[PATCH 15/16] x86/amd_smn: Add SMN offsets to exclusive region access

Yazen Ghannam
2024-10-23T17:22:30Z

From: Mario Limonciello <mario.limonciello@amd.com>

Offsets 0x60 and 0x64 are used internally by kernel drivers that call
the amd_smn_read() and amd_smn_write() functions. If userspace accesses
the regions at the same time as the kernel it may cause malfunctions in
drivers using the offsets.

Add these offsets to the exclusions so that the kernel is tainted if a
non locked down userspace tries to access them.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
---
arch/x86/kernel/amd_smn.c | 41 +++++++++++++++++++++++++++++++++++++++
1 file changed, 41 insertions(+)

diff --git a/arch/x86/kernel/amd_smn.c b/arch/x86/kernel/amd_smn.c
index 527dda8e3a2b..aeebd63234f9 100644
--- a/arch/x86/kernel/amd_smn.c
+++ b/arch/x86/kernel/amd_smn.c
@@ -14,6 +14,7 @@ static struct pci_dev **amd_roots;

/* Protect the PCI config register pairs used for SMN. */
static DEFINE_MUTEX(smn_mutex);
+static bool smn_exclusive;

#define SMN_INDEX_OFFSET 0x60
#define SMN_DATA_OFFSET 0x64
@@ -72,6 +73,9 @@ static int __amd_smn_rw(u8 i_off, u8 d_off, u16 node, u32 address, u32 *value, b

guard(mutex)(&smn_mutex);

+ if (!smn_exclusive)
+ return err;
+
err = pci_write_config_dword(root, i_off, address);
if (err) {
pr_warn("Error programming SMN address 0x%x.\n", address);
@@ -123,6 +127,39 @@ static int amd_cache_roots(void)
return 0;
}

+static int reserve_root_config_spaces(void)
+{
+ struct pci_dev *root = NULL;
+ struct pci_bus *bus = NULL;
+
+ while ((bus = pci_find_next_bus(bus))) {
+ /* Root device is Device 0 Function 0 on each Primary Bus. */
+ root = pci_get_slot(bus, 0);
+ if (!root)
+ continue;
+
+ if (root->vendor != PCI_VENDOR_ID_AMD &&
+ root->vendor != PCI_VENDOR_ID_HYGON)
+ continue;
+
+ pci_dbg(root, "Reserving PCI config space\n");
+
+ /*
+ * There are a few SMN index/data pairs and other registers
+ * that shouldn't be accessed by user space.
+ * So reserve the entire PCI config space for simplicity rather
+ * than covering specific registers piecemeal.
+ */
+ if (!pci_request_config_region_exclusive(root, 0, PCI_CFG_SPACE_SIZE, NULL)) {
+ pci_err(root, "Failed to reserve config space\n");
+ return -EEXIST;
+ }
+ }
+
+ smn_exclusive = true;
+ return 0;
+}
+
static int __init amd_smn_init(void)
{
int err;
@@ -139,6 +176,10 @@ static int __init amd_smn_init(void)
if (err)
return err;

+ err = reserve_root_config_spaces();
+ if (err)
+ return err;
+
return 0;
}

--
2.43.0