Re: [PATCH-for-5.0 04/12] qga: Restrict guest-file-read count to 48 MB to avoid crashes

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

On 4/15/20 2:34 PM, Daniel P. Berrangé wrote:
> On Tue, Apr 14, 2020 at 03:30:44PM +0200, Philippe Mathieu-Daudé wrote:
>> On [*] 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 (CVE-2018-12617), 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 48 MB.
>>
>> [*] https://www.mail-archive.com/qemu-devel@nongnu.org/msg693176.html
>>
>> Fixes: CVE-2018-12617
>> 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       | 9 ++++++++-
>>   2 files changed, 12 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.
> 
> s/10/48/
> 
>>   #
>>   # @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)
> 
> s/10/48/

Oops I totally missed those, thanks!

> 
>>   #
>>   # Returns: @GuestFileRead on success.
>>   #
>> diff --git a/qga/commands.c b/qga/commands.c
>> index 5611117372..efc8b90281 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"
>> @@ -24,6 +25,12 @@
>>   #define GUEST_EXEC_MAX_OUTPUT (16*1024*1024)
>>   /* Allocation and I/O buffer for reading guest-exec out_data/err_data - 4KB */
>>   #define GUEST_EXEC_IO_SIZE (4*1024)
>> +/*
>> + * Maximum file size to read - 48MB
>> + *
>> + * (48MB + Base64 3:4 overhead = JSON parser 64 MB limit)
>> + */
>> +#define GUEST_FILE_READ_COUNT_MAX (48 * MiB)
>>   
>>   /* Note: in some situations, like with the fsfreeze, logging may be
>>    * temporarilly disabled. if it is necessary that a command be able
>> @@ -560,7 +567,7 @@ GuestFileRead *qmp_guest_file_read(int64_t handle, bool has_count,
>>       }
>>       if (!has_count) {
>>           count = QGA_READ_COUNT_DEFAULT;
>> -    } else if (count < 0 || count >= UINT32_MAX) {
>> +    } else if (count < 0 || count > GUEST_FILE_READ_COUNT_MAX) {
>>           error_setg(errp, "value '%" PRId64 "' is invalid for argument count",
>>                      count);
>>           return NULL;
> 
> With the docs typos fixed:
> 
>    Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
> 
> 
> Regards,
> Daniel
>