[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] Why cannot HVM open a file as a floopy disk?
Akio Takebe writes ("[Xen-devel] Why cannot HVM open a file as a floopy disk?"): > I found a issue which we cannot open a file as a floppy disk in HVM guest. > If I specify fda="/dev/null", I can boot the HVM guest. > But if I specify fda="/root/floppy.img", > I cannot boot the HVM guest and xend hungs up. Unfortunately, it is not possible to autodetect raw formats safely. > Why did you need to change the line? > If it's not necessary, is the following patch acceptable? Here is a description of the problem which my patch fixes: Consider a raw disk image file which is writeable by a guest. (This is of course one very common usage model.) The guest can write anything it likes to the image file, including anything to the start of the file - where the cow header would be if it were a cow file. So it can, if it likes, write a cow header (qcow2 for example) to the start of its `virtual disk image'. Qemu's cow headers contain the pathname of the backing file, and the guest can of course name any file it likes. If this image, which is supposedly a raw image, is then opened by any tool which autoguesses the format, that tool will then spot the cow header written by the guest and access the backing file (in the context of the host) specified by the guest. Depending on the exact circumstances this can allow the guest to get copies of or even complete read access to any data of its choice in the host. Upstream qemu have fixed this problem in a half-hearted way and evidently their qemu-img is still vulnerable. We have changed the format-determination code in block.c so that any attempt to autodetect a format never returns `raw'; that means that any vulnerable code anywhere is instantly fixed although it may break some existing usages in cases where we haven't properly plumbed through a specification of the image format. Does anyone use Xen with non-raw floppy disk images in disk files ? If not then we can fix the problem by having the floppy driver device model explictly specify bdrv_raw to the block layer, eliminating the format auto-gessing. Ian. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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