[PATCH 1/9] iommu/vt-d: Fix no_iommu to disable platform optin

[Kevin Tian]

If user explicitly requests to disable iommu (via "iommu=off" or
"intel_iommu=off"), there is no reason to force enabling it due
to platform optin (for external-facing devices). User should be
aware of any security implication of doing so.

"intel_iommu=off" implements this policy by setting no_platform_optin
to skip platform optin in platform_optin_force_iommu().

However, "iommu=off" (no_iommu=1) doesn't set no_platform_optin
hence is broken in this aspect:

  - detect_intel_iommu() doesn't request ACS if no_iommu=1
  - platform_optin_force_iommu() forces iommu on if external-facing
    devices exist and no_platform_optin is not set

This leads to a bad configuration with ACS disabled while DMA
remapping is enabled.

Instead of setting no_platform_optin (will soon be removed) for
no_iommu=1, directly check no_iommu in platform_optin_force_iommu().

Fixes: 89a6079df791 ("iommu/vt-d: Force IOMMU on for platform opt in hint")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Kevin Tian <kevin.tian@xxxxxxxxx>
---
 drivers/iommu/intel/iommu.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index 4d0e65bc131d..9584ac0ed02f 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -2479,10 +2479,11 @@ static bool has_external_pci(void)
 
 static int __init platform_optin_force_iommu(void)
 {
-	if (!dmar_platform_optin() || no_platform_optin || !has_external_pci())
+	if (no_iommu || !dmar_platform_optin() || no_platform_optin ||
+	    !has_external_pci())
 		return 0;
 
-	if (no_iommu || dmar_disabled)
+	if (dmar_disabled)
 		pr_info("Intel-IOMMU force enabled due to platform opt in\n");
 
 	/*
@@ -2493,7 +2494,6 @@ static int __init platform_optin_force_iommu(void)
 		iommu_set_default_passthrough(false);
 
 	dmar_disabled = 0;
-	no_iommu = 0;
 
 	return 1;
 }
-- 
2.43.0