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

[Michael Roth]

Quoting Philippe Mathieu-Daudé (2020-04-15 08:02:18)
> 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!

I've rolled in the doc fix-ups and sent a pull request for patches 1-4

> 
> > 
> >>   #
> >>   # 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
> > 
>