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

[Philippe Mathieu-Daudé]

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/address@hidden/msg693176.html
> >
> > Fixes: CVE-2018-12617
> > 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       | 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 <address@hidden>
>
>
> Regards,
> Daniel