 * deal with execve()
280 */
281int exec_keys(struct task_struct *tsk)
282{
283 struct thread_group_cred *tgcred = NULL;
284 struct cred *new;
285
286#ifdef CONFIG_KEYS
287 tgcred = kmalloc(sizeof(*tgcred), GFP_KERNEL);
288 if (!tgcred)
289 return -ENOMEM;
290#endif
291
292 new = prepare_creds();
293 if (new < 0)
294 return -ENOMEM;
295
296 /* newly exec'd tasks don't get a thread keyring */
297 key_put(new->thread_keyring);
298 new->thread_keyring = NULL;
299
300 /* create a new per-thread-group creds for all this set of threads to
301 * share */
302 memcpy(tgcred, new->tgcred, sizeof(struct thread_group_cred));
303
304 atomic_set(&tgcred->usage, 1);
305 spin_lock_init(&tgcred->lock);
306
307 /* inherit the session keyring; new process keyring */
308 key_get(tgcred->session_keyring);
309 tgcred->process_keyring = NULL;
310
311 release_tgcred(new);
312 new->tgcred = tgcred;
313
314 commit_creds(new);
315 return 0;
316
317} /* end exec_keys() */
318
319/*****************************************************************************/
320/*
321 * the filesystem user ID changed 279 * the filesystem user ID changed
322 */ 280 */
323void key_fsuid_changed(struct task_struct *tsk) 281void key_fsuid_changed(struct task_struct *tsk)