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

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

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/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.

Oops, thanks :)

> 
> 
> Regards,
> Daniel
>