Re: [PATCH v3 2/2] x86/amd_node: Remove smn_exclusive
From: Jason Andryuk
Date: Tue Sep 01 2026 - 08:00:01 EST
On 2026-08-31 20:27, Borislav Petkov wrote:
On Mon, Aug 31, 2026 at 01:52:08PM -0400, Jason Andryuk wrote:Looks good to me.
No, the code and the comment are both correct as posted. It's just that I
wrote the comment reversed from the code to state the positive property we
want.
So if amd_smn_init() succeeds, that means we have registered the PCI config
regions of the PCI roots, including the SMN ones, as IORESOURCE_EXCLUSIVE and
they won't be exported to userspace through /dev/mem and what not.
So, any caller of __amd_smn_rw() should fail because it is coming from kernel
space and it can expect that userspace could interfere with the hw and thus
not a good idea.
So yes, this is what got me confused and the (!amd_roots) check is correct.
I'll change this whichever way you want. It's not a big deal, which is why
I added the comment like you requested originally.
I think I already "intercepted" your suggestion, see here:
From 1a1c4addc5a14e9ef2bb7d351a62ff4563d0f10a Mon Sep 17 00:00:00 2001
From: Jason Andryuk <jason.andryuk@xxxxxxx>
Date: Tue, 25 Aug 2026 17:48:03 -0400
Subject: [PATCH] x86/amd_node: Prevent 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 smn_exclusive 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.
Setting 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 avoid a potential
NULL pointer dereference.
[ bp: Reorg commit message, touchup comment. ]
Fixes: 77466b798d59 ("x86/amd_node: Remove dependency on AMD_NB")
Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Reviewed-by: Yazen Ghannam <yazen.ghannam@xxxxxxx>
Reviewed-by: Mario Limonciello (AMD) <superm1@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://patch.msgid.link/20260825214805.39148-3-jason.andryuk@xxxxxxx
Thanks,
Jason