x86/amd_node: Add support for debugfs access to SMN registers

There are certain registers on AMD Zen systems that can only be accessed
through SMN.

Introduce a new interface that provides debugfs files for accessing SMN.  As
this introduces the capability for userspace to manipulate the hardware in
unpredictable ways, taint the kernel when writing.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250130-wip-x86-amd-nb-cleanup-v4-3-b5cc997e471b@amd.com

diff --git a/arch/x86/kernel/amd_node.c b/arch/x86/kernel/amd_node.c
index ac571948cb353ffbfba04dd724c246c4f2e573af..b670fa85c61b4ba96daac7d3a2d1014e1af9cff2 100644 (file)
--- a/arch/x86/kernel/amd_node.c
+++ b/arch/x86/kernel/amd_node.c
@@ -8,6 +8,7 @@
  * Author: Yazen Ghannam <Yazen.Ghannam@amd.com>
  */
 
+#include <linux/debugfs.h>
 #include <asm/amd_node.h>
 
 /*
@@ -192,6 +193,87 @@ int __must_check amd_smn_hsmp_rdwr(u16 node, u32 address, u32 *value, bool write
 }
 EXPORT_SYMBOL_GPL(amd_smn_hsmp_rdwr);
 
+static struct dentry *debugfs_dir;
+static u16 debug_node;
+static u32 debug_address;
+
+static ssize_t smn_node_write(struct file *file, const char __user *userbuf,
+                             size_t count, loff_t *ppos)
+{
+       u16 node;
+       int ret;
+
+       ret = kstrtou16_from_user(userbuf, count, 0, &node);
+       if (ret)
+               return ret;
+
+       if (node >= amd_num_nodes())
+               return -ENODEV;
+
+       debug_node = node;
+       return count;
+}
+
+static int smn_node_show(struct seq_file *m, void *v)
+{
+       seq_printf(m, "0x%08x\n", debug_node);
+       return 0;
+}
+
+static ssize_t smn_address_write(struct file *file, const char __user *userbuf,
+                                size_t count, loff_t *ppos)
+{
+       int ret;
+
+       ret = kstrtouint_from_user(userbuf, count, 0, &debug_address);
+       if (ret)
+               return ret;
+
+       return count;
+}
+
+static int smn_address_show(struct seq_file *m, void *v)
+{
+       seq_printf(m, "0x%08x\n", debug_address);
+       return 0;
+}
+
+static int smn_value_show(struct seq_file *m, void *v)
+{
+       u32 val;
+       int ret;
+
+       ret = amd_smn_read(debug_node, debug_address, &val);
+       if (ret)
+               return ret;
+
+       seq_printf(m, "0x%08x\n", val);
+       return 0;
+}
+
+static ssize_t smn_value_write(struct file *file, const char __user *userbuf,
+                              size_t count, loff_t *ppos)
+{
+       u32 val;
+       int ret;
+
+       ret = kstrtouint_from_user(userbuf, count, 0, &val);
+       if (ret)
+               return ret;
+
+       add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+
+       ret = amd_smn_write(debug_node, debug_address, val);
+       if (ret)
+               return ret;
+
+       return count;
+}
+
+DEFINE_SHOW_STORE_ATTRIBUTE(smn_node);
+DEFINE_SHOW_STORE_ATTRIBUTE(smn_address);
+DEFINE_SHOW_STORE_ATTRIBUTE(smn_value);
+
 static int amd_cache_roots(void)
 {
        u16 node, num_nodes = amd_num_nodes();
@@ -239,6 +321,15 @@ static int reserve_root_config_spaces(void)
        return 0;
 }
 
+static bool enable_dfs;
+
+static int __init amd_smn_enable_dfs(char *str)
+{
+       enable_dfs = true;
+       return 1;
+}
+__setup("amd_smn_debugfs_enable", amd_smn_enable_dfs);
+
 static int __init amd_smn_init(void)
 {
        int err;
@@ -259,6 +350,14 @@ static int __init amd_smn_init(void)
        if (err)
                return err;
 
+       if (enable_dfs) {
+               debugfs_dir = debugfs_create_dir("amd_smn", arch_debugfs_dir);
+
+               debugfs_create_file("node",     0600, debugfs_dir, NULL, &smn_node_fops);
+               debugfs_create_file("address",  0600, debugfs_dir, NULL, &smn_address_fops);
+               debugfs_create_file("value",    0600, debugfs_dir, NULL, &smn_value_fops);
+       }
+
        return 0;
 }