x86/amd: fix legacy setting of SSBD on AMD Family 17h

The current logic to set SSBD on AMD Family 17h and Hygon Family 18h
processors requires that the setting of SSBD is coordinated at a core
level, as the setting is shared between threads.  Logic was introduced
to keep track of how many threads require SSBD active in order to
coordinate it, such logic relies on using a per-core counter of
threads that have SSBD active.

Given the current logic, it's possible for a guest to under or
overflow the thread counter, because each write to VIRT_SPEC_CTRL.SSBD
by the guest gets propagated to the helper that does the per-core
active accounting.  Overflowing the counter is not so much of an
issue, as this would just make SSBD sticky.

Underflowing however is more problematic: on non-debug Xen builds a
guest can perform empty writes to VIRT_SPEC_CTRL that would cause the
counter to underflow and thus the value gets saturated to the max
value of unsigned int.  At which points attempts from any thread to
set VIRT_SPEC_CTRL.SSBD won't get propagated to the hardware anymore,
because the logic will see that the counter is greater than 1 and
assume that SSBD is already active, effectively loosing the setting
of SSBD and the protection it provides.

Fix this by introducing a per-CPU variable that keeps track of whether
the current thread has legacy SSBD active or not, and thus only
attempt to propagate the value to the hardware once the thread
selected value changes.

This is XSA-431 / CVE-2022-42336

Fixes: b2030e6730a2 ('amd/virt_ssbd: set SSBD at vCPU context switch')
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/xen/arch/x86/cpu/amd.c b/xen/arch/x86/cpu/amd.c
index caafe44740..9a1a3858ed 100644
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -783,12 +783,23 @@ bool __init amd_setup_legacy_ssbd(void)
 	return true;
 }
 
+/*
+ * legacy_ssbd is always initialized to false because when SSBD is set
+ * from the command line guest attempts to change it are a no-op (see
+ * amd_set_legacy_ssbd()), whereas when SSBD is inactive hardware will
+ * be forced into that mode (see amd_init_ssbd()).
+ */
+static DEFINE_PER_CPU(bool, legacy_ssbd);
+
+/* Must be called only when the SSBD setting needs toggling. */
 static void core_set_legacy_ssbd(bool enable)
 {
 	const struct cpuinfo_x86 *c = &current_cpu_data;
 	struct ssbd_ls_cfg *status;
 	unsigned long flags;
 
+	BUG_ON(this_cpu(legacy_ssbd) == enable);
+
 	if ((c->x86 != 0x17 && c->x86 != 0x18) || c->x86_num_siblings <= 1) {
 		BUG_ON(!set_legacy_ssbd(c, enable));
 		return;
@@ -816,12 +827,17 @@ void amd_set_legacy_ssbd(bool enable)
 		 */
 		return;
 
+	if (this_cpu(legacy_ssbd) == enable)
+		return;
+
 	if (cpu_has_virt_ssbd)
 		wrmsr(MSR_VIRT_SPEC_CTRL, enable ? SPEC_CTRL_SSBD : 0, 0);
 	else if (amd_legacy_ssbd)
 		core_set_legacy_ssbd(enable);
 	else
 		ASSERT_UNREACHABLE();
+
+	this_cpu(legacy_ssbd) = enable;
 }
 
 /*