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authorKees Cook <keescook@chromium.org>2014-06-05 00:23:17 -0700
committerKees Cook <keescook@google.com>2016-04-08 12:32:15 -0700
commitd2bcc8c206ae10c300a23730b9df4de7df6d6a3c (patch)
treedf0debaf64bd8600e9049abd1513e0d11940b2c1
parent3e4de20ab65d8286f340436fdaef8b406790fb30 (diff)
downloadqcom-msm-v3.10-d2bcc8c206ae10c300a23730b9df4de7df6d6a3c.tar.gz
BACKPORT: seccomp: implement SECCOMP_FILTER_FLAG_TSYNC
Applying restrictive seccomp filter programs to large or diverse codebases often requires handling threads which may be started early in the process lifetime (e.g., by code that is linked in). While it is possible to apply permissive programs prior to process start up, it is difficult to further restrict the kernel ABI to those threads after that point. This change adds a new seccomp syscall flag to SECCOMP_SET_MODE_FILTER for synchronizing thread group seccomp filters at filter installation time. When calling seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_TSYNC, filter) an attempt will be made to synchronize all threads in current's threadgroup to its new seccomp filter program. This is possible iff all threads are using a filter that is an ancestor to the filter current is attempting to synchronize to. NULL filters (where the task is running as SECCOMP_MODE_NONE) are also treated as ancestors allowing threads to be transitioned into SECCOMP_MODE_FILTER. If prctrl(PR_SET_NO_NEW_PRIVS, ...) has been set on the calling thread, no_new_privs will be set for all synchronized threads too. On success, 0 is returned. On failure, the pid of one of the failing threads will be returned and no filters will have been applied. The race conditions against another thread are: - requesting TSYNC (already handled by sighand lock) - performing a clone (already handled by sighand lock) - changing its filter (already handled by sighand lock) - calling exec (handled by cred_guard_mutex) The clone case is assisted by the fact that new threads will have their seccomp state duplicated from their parent before appearing on the tasklist. Holding cred_guard_mutex means that seccomp filters cannot be assigned while in the middle of another thread's exec (potentially bypassing no_new_privs or similar). The call to de_thread() may kill threads waiting for the mutex. Changes across threads to the filter pointer includes a barrier. Based on patches by Will Drewry. Suggested-by: Julien Tinnes <jln@chromium.org> Signed-off-by: Kees Cook <keescook@chromium.org> Reviewed-by: Oleg Nesterov <oleg@redhat.com> Reviewed-by: Andy Lutomirski <luto@amacapital.net> Bug: 28020023 Patchset: seccomp (cherry picked from kernel/msm commit 6108b31b1d5cc6fa8f301a6e1fe517ec3e4ad6de) Signed-off-by: Kees Cook <keescook@google.com> Change-Id: I84acf30942a4a8527c55fd132e62f34c6f29f3d7
-rw-r--r--fs/exec.c2
-rw-r--r--include/linux/seccomp.h2
-rw-r--r--include/uapi/linux/seccomp.h3
-rw-r--r--kernel/seccomp.c135
4 files changed, 140 insertions, 2 deletions
diff --git a/fs/exec.c b/fs/exec.c
index 02ad43ce2f6..cb7f31c71c6 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1220,7 +1220,7 @@ EXPORT_SYMBOL(install_exec_creds);
/*
* determine how safe it is to execute the proposed program
* - the caller must hold ->cred_guard_mutex to protect against
- * PTRACE_ATTACH
+ * PTRACE_ATTACH or seccomp thread-sync
*/
static int check_unsafe_exec(struct linux_binprm *bprm)
{
diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index 9ab63a574d4..9687691799f 100644
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -3,6 +3,8 @@
#include <uapi/linux/seccomp.h>
+#define SECCOMP_FILTER_FLAG_MASK (SECCOMP_FILTER_FLAG_TSYNC)
+
#ifdef CONFIG_SECCOMP
#include <linux/thread_info.h>
diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
index b258878ba75..0f238a43ff1 100644
--- a/include/uapi/linux/seccomp.h
+++ b/include/uapi/linux/seccomp.h
@@ -14,6 +14,9 @@
#define SECCOMP_SET_MODE_STRICT 0
#define SECCOMP_SET_MODE_FILTER 1
+/* Valid flags for SECCOMP_SET_MODE_FILTER */
+#define SECCOMP_FILTER_FLAG_TSYNC 1
+
/*
* All BPF programs must return a 32-bit value.
* The bottom 16-bits are for optional return data.
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 5f2962e4aee..ebdaaf427de 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -26,6 +26,7 @@
#ifdef CONFIG_SECCOMP_FILTER
#include <asm/syscall.h>
#include <linux/filter.h>
+#include <linux/pid.h>
#include <linux/ptrace.h>
#include <linux/security.h>
#include <linux/tracehook.h>
@@ -251,6 +252,114 @@ static inline void seccomp_assign_mode(struct task_struct *task,
}
#ifdef CONFIG_SECCOMP_FILTER
+/* Returns 1 if the parent is an ancestor of the child. */
+static int is_ancestor(struct seccomp_filter *parent,
+ struct seccomp_filter *child)
+{
+ /* NULL is the root ancestor. */
+ if (parent == NULL)
+ return 1;
+ for (; child; child = child->prev)
+ if (child == parent)
+ return 1;
+ return 0;
+}
+
+/**
+ * seccomp_can_sync_threads: checks if all threads can be synchronized
+ *
+ * Expects sighand and cred_guard_mutex locks to be held.
