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CONFIG_EFI_DISABLE_PCI_DMA: Clear Busmaster bit on PCI bridges during ExitBootServices()

General informations

The Linux kernel configuration item CONFIG_EFI_DISABLE_PCI_DMA:

Help text

Disable the busmaster bit in the control register on all PCI bridges while calling ExitBootServices() and passing control to the runtime kernel. System firmware may configure the IOMMU to prevent malicious PCI devices from being able to attack the OS via DMA. However, since firmware can't guarantee that the OS is IOMMU-aware, it will tear down IOMMU configuration when ExitBootServices() is called. This leaves a window between where a hostile device could still cause damage before Linux configures the IOMMU again.

If you say Y here, the EFI stub will clear the busmaster bit on all PCI bridges before ExitBootServices() is called. This will prevent any malicious PCI devices from being able to perform DMA until the kernel reenables busmastering after configuring the IOMMU.

This option will cause failures with some poorly behaved hardware and should not be enabled without testing. The kernel commandline options "efi=disable_early_pci_dma" or "efi=no_disable_early_pci_dma" may be used to override this option.

Hardware

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