linux-kernel - [PATCH v9 10/11] seccomp: allow mode setting across threads

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Date:	Fri, 27 Jun 2014 16:22:59 -0700
From:	Kees Cook <keescook@...omium.org>
To:	linux-kernel@...r.kernel.org
Cc:	Kees Cook <keescook@...omium.org>, Oleg Nesterov <oleg@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Will Drewry <wad@...omium.org>,
	Julien Tinnes <jln@...omium.org>,
	David Drysdale <drysdale@...gle.com>,
	linux-api@...r.kernel.org, x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-mips@...ux-mips.org,
	linux-arch@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: [PATCH v9 10/11] seccomp: allow mode setting across threads

This changes the mode setting helper to allow threads to change the
seccomp mode from another thread. We must maintain barriers to keep
TIF_SECCOMP synchronized with the rest of the seccomp state.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 kernel/seccomp.c |   27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index e1ff2c193190..7bbcb9ed16df 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -207,12 +207,18 @@ static inline bool seccomp_check_mode(unsigned long seccomp_mode)
 	return true;
 }
 
-static inline void seccomp_assign_mode(unsigned long seccomp_mode)
+static inline void seccomp_assign_mode(struct task_struct *task,
+				       unsigned long seccomp_mode)
 {
-	BUG_ON(!spin_is_locked(&current->sighand->siglock));
+	BUG_ON(!spin_is_locked(&task->sighand->siglock));
 
-	current->seccomp.mode = seccomp_mode;
-	set_tsk_thread_flag(current, TIF_SECCOMP);
+	task->seccomp.mode = seccomp_mode;
+	/*
+	 * Make sure TIF_SECCOMP cannot be set before the mode (and
+	 * filter) is set.
+	 */
+	smp_mb__before_atomic();
+	set_tsk_thread_flag(task, TIF_SECCOMP);
 }
 
 #ifdef CONFIG_SECCOMP_FILTER
@@ -433,12 +439,17 @@ static int mode1_syscalls_32[] = {
 
 int __secure_computing(int this_syscall)
 {
-	int mode = current->seccomp.mode;
 	int exit_sig = 0;
 	int *syscall;
 	u32 ret;
 
-	switch (mode) {
+	/*
+	 * Make sure that any changes to mode from another thread have
+	 * been seen after TIF_SECCOMP was seen.
+	 */
+	rmb();
+
+	switch (current->seccomp.mode) {
 	case SECCOMP_MODE_STRICT:
 		syscall = mode1_syscalls;
 #ifdef CONFIG_COMPAT
@@ -543,7 +554,7 @@ static long seccomp_set_mode_strict(void)
 #ifdef TIF_NOTSC
 	disable_TSC();
 #endif
-	seccomp_assign_mode(seccomp_mode);
+	seccomp_assign_mode(current, seccomp_mode);
 	ret = 0;
 
 out:
@@ -593,7 +604,7 @@ static long seccomp_set_mode_filter(unsigned int flags,
 	/* Do not free the successfully attached filter. */
 	prepared = NULL;
 
-	seccomp_assign_mode(seccomp_mode);
+	seccomp_assign_mode(current, seccomp_mode);
 out:
 	spin_unlock_irq(&current->sighand->siglock);
 	seccomp_filter_free(prepared);
-- 
1.7.9.5

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