On 10.12.2009, at 11:27, Michael S. Tsirkin wrote: > On Thu, Dec 10, 2009 at 11:08:58AM +0100, Alexander Graf wrote: >> >> 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 > > This might break existing applications. > We don't want that.
Well currently you can't mmap the resource at all without at least r/w rights on the file, right? But yeah, we'd probably change behavior that could break someone - sigh. Alex-- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
