On 01/11/2018 16:50, Jan Beulich wrote: >>>> Juergen Gross <jgross@xxxxxxxx> 11/01/18 3:23 PM >>> >> On 01/11/2018 15:18, Jan Beulich wrote: >>>>>> Juergen Gross <jgross@xxxxxxxx> 11/01/18 1:34 PM >>> >>>> Currently the size of hypercall buffers allocated via >>>> /dev/xen/hypercall is limited to a default of 64 memory pages. For live >>>> migration of guests this might be too small as the page dirty bitmask >>>> needs to be sized according to the size of the guest. This means >>>> migrating a 8GB sized guest is already exhausting the default buffer >>>> size for the dirty bitmap. >>>> >>>> There is no sensible way to set a sane limit, so just remove it >>>> completely. The device node's usage is limited to root anyway, so there >>>> is no additional DOS scenario added by allowing unlimited buffers. >>> >>> But is this setting of permissions what we want long term? What about a >>> de-privileged qemu, which still needs to be able to issue at least dm-op >>> hypercalls? >> >> Wouldn't that qemu have opened the node while still being privileged? > > Possibly, but how does this help? As soon as it's unprivileged it must not > be able to hog resources anymore. > > Anyway, with Andrew's reply my point may be irrelevant, but I have to > admit I'm not entirely sure. I guess we want Xen tools to close /dev/xen/hypercall (or more precise: don't dup2() it) when qemu is de-privileging itself. This will make it very clear that it can't hog memory via mmap(). When you are fine with that I'll send a Xen patch for this. Juergen
