linux-kernel - Re: [PATCH 4/4] iommu/vt-d: Do access checks before calling handle_mm

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Date:	Tue, 10 Nov 2015 14:45:56 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	Joerg Roedel <joro@...tes.org>, iommu@...ts.linux-foundation.org
Cc:	Oded Gabbay <oded.gabbay@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Joerg Roedel <jroedel@...e.de>,
	jbarnes@...tuousgeek.org
Subject: Re: [PATCH 4/4] iommu/vt-d: Do access checks before calling
 handle_mm_fault()

On Tue, 2015-11-10 at 14:26 +0100, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.de>
> 
> Not doing so is a bug and might trigger a BUG_ON in
> handle_mm_fault(). So add the proper permission checks
> before calling into mm code.
> 
> Signed-off-by: Joerg Roedel <jroedel@...e.de>

> +static bool access_error(struct vm_area_struct *vma, struct
> page_req_dsc *req)
> +{
> +	return !((req->rd_req  && (vma->vm_flags & VM_READ))  ||
> +		 (req->wr_req  && (vma->vm_flags & VM_WRITE)) ||
> +		 (req->exe_req && (vma->vm_flags & VM_EXEC)));
> +}
> +

This is a TLB fill request from the device — can it not be asking for
*all* of read, write and exec privs? And you allow it to succeed if any
*one* of the permissions that it asks for is available?

Even if we don't see read+write in the same request, the VT-d spec does
seem quite clear that we *will* see read+exec (§7.5.1.1 p7-17 of v2.3:

• Execute Requested: If the PASID Present, Read Requested and Execute
  Requested fields are all 1, the request-with-PASID that encountered 
  the recoverable fault that resulted in this page request, requires 
  execute access to the page.

Also, I'm afraid my Skylake box blew up the day I sent the pull request
to Linus so I'm unable to test until I've sorted out a replacement.
Jesse should be able to though...

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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