>>> On 01.11.18 at 17:27, <jgross@xxxxxxxx> wrote: > 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. If that doesn't prevent the process from making the hypercalls it is permitted to do (I have to admit I don't recall if there are any still needed besides the dmop ones), sure. Jan
