Re: [XEN PATCH] tools/misc: xen-hvmcrash: Inject #DF instead of overwriting RIP

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 03/06/2024 3:59 pm, Matthew Barnes wrote:
> xen-hvmcrash would previously save records, overwrite the instruction
> pointer with a bogus value, and then restore them to crash a domain
> just enough to cause the guest OS to memdump.
>
> This approach is found to be unreliable when tested on a guest running
> Windows 10 x64, with some executions doing nothing at all.
>
> Another approach would be to trigger NMIs. This approach is found to be
> unreliable when tested on Linux (Ubuntu 22.04), as Linux will ignore
> NMIs if it is not configured to handle such.
>
> Injecting a double fault abort to all vCPUs is found to be more
> reliable at crashing and invoking memdumps from Windows and Linux
> domains.

Why every CPU?

We never did that before, and I don't see any it ought to be necessary
now either.



> diff --git a/tools/misc/xen-hvmcrash.c b/tools/misc/xen-hvmcrash.c
> index 1d058fa40a47..8ef1beb388f8 100644
> --- a/tools/misc/xen-hvmcrash.c
> +++ b/tools/misc/xen-hvmcrash.c
> @@ -38,22 +38,21 @@
>  #include <sys/stat.h>
>  #include <arpa/inet.h>
>  
> +#define XC_WANT_COMPAT_DEVICEMODEL_API

Please don't introduce this.  We want to purge it from the codebase, not
propagate it.

You want to open and use a libxendevicemodel handle.  (Sadly you also
need a xenctrl handle too, until we sort out the userspace ABIs).

>  #include <xenctrl.h>
>  #include <xen/xen.h>
>  #include <xen/domctl.h>
>  #include <xen/hvm/save.h>
>  
> +#define X86_ABORT_DF 8

#include <xen/asm/x86-defns.h>

and use X86_EXC_DF.

~Andrew