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Subject[PATCH V6 34/49] x86/entry: Add the C version ist_switch_to_kernel_gsbase()
Date
From: Lai Jiangshan <laijs@linux.alibaba.com>

It implements the second half of paranoid_entry() whose functionality
is to switch to kernel gsbase.

Not functional difference intended.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
arch/x86/entry/entry64.c | 49 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 49 insertions(+)

diff --git a/arch/x86/entry/entry64.c b/arch/x86/entry/entry64.c
index 3ec145c38e9e..60c37dbe650b 100644
--- a/arch/x86/entry/entry64.c
+++ b/arch/x86/entry/entry64.c
@@ -219,3 +219,52 @@ static __always_inline unsigned long get_percpu_base(void)
return pcpu_unit_offsets;
}
#endif
+
+/*
+ * Handle GSBASE depends on the availability of FSGSBASE.
+ *
+ * Without FSGSBASE the kernel enforces that negative GSBASE
+ * values indicate kernel GSBASE. With FSGSBASE no assumptions
+ * can be made about the GSBASE value when entering from user
+ * space.
+ */
+static __always_inline unsigned long ist_switch_to_kernel_gsbase(void)
+{
+ int ret = 1;
+ unsigned long gsbase;
+
+ if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
+ /*
+ * Read the current GSBASE for return.
+ * Retrieve and set the current CPUs kernel GSBASE.
+ *
+ * The unconditional write to GS base below ensures that
+ * no subsequent loads based on a mispredicted GS base can
+ * happen, therefore no LFENCE is needed here.
+ */
+ gsbase = rdgsbase();
+ wrgsbase(get_percpu_base());
+ return gsbase;
+ }
+
+ gsbase = __rdmsr(MSR_GS_BASE);
+
+ /*
+ * The kernel-enforced convention is a negative GSBASE indicates
+ * a kernel value. No SWAPGS needed on entry and exit.
+ */
+ if ((long)gsbase >= 0) {
+ /* User GSBASE active, SWAPGS required on exit */
+ ret = 0;
+ native_swapgs();
+ }
+
+ /*
+ * The above ist_switch_to_kernel_cr3() doesn't do an unconditional
+ * CR3 write, even in the PTI case. So do an lfence to prevent GS
+ * speculation, regardless of whether PTI is enabled.
+ */
+ fence_swapgs_kernel_entry();
+
+ return ret;
+}
--
2.19.1.6.gb485710b
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