From: Jann Horn <jann@thejh.net>
To: Alexander Viro <viro@zeniv.linux.org.uk>,
Roland McGrath <roland@hack.frob.com>,
Oleg Nesterov <oleg@redhat.com>,
John Johansen <john.johansen@canonical.com>,
James Morris <james.l.morris@oracle.com>,
"Serge E. Hallyn" <serge@hallyn.com>,
Paul Moore <aul@paul-moore.com>,
Stephen Smalley <sds@tycho.nsa.gov>,
Eric Paris <eparis@parisplace.org>,
Casey Schaufler <casey@schaufler-ca.com>,
Kees Cook <keescook@chromium.org>,
Andrew Morton <akpm@linux-foundation.org>,
Janis Danisevskis <jdanis@google.com>,
Seth Forshee <seth.forshee@canonical.com>,
"Eric W. Biederman" <ebiederm@xmission.com>,
Thomas Gleixner <tglx@linutronix.de>,
Benjamin LaHaise <bcrl@kvack.org>,
Ben Hutchings <ben@decadent.org.uk>,
Andy Lutomirski <luto@amacapital.net>,
Linus Torvalds <torvalds@linux-foundation.org>,
Krister Johansen <kjlx@templeofstupid.com>
Cc: linux-fsdevel@vger.kernel.org,
linux-security-module@vger.kernel.org, security@kernel.org
Subject: [PATCH v3 1/8] exec: introduce cred_guard_light
Date: Sun, 30 Oct 2016 22:46:31 +0100 [thread overview]
Message-ID: <1477863998-3298-2-git-send-email-jann@thejh.net> (raw)
In-Reply-To: <1477863998-3298-1-git-send-email-jann@thejh.net>
This is a new per-threadgroup lock that can often be taken instead of
cred_guard_mutex and has less deadlock potential. I'm doing this because
Oleg Nesterov mentioned the potential for deadlocks, in particular if a
debugged task is stuck in execve, trying to get rid of a ptrace-stopped
thread, and the debugger attempts to inspect procfs files of the debugged
task.
The binfmt handlers (in particular for elf_fdpic and flat) might still
call VFS read and mmap operations on the binary with the lock held, but
not open operations (as is the case with cred_guard_mutex).
An rwlock would be more appropriate here, but apparently those don't
have _killable variants of the locking functions?
This is a preparation patch for using proper locking in more places.
Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Jann Horn <jann@thejh.net>
---
fs/exec.c | 15 ++++++++++++++-
include/linux/init_task.h | 1 +
include/linux/sched.h | 10 ++++++++++
kernel/fork.c | 1 +
kernel/ptrace.c | 10 ++++++++++
5 files changed, 36 insertions(+), 1 deletion(-)
diff --git a/fs/exec.c b/fs/exec.c
index 4e497b9ee71e..67b76cb319d8 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1243,6 +1243,10 @@ int flush_old_exec(struct linux_binprm * bprm)
if (retval)
goto out;
+ retval = mutex_lock_killable(¤t->signal->cred_guard_light);
+ if (retval)
+ goto out;
+
/*
* Must be called _before_ exec_mmap() as bprm->mm is
* not visibile until then. This also enables the update
@@ -1256,7 +1260,7 @@ int flush_old_exec(struct linux_binprm * bprm)
acct_arg_size(bprm, 0);
retval = exec_mmap(bprm->mm);
if (retval)
- goto out;
+ goto out_unlock;
bprm->mm = NULL; /* We're using it now */
@@ -1268,6 +1272,8 @@ int flush_old_exec(struct linux_binprm * bprm)
return 0;
+out_unlock:
+ mutex_unlock(¤t->signal->cred_guard_light);
out:
return retval;
}
@@ -1391,6 +1397,7 @@ void install_exec_creds(struct linux_binprm *bprm)
* credentials; any time after this it may be unlocked.
