Re: [PATCH v27 23/31] x86/cet/shstk: Add user-mode shadow stack support

From: Yu, Yu-cheng
Date: Tue Jul 20 2021 - 13:16:22 EST

On 7/19/2021 11:23 AM, Edgecombe, Rick P wrote:
On Fri, 2021-05-21 at 15:12 -0700, Yu-cheng Yu wrote:
Introduce basic shadow stack enabling/disabling/allocation routines.
A task's shadow stack is allocated from memory with VM_SHADOW_STACK
flag
and has a fixed size of min(RLIMIT_STACK, 4GB).

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>

[...]

diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
new file mode 100644
index 000000000000..5ea2b494e9f9
--- /dev/null
+++ b/arch/x86/kernel/shstk.c

[...]

+int shstk_setup(void)
+{
+       struct thread_shstk *shstk = &current->thread.shstk;
+       unsigned long addr, size;
+
+       if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+               return -EOPNOTSUPP;
The only caller of this will skip it if
!cpu_feature_enabled(X86_FEATURE_SHSTK), so this is dead logic. Same
pattern in the IBT patch.


Indeed that is the case. We can simply remove the test of X86_FEATURE_SHSTK.

Thanks,
Yu-cheng