Re: [Qemu-devel] [PATCH] vnc: tight: Fix crash after 2GB of output

[Michael Tokarev <mjt@tls.msk.ru>]

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04.03.2011 11:56, Corentin Chary wrote:
>>>
>>>     bytes = zstream->total_out - previous_out;
> 
> Good catch
> 
>> total_out isn't used by zlib internally, so if the resulting
>> "total" counter is not needed in qemu, we can just zero-out
>> the total_out in this function before calling zlib, and
>> use the resulting value directly as "bytes", without
>> saving its previous value in previous_out.  Something like
>> the attached patch does.
> 
> If you're certain that total_out is not used by zlib, could you also
> send a patch for zlib encoding please ? (vnc-enc-zlib.c)
> Thanks,

Yes, I noticed this too (the same code is in enc-zlib), and mentioned
this in my previous email.

The attached slightly different patch fixes both places and fixes
them for good (hopefully anyway).  Runtime-tested for the tight
case, but honestly, I didn't wait for 2G of output ;)

Thanks!

/mjt

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fix 2Gb integer overflow in in VNC tight and zlib encodings

As found by Roland Dreier <roland@purestorage.com> (excellent
catch!), when amount of VNC compressed data produced by zlib
and sent to client exceeds 2Gb, integer overflow occurs because
currently, we calculate amount of data produced at each step by
comparing saved total_out with new total_out, and total_out is
something which grows without bounds.  Compare it with previous
avail_out instead of total_out, and leave total_out alone.

The same code is used in vnc-enc-tight.c and vnc-enc-zlib.c,
so fix both cases.

There, there's no actual need to save previous_out value, since
capacity-offset (which is how that value is calculated) stays
the same so it can be recalculated again after call to deflate(),
but whole thing becomes less readable this way.

Reported-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

diff --git a/ui/vnc-enc-tight.c b/ui/vnc-enc-tight.c
index 2522936..87fdf35 100644
--- a/ui/vnc-enc-tight.c
+++ b/ui/vnc-enc-tight.c
@@ -868,8 +868,8 @@ static int tight_compress_data(VncState *vs, int stream_id, size_t bytes,
     zstream->avail_in = vs->tight.tight.offset;
     zstream->next_out = vs->tight.zlib.buffer + vs->tight.zlib.offset;
     zstream->avail_out = vs->tight.zlib.capacity - vs->tight.zlib.offset;
+    previous_out = zstream->avail_out;
     zstream->data_type = Z_BINARY;
-    previous_out = zstream->total_out;
 
     /* start encoding */
     if (deflate(zstream, Z_SYNC_FLUSH) != Z_OK) {
@@ -878,7 +878,8 @@ static int tight_compress_data(VncState *vs, int stream_id, size_t bytes,
     }
 
     vs->tight.zlib.offset = vs->tight.zlib.capacity - zstream->avail_out;
-    bytes = zstream->total_out - previous_out;
+    /* ...how much data has actually been produced by deflate() */
+    bytes = previous_out - zstream->avail_out;
 
     tight_send_compact_size(vs, bytes);
     vnc_write(vs, vs->tight.zlib.buffer, bytes);
diff --git a/ui/vnc-enc-zlib.c b/ui/vnc-enc-zlib.c
index 3c6e6ab..e32e4cd 100644
--- a/ui/vnc-enc-zlib.c
+++ b/ui/vnc-enc-zlib.c
@@ -103,8 +103,8 @@ static int vnc_zlib_stop(VncState *vs)
     zstream->avail_in = vs->zlib.zlib.offset;
     zstream->next_out = vs->output.buffer + vs->output.offset;
     zstream->avail_out = vs->output.capacity - vs->output.offset;
+    previous_out = zstream->avail_out;
     zstream->data_type = Z_BINARY;
-    previous_out = zstream->total_out;
 
     // start encoding
     if (deflate(zstream, Z_SYNC_FLUSH) != Z_OK) {
@@ -113,7 +113,7 @@ static int vnc_zlib_stop(VncState *vs)
     }
 
     vs->output.offset = vs->output.capacity - zstream->avail_out;
-    return zstream->total_out - previous_out;
+    return previous_out - zstream->avail_out;
 }
 
 int vnc_zlib_send_framebuffer_update(VncState *vs, int x, int y, int w, int h)