[FFmpeg-devel] [PATCH v4] avdevice/xcbgrab: check return values of xcb query functions

[Moritz Barsnick]

Since xcbgrab is getting some attention recently...

Fixes a segfault, as reported in #7312.

To reproduce:
Terminal 1:
$ Xvfb :1 -nolisten tcp -screen 0 800x600x24
Terminal 2:
$ ffmpeg -f x11grab -i :1 -f null -
or rather
$ gdb -ex r --args ffmpeg_g -f x11grab -i :1 -f null -
Then terminate Xvfb while ffmpeg is running.

Cheers,
Moritz
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From 3bbf40dd08bb67e993cca97880aec032644fd02b Mon Sep 17 00:00:00 2001
From: Moritz Barsnick <barsnick at gmx.net>
Date: Fri, 10 Jul 2020 13:26:55 +0200
Subject: [PATCH] avdevice/xcbgrab: check return values of xcb query functions

Fixes #7312, segmentation fault on close of X11 server

xcb_query_pointer_reply() and xcb_get_geometry_reply() can return NULL
if e.g. the X server closes or the connection is lost. This needs to
be checked in order to cleanly exit, because the returned pointers are
dereferenced later.

Furthermore, their return values need to be free()d, also in error
code paths.

Signed-off-by: Moritz Barsnick <barsnick at gmx.net>
---
 libavdevice/xcbgrab.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/libavdevice/xcbgrab.c b/libavdevice/xcbgrab.c
index 6f6b2dbf15..be4e0d14f9 100644
--- a/libavdevice/xcbgrab.c
+++ b/libavdevice/xcbgrab.c
@@ -346,8 +346,10 @@ static void xcbgrab_draw_mouse(AVFormatContext *s, AVPacket *pkt,
         return;

     cursor = xcb_xfixes_get_cursor_image_cursor_image(ci);
-    if (!cursor)
+    if (!cursor) {
+        free(ci);
         return;
+    }

     cx = ci->x - ci->xhot;
     cy = ci->y - ci->yhot;
@@ -425,7 +427,16 @@ static int xcbgrab_read_packet(AVFormatContext *s, AVPacket *pkt)
         pc  = xcb_query_pointer(c->conn, c->screen->root);
         gc  = xcb_get_geometry(c->conn, c->screen->root);
         p   = xcb_query_pointer_reply(c->conn, pc, NULL);
+        if (!p) {
+            av_log(c, AV_LOG_ERROR, "Failed to query xcb pointer\n");
+            return AVERROR_EXTERNAL;
+        }
         geo = xcb_get_geometry_reply(c->conn, gc, NULL);
+        if (!geo) {
+            av_log(c, AV_LOG_ERROR, "Failed to get xcb geometry\n");
+            free(p);
+            return AVERROR_EXTERNAL;
+        }
     }

     if (c->follow_mouse && p->same_screen)
--
2.26.2