On 10.12.2009, at 10:52, Alexander Graf wrote: > > On 10.12.2009, at 10:43, Michael S. Tsirkin wrote: > >> On Thu, Dec 10, 2009 at 07:16:04AM +0200, Muli Ben-Yehuda wrote: >>> On Wed, Dec 09, 2009 at 06:38:54PM +0100, Alexander Graf wrote: >>> >>>> While trying to get device passthrough working with an emulex hba, >>>> kvm refused to pass it through because it has a BAR of 256 bytes: >>>> >>>> Region 0: Memory at d2100000 (64-bit, non-prefetchable) [size=4K] >>>> Region 2: Memory at d2101000 (64-bit, non-prefetchable) [size=256] >>>> Region 4: I/O ports at b100 [size=256] >>>> >>>> Since the page boundary is an arbitrary optimization to allow 1:1 >>>> mapping of physical to virtual addresses, we can still take the old >>>> MMIO callback route. >>>> >>>> So let's add a second code path that allows for size & 0xFFF != 0 >>>> sized regions by looping it through userspace. >>> >>> That makes sense in general *but* the 4K-aligned check isn't just an >>> optimization, it also has a security implication. Consider the >>> theoretical case where has a multi-function device has BARs for two >>> functions on the same page (within a 4K boundary), and each function >>> is assigned to a different guest. With your current patch both guests >>> will be able to write to each other's BARs. Another case is where a >>> device has a bug and you must not write beyond the BAR or Bad Things >>> Happen. With this patch an *unprivileged* guest could exploit that bug >>> and make bad things happen. >>> >>> This can be fixed if the slow userspace mmio path checks that all MMIO >>> accesses by a guest fall within the portion of the page that is >>> assigned to it. >> >> This patch seems to implement range checks correctly, >> let me know if I am missing something. >> >> One also notes that we currently link qemu with libpci >> which I think requires admin cap to work. >> However, in the future we might extend this to >> also support getting device fds over a unix socket >> from a higher priviledged process. >> >> If or when this is done, we will have to be >> extra careful when passing >> device file descriptor to an unpriveledged qemu process if >> the BARs are less than full page in size: mapping >> such BAR will allow qemu access outside this BAR. >> >> A possible solution to this problem >> if/when it arises would be adding yet another sysfs file >> for each resource, which would allow read/write but not >> mmap access, and perform range checks in the kernel. > > Sounds like the best solution to this problem, yeah. Though we'd only need those for non-page-boundary BARs. So I guess the best would be to always export them from the kernel, but only use them when BAR & (PAGE_SIZE-1). Hm, or add read/write fd functions that always do boundary checks to the existing interface and only allow mmap on size & PAGE_SIZE. Or only allow non-aligned mmap when the admin cap is present. Alex-- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
