[PULL for-5.0 3/5] qga-win: prevent crash when executing guest-file-read with large count

[Michael Roth <mdroth@linux.vnet.ibm.com>]

From: Basil Salman <basil@daynix.com>

guest-file-read command is currently implemented to read from a
file handle count number of bytes. when executed with a very large count number
qemu-ga crashes.
after some digging turns out that qemu-ga crashes after trying to allocate
a buffer large enough to save the data read in it, the buffer was allocated using
g_malloc0 which is not fail safe, and results a crash in case of failure.
g_malloc0 was replaced with g_try_malloc0() which returns NULL on failure,
A check was added for that case in order to prevent qemu-ga from crashing
and to send a response to the qemu-ga client accordingly.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1594054

Signed-off-by: Basil Salman <basil@daynix.com>
Reported-by: Fakhri Zulkifli <mohdfakhrizulkifli@gmail.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
---
 qga/commands-win32.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/qga/commands-win32.c b/qga/commands-win32.c
index 9c744d6405..b49920e201 100644
--- a/qga/commands-win32.c
+++ b/qga/commands-win32.c
@@ -343,7 +343,13 @@ GuestFileRead *qmp_guest_file_read(int64_t handle, bool has_count,
     }
 
     fh = gfh->fh;
-    buf = g_malloc0(count+1);
+    buf = g_try_malloc0(count + 1);
+    if (!buf) {
+        error_setg(errp,
+                   "failed to allocate sufficient memory "
+                   "to complete the requested service");
+        return NULL;
+    }
     is_ok = ReadFile(fh, buf, count, &read_count, NULL);
     if (!is_ok) {
         error_setg_win32(errp, GetLastError(), "failed to read file");
-- 
2.17.1