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Re: [PATCH v2 1/2] x86/pci: prevent cross-device accesses in pci_mmcfg_{read,write}()
- To: Roger Pau Monne <roger@xxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 7 Aug 2026 08:22:44 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Fri, 07 Aug 2026 06:23:01 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 06.08.2026 17:26, Roger Pau Monne wrote:
> Introduce a specific check that prevents an accesses from spilling across
> two devices.
>
> Signed-off-by: Roger Pau Monné <roger@xxxxxxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
albeit with a remark:
> --- a/xen/arch/x86/x86_64/mmconfig_64.c
> +++ b/xen/arch/x86/x86_64/mmconfig_64.c
> @@ -61,7 +61,8 @@ int pci_mmcfg_read(unsigned int seg, unsigned int bus,
> char __iomem *addr;
>
> /* Why do we have this when nobody checks it. How about a BUG()!? -AK */
> - if (unlikely((bus > 255) || (devfn > 255) || (reg > 4095))) {
> + if (unlikely((bus > 255) || (devfn > 255) ||
> + (reg + len > PCI_CFG_SPACE_EXP_SIZE))) {
> err: *value = -1;
> return -EINVAL;
> }
> @@ -91,7 +92,8 @@ int pci_mmcfg_write(unsigned int seg, unsigned int bus,
> char __iomem *addr;
>
> /* Why do we have this when nobody checks it. How about a BUG()!? -AK */
> - if (unlikely((bus > 255) || (devfn > 255) || (reg > 4095)))
> + if (unlikely((bus > 255) || (devfn > 255) ||
> + (reg + len > PCI_CFG_SPACE_EXP_SIZE)))
> return -EINVAL;
>
> addr = pci_dev_base(seg, bus, devfn);
In both cases the unlikely() uses won't have the intended effect, from all I
know. They would help as used here only if the compiler managed to fold all
three parts of the ||-expression into a single conditional branch, which I
don't think it would end up doing.
Jan
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