Re: kernel NULL pointer dereference in quirk_clear_strap_no_soft_reset_dev2_f0 -> amd_smn_read

[Marek Marczykowski-Górecki]

On Mon, Jan 12, 2026 at 08:47:50PM -0600, Mario Limonciello wrote:
> 
> 
> On 1/12/2026 7:01 PM, Marek Marczykowski-Górecki wrote:
> > Hi,
> > 
> > I've got a report that kernel 6.17.9 crashes when running a Xen HVM domU
> > with AMD Raphael/Granite Ridge USB controller passed through.
> > It worked correctly in 6.12.59. Between those versions, I don't see any
> > relevant change to quirk_clear_strap_no_soft_reset_dev2_f0() function,
> > but the AMD node driver did got some changes, so my guess is one of them
> > is to blame. I know the good-bad range is huge, but there aren't that
> > many changes to the AMD node driver in this range.
> 
> Is this perhaps a case that only the USB controller was passed through but
> that the root controller wasn't?  That would lead to a case that
> amd_smn_init() was never called and thus amd_roots was not initialized
> properly.
> 
> So it would be a NULL pointer deref.  If that's correct, something like this
> should work to avoid it.
> 
> diff --git a/arch/x86/kernel/amd_node.c b/arch/x86/kernel/amd_node.c
> index 3d0a4768d603c..894823b444d47 100644
> --- a/arch/x86/kernel/amd_node.c
> +++ b/arch/x86/kernel/amd_node.c
> @@ -91,6 +91,11 @@ 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;
> 
> +       if (!amd_roots) {
> +               pr_warn("AMD SMN roots not initialized.\n");
> +               return err;
> +       }
> +
>         root = amd_roots[node];
>         if (!root)
>                 return err;

Thanks, I finally got confirmation from affected user that this patch
fixes the issue. From what I understand, adbf61cc47cb ("x86/acpi/boot: Correct
acpi_is_processor_usable() check again") was not enough.

> > Original report at (with full kernel log etc): https://forum.qubes-os.org/t/yet-another-usb-keyboard-thread/38355/8

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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