linux-kernel - Re: [PATCH next] iommu/vt-d: Use shallowest supported table depth in sagaw

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <20251127183210.GC738503@ziepe.ca>
Date: Thu, 27 Nov 2025 14:32:10 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Calvin Owens <calvin@...nvd.org>
Cc: linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	Lu Baolu <baolu.lu@...ux.intel.com>,
	Nicolin Chen <nicolinc@...dia.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH next] iommu/vt-d: Use shallowest supported table depth in
 sagaw

On Thu, Nov 27, 2025 at 09:06:35AM -0800, Calvin Owens wrote:
> The failing condition in paging_domain_compatible_second_stage() is:
> 
>     /* Page table level is supported. */
>     if (!(cap_sagaw(iommu->cap) & BIT(pt_info.aw)))
>         return -EINVAL;
> 
> This happens because, for many domains on this machine, MGAW=39 but
> SAGAW=0x04: that claims a 39-bit maximum address width, but also claims
> to only support 48-bit/4-level paging, which seems odd.

This logic was intended to deal with this:

static int compute_vasz_lg2_ss(struct intel_iommu *iommu)
{
	unsigned int sagaw = cap_sagaw(iommu->cap);
	unsigned int mgaw = cap_mgaw(iommu->cap);

	/*
	 * Find the largest table size that both the mgaw and sagaw support.
	 * This sets both the number of table levels and the valid range of
	 * IOVA.
	 */
	if (mgaw >= 48 && (sagaw & BIT(3)))
		return min(57, mgaw);
	else if (mgaw >= 39 && (sagaw & BIT(2)))
		return min(48, mgaw);
	else if (mgaw >= 30 && (sagaw & BIT(1)))
		return min(39, mgaw);

Though it is not right..

I didn't consider that this would come up, the only way to solve it is
to pass in more information so the tree can be higher than the vasz
requires..

Can you try these two commits?

https://github.com/jgunthorpe/linux/commits/iommu_pt_vtd/

Thanks,
Jason