[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 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"
 
 /* Maximum captured guest-exec out_data/err_data - 16MB */
 #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 - 10MB */
+#define GUEST_FILE_READ_COUNT_MAX (10 * MiB)
 
 /* Note: in some situations, like with the fsfreeze, logging may be
  * temporarilly disabled. if it is necessary that a command be able
@@ -559,7 +563,7 @@ 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;
-- 
2.21.1