*/
security_bprm_committed_creds(bprm);
+ mutex_unlock(¤t->signal->cred_guard_light);
mutex_unlock(¤t->signal->cred_guard_mutex);
}
EXPORT_SYMBOL(install_exec_creds);
@@ -1758,6 +1765,12 @@ static int do_execveat_common(int fd, struct filename *filename,
return retval;
out:
+ if (!bprm->mm && bprm->cred) {
+ /* failure after flush_old_exec(), but before
+ * install_exec_creds()
+ */
+ mutex_unlock(¤t->signal->cred_guard_light);
+ }
if (bprm->mm) {
acct_arg_size(bprm, 0);
mmput(bprm->mm);
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 325f649d77ff..c6819468e79a 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -60,6 +60,7 @@ extern struct fs_struct init_fs;
INIT_PREV_CPUTIME(sig) \
.cred_guard_mutex = \
__MUTEX_INITIALIZER(sig.cred_guard_mutex), \
+ .cred_guard_light = __MUTEX_INITIALIZER(sig.cred_guard_light) \
}
extern struct nsproxy init_nsproxy;
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 348f51b0ec92..0ccb379895b3 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -812,6 +812,16 @@ struct signal_struct {
struct mutex cred_guard_mutex; /* guard against foreign influences on
* credential calculations
* (notably. ptrace) */
+ /*
+ * Lightweight version of cred_guard_mutex; used to prevent race
+ * conditions where a user can gain information about the post-execve
+ * state of a task to which access should only be granted pre-execve.
+ * Hold this mutex while performing remote task inspection associated
+ * with a security check.
+ * This mutex MUST NOT be used in cases where anything changes about
+ * the security properties of a running execve().
+ */
+ struct mutex cred_guard_light;
};
/*
diff --git a/kernel/fork.c b/kernel/fork.c
index 623259fc794d..d0e1d6fa4d00 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1361,6 +1361,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
current->signal->is_child_subreaper;
mutex_init(&sig->cred_guard_mutex);
+ mutex_init(&sig->cred_guard_light);
return 0;
}
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index e6474f7272ec..c3312e9e0078 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -285,6 +285,16 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
return security_ptrace_access_check(task, mode);
}
+/*
+ * NOTE: When you call this function, you need to ensure that the target task
+ * can't acquire (via setuid execve) credentials between the ptrace access
+ * check and the privileged access. The recommended way to do this is to hold
+ * one of task->signal->{cred_guard_mutex,cred_guard_light} while calling this
+ * function and performing the requested access.
+ *
+ * This function may only be used if access is requested in the name of
+ * current_cred().
+ */
bool ptrace_may_access(struct task_struct *task, unsigned int mode)
{
int err;
--
2.1.4
next prev parent reply other threads:[~2016-10-30 21:46 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-10-30 21:46 [PATCH v3 0/8] Various fixes related to ptrace_may_access() Jann Horn
2016-10-30 21:46 ` Jann Horn [this message]
2016-11-02 18:18 ` [PATCH v3 1/8] exec: introduce cred_guard_light Oleg Nesterov
2016-11-02 20:50 ` Jann Horn
2016-11-02 21:38 ` Ben Hutchings
2016-11-02 21:54 ` Jann Horn
2016-11-03 18:12 ` Oleg Nesterov
2016-11-03 21:17 ` Jann Horn
2016-11-04 13:26 ` Eric W. Biederman
2016-11-04 15:00 ` Eric W. Biederman
2016-11-04 18:04 ` Oleg Nesterov
2016-11-04 18:45 ` Oleg Nesterov
2016-11-05 14:56 ` Oleg Nesterov
2016-11-09 0:34 ` Eric W. Biederman
2016-11-16 20:03 ` Eric W. Biederman
2016-11-08 22:02 ` Kees Cook
2016-11-08 22:46 ` Eric W. Biederman
2016-11-08 22:56 ` Benjamin LaHaise
2016-11-08 23:33 ` Eric W. Biederman
2016-10-30 21:46 ` [PATCH v3 2/8] exec: add privunit to task_struct Jann Horn
2016-10-30 21:46 ` [PATCH v3 3/8] proc: use open()-time creds for ptrace checks Jann Horn
2016-10-30 21:46 ` [PATCH v3 4/8] futex: don't leak robust_list pointer Jann Horn
2016-10-30 21:46 ` [PATCH v3 5/8] proc: lock properly in ptrace_may_access callers Jann Horn
2016-10-30 21:46 ` [PATCH v3 6/8] fs/proc: fix attr access check Jann Horn
2016-10-30 21:46 ` [PATCH v3 7/8] proc: fix timerslack_ns handling Jann Horn
2016-10-30 21:46 ` [PATCH v3 8/8] Documentation: add security/ptrace_checks.txt Jann Horn
2016-11-01 23:57 ` [PATCH v3 0/8] Various fixes related to ptrace_may_access() Linus Torvalds
2016-11-02 18:38 ` Oleg Nesterov
2016-11-02 21:40 ` Jann Horn
2016-11-03 19:09 ` Andrew Morton
2016-11-03 20:01 ` Jann Horn
2016-11-04 0:57 ` James Morris
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