[FFmpeg-devel] [PATCH v5] avdevice/xcbgrab: check return values of xcb query functions

[Moritz Barsnick]

On Sun, Jul 12, 2020 at 10:54:45 -0400, Andriy Gelman wrote:
> On Fri, 10. Jul 21:13, Moritz Barsnick wrote:
> > 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.
>
> The rest of the av_log calls use AVFormatContext*.
> You may want to update to be consistent.

Good point. I didn't mind the "xcbgrab indev" vs. "x11grab", but it's
indeed inconsistent. Updated patch attached.

Moritz
-------------- next part --------------
From 269b43209394c0eceb83f5ae384792c32305333a Mon Sep 17 00:00:00 2001
From: Moritz Barsnick <barsnick at gmx.net>
Date: Tue, 14 Jul 2020 14:07:33 +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..8bc320d055 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(s, 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(s, AV_LOG_ERROR, "Failed to get xcb geometry\n");
+            free(p);
+            return AVERROR_EXTERNAL;
+        }
     }

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