Re: [PATCH v15 05/26] x86/cet/shstk: Add Kconfig option for user-mode Shadow Stack

On 11/30/2020 10:15 AM, Borislav Petkov wrote:

On Sat, Nov 28, 2020 at 08:23:59AM -0800, Yu, Yu-cheng wrote:

quoted

We have X86_BRANCH_TRACKING_USER too.  My thought was, X86_CET means any of
kernel/user shadow stack/ibt.

It is not about what it means - it is what you're going to use/need. You have
ifdeffery both with X86_CET and X86_SHADOW_STACK_USER.

This one

+#ifdef CONFIG_X86_SHADOW_STACK_USER
+#define DISABLE_SHSTK	0
+#else
+#define DISABLE_SHSTK	(1 << (X86_FEATURE_SHSTK & 31))
+#endif

for example, is clearly wrong and wants to be #ifdef CONFIG_X86_CET, for
example. Unless I'm missing something totally obvious.

Logically, enabling IBT without shadow stack does not make sense, but 
these features have different CPUIDs, and CONFIG_X86_SHADOW_STACK_USER 
and CONFIG_X86_BRANCH_TRACKING_USER can be selected separately.

Do we want to have only one selection for both features?  In other 
words, we turn on both or neither.

Thanks,
Yu-cheng

In any case, you need to analyze what Kconfig defines the code will
need and to what they belong and add only the minimal subset needed.
Our Kconfig symbols space is already nuts so adding more needs to be
absolutely justified.

Thx.