On 12/1/2023 1:26 AM, Maxim Levitsky wrote:
On Fri, 2023-11-24 at 00:53 -0500, Yang Weijiang wrote:
Remove XFEATURE_CET_USER entry from dependency array as the entry doesn't
reflect true dependency between CET features and the user xstate bit.
Enable the bit in fpu_kernel_cfg.max_features when either SHSTK or IBT is
available.
Both user mode shadow stack and indirect branch tracking features depend
on XFEATURE_CET_USER bit in XSS to automatically save/restore user mode
xstate registers, i.e., IA32_U_CET and IA32_PL3_SSP whenever necessary.
Note, the issue, i.e., CPUID only enumerates IBT but no SHSTK is resulted
from CET KVM series which synthesizes guest CPUIDs based on userspace
settings,in real world the case is rare. In other words, the exitings
dependency check is correct when only user mode SHSTK is available.
Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Tested-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
---
arch/x86/kernel/fpu/xstate.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 73f6bc00d178..6e50a4251e2b 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -73,7 +73,6 @@ static unsigned short xsave_cpuid_features[] __initdata = {
[XFEATURE_PT_UNIMPLEMENTED_SO_FAR] = X86_FEATURE_INTEL_PT,
[XFEATURE_PKRU] = X86_FEATURE_OSPKE,
[XFEATURE_PASID] = X86_FEATURE_ENQCMD,
- [XFEATURE_CET_USER] = X86_FEATURE_SHSTK,
[XFEATURE_XTILE_CFG] = X86_FEATURE_AMX_TILE,
[XFEATURE_XTILE_DATA] = X86_FEATURE_AMX_TILE,
};
@@ -798,6 +797,14 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
fpu_kernel_cfg.max_features &= ~BIT_ULL(i);
}
+ /*
+ * CET user mode xstate bit has been cleared by above sanity check.
+ * Now pick it up if either SHSTK or IBT is available. Either feature
+ * depends on the xstate bit to save/restore user mode states.
+ */
+ if (boot_cpu_has(X86_FEATURE_SHSTK) || boot_cpu_has(X86_FEATURE_IBT))
+ fpu_kernel_cfg.max_features |= BIT_ULL(XFEATURE_CET_USER);
+
if (!cpu_feature_enabled(X86_FEATURE_XFD))
fpu_kernel_cfg.max_features &= ~XFEATURE_MASK_USER_DYNAMIC;
I am curious:
Any reason why my review feedback was not applied even though you did agree
that it is reasonable?
My apology! I changed the patch per you feedback but found XFEATURE_CET_USER didn't
work before sending out v7 version, after a close look at the existing code:
for (i = 0; i < ARRAY_SIZE(xsave_cpuid_features); i++) {
unsigned short cid = xsave_cpuid_features[i];
/* Careful: X86_FEATURE_FPU is 0! */
if ((i != XFEATURE_FP && !cid) || !boot_cpu_has(cid))
fpu_kernel_cfg.max_features &= ~BIT_ULL(i);
}
With removal of XFEATURE_CET_USER entry from xsave_cpuid_features, actually
above check will clear the bit from fpu_kernel_cfg.max_features. So now I need
to add it back conditionally.
Your sample code is more consistent with existing code in style, but I don't want to
hack into the loop and handle XFEATURE_CET_USER specifically. Just keep the handling
and rewording the comments which is also straightforward.
https://lore.kernel.org/lkml/c72dfaac-1622-94cf-a81d-9d7ed81b2f55@xxxxxxxxx/
Best regards,
Maxim Levitsky
