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Re: [XEN PATCH] xen/vpci: Fix UB in mask_write


  • To: Mykyta Poturai <Mykyta_Poturai@xxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Thu, 7 Nov 2024 10:25:02 +0100
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  • Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Thu, 07 Nov 2024 09:25:14 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 06.11.2024 13:26, Mykyta Poturai wrote:
> On 06.11.2024 10:07, Roger Pau Monné wrote:>
>> Wait - how can msi->vectors ever be 0?  AFAICT there's no way in the
>> MSI logic to configure 0 vectors, there will always be at least 1 vector
>> enabled.
>>
>> Maybe what you want, if this fix is for compliance reasons, is an
>> assert unreachable that msi->vectors > 0?
> 
> I did some investigation and figured out that the value of 0 is being
> set by guest writing to msi_control_reg. As far as I understand, the
> control_write() function only checks that vectors are not greater than
> the maximum allowed value, but does not check for 0.

How that? How could it even check for 0, when 0 isn't possible? Quoting
the code there:

    unsigned int vectors = min_t(uint8_t,
                                 1u << MASK_EXTR(val, PCI_MSI_FLAGS_QSIZE),
                                 pdev->msi_maxvec);

"val" in the guest written value. As that's used as a shift count, how
could 0 result there? The only way I can see 0 ending up in vectors is
when pdev->msi_maxvec was still zero. Yet that's then a bug in device
initialization.

Jan



 


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