xcb: Fix access to shm for X server running from another user

Use ShmCreateSegment call, that was added in MIT-SHM 1.2, to
create shared memory segments on the server side. It returns
a POSIX shared memory object that is used to mmap memory.
It's in effect a file descriptor that is passed through the
X server socket and thus avoids permission checks.
On the other hand this scheme is more secure, because the
file descriptor, and thus the shared memory, are accessible
only by the X server and the application.

Task-number: QTBUG-46017
Change-Id: I202eead9d01aee2ab5b65f4f74f4c13da7cb2239
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>

1 files changed, 9 insertions, 0 deletions

diff --git a/src/plugins/platforms/xcb/qxcbconnection.cpp b/src/plugins/platforms/xcb/qxcbconnection.cpp
index e8bb97c6af..d0868f44c0 100644
--- a/src/plugins/platforms/xcb/qxcbconnection.cpp
+++ b/src/plugins/platforms/xcb/qxcbconnection.cpp
@@ -2103,6 +2103,15 @@ void QXcbConnection::initializeShm()
     }
 
     has_shm = true;
+
+    auto shm_query = Q_XCB_REPLY(xcb_shm_query_version, m_connection);
+    if (!shm_query) {
+        qWarning("QXcbConnection: Failed to request MIT-SHM version");
+        return;
+    }
+
+    has_shm_fd = (shm_query->major_version == 1 && shm_query->minor_version >= 2) ||
+                  shm_query->major_version > 1;
 }
 
 void QXcbConnection::initializeXFixes()