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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Mar 31, 2020 at 04:06:38PM +0200, Philippe Mathieu-Daudé wrote:
> On https://www.mail-archive.com/qemu-devel@nongnu.org/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 <mohdfakhrizulkifli@gmail.com>
> Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>  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.


Regards,
Daniel
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