On Mon, Dec 2, 2019 at 4:41 PM Matthew Garrett <matthewgarrett@xxxxxxxxxx> wrote: > > Add an option to disable the busmaster bit in the control register on > all PCI bridges before 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 CONFIG_EFI_NO_BUSMASTER is enabled or the "disable_busmaster=1" > commandline argument is passed, 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. I hate to be an obnoxious bikeshedder, but I really dislike the "disable_busmaster" name. I read this and $SUBJECT as "for some reason, the admin wants to operate the system with busmastering off". What you really want is something more like "disable busmastering before IOMMU initialization". Maybe "iommu.disable_busmaster_before_init"? Similarly, EFI_NO_BUSMASTER sounds like a permanent state of affairs. Would a similar patch apply to non-EFI boot? That is, in a BIOS boot, is busmastering on when the kernel is loaded? --Andy
