Linux: x86/amd_node: Fix potential NULL pointer dereference

In the Linux kernel, the following vulnerability has been resolved:

x86/amd_node: Fix potential NULL pointer dereference

amd_smn_read/write() are exported functions around __amd_smn_rw(), so they are always available even if amd_smn_init() fails. In that case, 'amd_roots' is NULL and __amd_smn_rw() will access uninitialized memory.

Then, commit:

83518453074d ("x86/amd_node: Add SMN offsets to exclusive region access")

added the 'smn_exclusive' flag, which indicated the calls to pci_request_config_region_exclusive() succeeded, to prevent concurrent 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'. The setting of 'smn_exclusive' moved to the end of amd_smn_init(), after 'amd_roots' is allocated. It became redundant and can be removed.

Replace 'smn_exclusive' with directly checking 'amd_roots', to fix a potential NULL pointer dereference and to simplify the logic.

[ bp: Reorg commit message, touchup comment. ] [ mingo: Rebase & further touchups. ]