Re: [Xen-devel] [PATCH v2 for-4.6] libxl: handle read-only drives with qemu-xen

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Stefano Stabellini writes ("[PATCH v2 for-4.6] libxl: handle read-only drives 
with qemu-xen"):
> The current libxl code doesn't deal with read-only drives at all.
> 
> Upstream QEMU and qemu-xen only support read-only cdrom drives: make
> sure to specify "readonly=on" for cdrom drives and return error in case
> the user requested a non-cdrom read-only drive.

Acked-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>

I have determined by experiment that cdroms are always marked
readonly.  My qemu-dm opened a test iso, which I had expressed to be
passed as a cdrom, readonly.

So maybe the guest would be able to get qemu to write to it, but this
would just result in errors (or maybe a crash somewhere in qemu).  I
was not able to easily persuade Linux to write to the cdrom device.

> -                        (gc, 
> "if=ide,index=%d,media=cdrom,cache=writeback,id=ide-%i",
> -                         disk, dev_number);
> +                        (gc, 
> "if=ide,index=%d,readonly=%s,media=cdrom,cache=writeback,id=ide-%i",
> +                         disk, disks[i].readwrite ? "off" : "on", 
> dev_number);

I don't know what the effect of this would be on qemu iff
readwrite==0.  I think it might be ineffective.  But that is certainly
not a security problem.

I have just sent a draft of the advisory.

Ian.

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