Re: [PATCH v6 1/9] kernel: Support TIF_SYSCALL_INTERCEPT flag

Christian Brauner [off-list ref] writes:

On Fri, Sep 04, 2020 at 04:31:39PM -0400, Gabriel Krisman Bertazi wrote:

quoted

index afe01e232935..3511c98a7849 100644

--- a/include/linux/sched.h
+++ b/include/linux/sched.h

@@ -959,7 +959,11 @@ struct task_struct {
 	kuid_t				loginuid;
 	unsigned int			sessionid;
 #endif
-	struct seccomp			seccomp;
+
+	struct {
+		unsigned int			syscall_intercept;
+		struct seccomp			seccomp;
+	};

If there's no specific reason to do this I'd not wrap this in an
anonymous struct. It doesn't really buy anything and there doesn't seem
to be  precedent in struct task_struct right now. Also, if this somehow
adds padding it seems you might end up increasing the size of struct
task_struct more than necessary by accident? (I might be wrong
though.)

Hi Christian,

Thanks for your review on this and on the other patches of this series.

I wrapped these to prevent struct layout randomization from separating
the flags field from seccomp, as they are going to be used together and
I was trying to reduce overhead to seccomp entry due to two cache misses
when reading this structure.  Measuring it seccomp_benchmark didn't show
any difference with the unwrapped version, so perhaps it was a bit of
premature optimization?

quoted

diff --git a/include/linux/syscall_intercept.h b/include/linux/syscall_intercept.h
new file mode 100644
index 000000000000..725d157699da
--- /dev/null
+++ b/include/linux/syscall_intercept.h

@@ -0,0 +1,70 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2020 Collabora Ltd.
+ */
+#ifndef _SYSCALL_INTERCEPT_H
+#define _SYSCALL_INTERCEPT_H
+
+#include <linux/sched.h>
+#include <linux/sched/signal.h>
+#include <linux/thread_info.h>
+
+#define SYSINT_SECCOMP		0x1

<bikeshed>

Can we maybe use a better name for this? I noone minds the extra
characters I'd suggest:
SYSCALL_INTERCEPT_SECCOMP
or
SYS_INTERCEPT_SECCOMP

</bikeshed>

will do.

Thanks,

-- 
Gabriel Krisman Bertazi

On Tue, Sep 08, 2020 at 12:59:49AM -0400, Gabriel Krisman Bertazi wrote:

Christian Brauner [off-list ref] writes:

quoted

On Fri, Sep 04, 2020 at 04:31:39PM -0400, Gabriel Krisman Bertazi wrote:

quoted

index afe01e232935..3511c98a7849 100644

--- a/include/linux/sched.h
+++ b/include/linux/sched.h

@@ -959,7 +959,11 @@ struct task_struct {
 	kuid_t				loginuid;
 	unsigned int			sessionid;
 #endif
-	struct seccomp			seccomp;
+
+	struct {
+		unsigned int			syscall_intercept;
+		struct seccomp			seccomp;
+	};

If there's no specific reason to do this I'd not wrap this in an
anonymous struct. It doesn't really buy anything and there doesn't seem
to be  precedent in struct task_struct right now. Also, if this somehow
adds padding it seems you might end up increasing the size of struct
task_struct more than necessary by accident? (I might be wrong
though.)

Hi Christian,

Thanks for your review on this and on the other patches of this series.

I wrapped these to prevent struct layout randomization from separating
the flags field from seccomp, as they are going to be used together and
I was trying to reduce overhead to seccomp entry due to two cache misses
when reading this structure.  Measuring it seccomp_benchmark didn't show
any difference with the unwrapped version, so perhaps it was a bit of
premature optimization?

That should not be a thing to think about here. Structure randomization
already has a mode to protect against cache line issues. I would leave
this as just a new member; no wrapping struct.

-- 
Kees Cook