Re: [PATCH-for-5.0 v2 4/4] qga: Restrict guest-file-read count to 10 MB to avoid crashes

[Philippe Mathieu-Daudé]

On 3/31/20 4:15 PM, Daniel P. Berrangé wrote:
> On Tue, Mar 31, 2020 at 04:06:38PM +0200, Philippe Mathieu-Daudé wrote:
> > On https://www.mail-archive.com/address@hidden/msg693176.html
> > Daniel Berrangé commented:
> >
> >    The QEMU guest agent protocol is not sensible way to access huge
> >    files inside the guest. It requires the inefficient process of
> >    reading the entire data into memory than duplicating it again in
> >    base64 format, and then copying it again in the JSON serializer /
> >    monitor code.
> >
> >    For arbitrary general purpose file access, especially for large
> >    files, use a real file transfer program or use a network block
> >    device, not the QEMU guest agent.
> >
> > To avoid bug reports as BZ#1594054, follow his suggestion to put a
> > low, hard limit on "count" in the guest agent QAPI schema, and don't
> > allow count to be larger than 10 MB.
> >
> > Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1594054
> > Reported-by: Fakhri Zulkifli <address@hidden>
> > Suggested-by: Daniel P. Berrangé <address@hidden>
> > Signed-off-by: Philippe Mathieu-Daudé <address@hidden>
> > ---
> >   qga/qapi-schema.json | 6 ++++--
> >   qga/commands.c       | 6 +++++-
> >   2 files changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/qga/qapi-schema.json b/qga/qapi-schema.json
> > index f6fcb59f34..7758d9daf8 100644
> > --- a/qga/qapi-schema.json
> > +++ b/qga/qapi-schema.json
> > @@ -266,11 +266,13 @@
> >   ##
> >   # @guest-file-read:
> >   #
> > -# Read from an open file in the guest. Data will be base64-encoded
> > +# Read from an open file in the guest. Data will be base64-encoded.
> > +# As this command is just for limited, ad-hoc debugging, such as log
> > +# file access, the number of bytes to read is limited to 10 MB.
> >   #
> >   # @handle: filehandle returned by guest-file-open
> >   #
> > -# @count: maximum number of bytes to read (default is 4KB)
> > +# @count: maximum number of bytes to read (default is 4KB, maximum is 10MB)
> >   #
> >   # Returns: @GuestFileRead on success.
> >   #
> > diff --git a/qga/commands.c b/qga/commands.c
> > index 8ee1244ebb..c130d1b0f5 100644
> > --- a/qga/commands.c
> > +++ b/qga/commands.c
> > @@ -11,6 +11,7 @@
> >    */
> >   
> >   #include "qemu/osdep.h"
> > +#include "qemu/units.h"
> >   #include "guest-agent-core.h"
> >   #include "qga-qapi-commands.h"
> >   #include "qapi/error.h"
> > @@ -18,11 +19,14 @@
> >   #include "qemu/base64.h"
> >   #include "qemu/cutils.h"
> >   #include "qemu/atomic.h"
> > +#include "commands-common.h"
>
> This needs to be in the previous patch AFAICT.

Oops, thanks :)

> Regards,
> Daniel