+ *
+ * Returns 0 on success, -ve on error, or the pid of a thread which was
+ * either not in the correct seccomp mode or it did not have an ancestral
+ * seccomp filter.
+ */
+static inline pid_t seccomp_can_sync_threads(void)
+{
+ struct task_struct *thread, *caller;
+
+ BUG_ON(!mutex_is_locked(&current->signal->cred_guard_mutex));
+ BUG_ON(!spin_is_locked(&current->sighand->siglock));
+
+ /* Validate all threads being eligible for synchronization. */
+ caller = current;
+ for_each_thread(caller, thread) {
+ pid_t failed;
+
+ /* Skip current, since it is initiating the sync. */
+ if (thread == caller)
+ continue;
+
+ if (thread->seccomp.mode == SECCOMP_MODE_DISABLED ||
+ (thread->seccomp.mode == SECCOMP_MODE_FILTER &&
+ is_ancestor(thread->seccomp.filter,
+ caller->seccomp.filter)))
+ continue;
+
+ /* Return the first thread that cannot be synchronized. */
+ failed = task_pid_vnr(thread);
+ /* If the pid cannot be resolved, then return -ESRCH */
+ if (unlikely(WARN_ON(failed == 0)))
+ failed = -ESRCH;
+ return failed;
+ }
+
+ return 0;
+}
+
+/**
+ * seccomp_sync_threads: sets all threads to use current's filter
+ *
+ * Expects sighand and cred_guard_mutex locks to be held, and for
+ * seccomp_can_sync_threads() to have returned success already
+ * without dropping the locks.
+ *
+ */
+static inline void seccomp_sync_threads(void)
+{
+ struct task_struct *thread, *caller;
+
+ BUG_ON(!mutex_is_locked(&current->signal->cred_guard_mutex));
+ BUG_ON(!spin_is_locked(&current->sighand->siglock));
+
+ /* Synchronize all threads. */
+ caller = current;
+ for_each_thread(caller, thread) {
+ /* Skip current, since it needs no changes. */
+ if (thread == caller)
+ continue;
+
+ /* Get a task reference for the new leaf node. */
+ get_seccomp_filter(caller);
+ /*
+ * Drop the task reference to the shared ancestor since
+ * current's path will hold a reference. (This also
+ * allows a put before the assignment.)
+ */
+ put_seccomp_filter(thread);
+ smp_store_release(&thread->seccomp.filter,
+ caller->seccomp.filter);
+ /*
+ * Opt the other thread into seccomp if needed.
+ * As threads are considered to be trust-realm
+ * equivalent (see ptrace_may_access), it is safe to
+ * allow one thread to transition the other.
+ */
+ if (thread->seccomp.mode == SECCOMP_MODE_DISABLED) {
+ /*
+ * Don't let an unprivileged task work around
+ * the no_new_privs restriction by creating
+ * a thread that sets it up, enters seccomp,
+ * then dies.
+ */
+ if (task_no_new_privs(caller))
+ task_set_no_new_privs(thread);
+
+ seccomp_assign_mode(thread, SECCOMP_MODE_FILTER);
+ }
+ }
+}
+
/**
* seccomp_prepare_filter: Prepares a seccomp filter for use.
* @fprog: BPF program to install
@@ -366,6 +475,15 @@ static long seccomp_attach_filter(unsigned int flags,
if (total_insns > MAX_INSNS_PER_PATH)
return -ENOMEM;
+ /* If thread sync has been requested, check that it is possible. */
+ if (flags & SECCOMP_FILTER_FLAG_TSYNC) {
+ int ret;
+
+ ret = seccomp_can_sync_threads();
+ if (ret)
+ return ret;
+ }
+
/*
* If there is an existing filter, make it the prev and don't drop its
* task reference.
@@ -373,6 +491,10 @@ static long seccomp_attach_filter(unsigned int flags,
filter->prev = current->seccomp.filter;
current->seccomp.filter = filter;
+ /* Now that the new filter is in place, synchronize to all threads. */
+ if (flags & SECCOMP_FILTER_FLAG_TSYNC)
+ seccomp_sync_threads();
+
return 0;
}
@@ -591,7 +713,7 @@ static long seccomp_set_mode_filter(unsigned int flags,
long ret = -EINVAL;
/* Validate flags. */
- if (flags != 0)
+ if (flags & ~SECCOMP_FILTER_FLAG_MASK)
return -EINVAL;
/* Prepare the new filter before holding any locks. */
@@ -599,6 +721,14 @@ static long seccomp_set_mode_filter(unsigned int flags,
if (IS_ERR(prepared))
return PTR_ERR(prepared);
+ /*
+ * Make sure we cannot change seccomp or nnp state via TSYNC
+ * while another thread is in the middle of calling exec.
+ */
+ if (flags & SECCOMP_FILTER_FLAG_TSYNC &&
+ mutex_lock_killable(&current->signal->cred_guard_mutex))
+ goto out_free;
+
spin_lock_irq(&current->sighand->siglock);
if (!seccomp_may_assign_mode(seccomp_mode))
@@ -613,6 +743,9 @@ static long seccomp_set_mode_filter(unsigned int flags,
seccomp_assign_mode(current, seccomp_mode);
out:
spin_unlock_irq(&current->sighand->siglock);
+ if (flags & SECCOMP_FILTER_FLAG_TSYNC)
+ mutex_unlock(&current->signal->cred_guard_mutex);
+out_free:
seccomp_filter_free(prepared);
return ret;
}