[PATCH v3 2/2] x86/amd_node: Remove smn_exclusive

[Jason Andryuk <jason.andryuk@amd.com>]

amd_smn_read/write() are exported functions around __amd_smn_rw(), so
they are always available even if amd_smn_init() fails.  smn_exclusive
would prevent access to __amd_smn_rw(), but it is placed too late.  If
amd_smn_init() failed, amd_roots is NULL and __amd_smn_rw() will fault
over it.  Replace smn_exclusive with directly checking amd_roots to
avoid the NULL pointer dereference.

commit 83518453074d ("x86/amd_node: Add SMN offsets to exclusive region
access") added smn_exclusive which indicated the calls to
pci_request_config_region_exclusive() succeeded to prevent userspace
access.

commit 0a4b61d9c2e4 ("x86/amd_node: Fix AMD root device caching")
re-ordered initialization so pci_request_config_region_exclusive() is
called earlier and a failure exits amd_smn_init() before allocating
amd_roots.  Setting smn_exclusive moved to the end of amd_smn_init(),
after amd_roots is allocated.  smn_exclusive became redundant to
amd_roots and can be removed.

Add a comment stating how amd_roots replaces smn_exclusive's purpose.

Fixes: 77466b798d59 ("x86/amd_node: Remove dependency on AMD_NB")
Cc: stable@vger.kernel.org
Reviewed-by: Yazen Ghannam <yazen.ghannam@amd.com>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
---
Fixes is the introduction of amd_roots, which could be a NULL deref.

v3:
No change

v2:
R-b Yazen and Mario
Add comment about amd_root taking smn_exclusive's purpose to indicate
userspace access is prevented.
Expand commit message
---
 arch/x86/kernel/amd_node.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/amd_node.c b/arch/x86/kernel/amd_node.c
index 408b9fd48349..833e5eda5d91 100644
--- a/arch/x86/kernel/amd_node.c
+++ b/arch/x86/kernel/amd_node.c
@@ -38,7 +38,6 @@ 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
@@ -91,11 +90,15 @@ static int __amd_smn_rw(u8 i_off, u8 d_off, u16 node, u32 address, u32 *value, b
 	if (node >= amd_num_nodes())
 		return err;
 
-	root = amd_roots[node];
-	if (!root)
+	/*
+	 * non-NULL amd_roots indicates pci_request_config_region_exclusive()
+	 * succeeded and userspace cannot access the registers.
+	 */
+	if (!amd_roots)
 		return err;
 
-	if (!smn_exclusive)
+	root = amd_roots[node];
+	if (!root)
 		return err;
 
 	guard(mutex)(&smn_mutex);
@@ -313,8 +316,6 @@ static int __init amd_smn_init(void)
 		debugfs_create_file("value",	0600, debugfs_dir, NULL, &smn_value_fops);
 	}
 
-	smn_exclusive = true;
-
 	return 0;
 }
 
-- 
2.55.0