render: Fix out of bounds access in SProcRenderCompositeGlyphs()
ZDI-CAN-14192, CVE-2021-4008
This vulnerability was discovered and the fix was suggested by:
Jan-Niklas Sohn working with Trend Micro Zero Day Initiative
Signed-off-by: Povilas Kanapickas <povilas@radix.lt>
(cherry picked from commit ebce7e2d80)
			
			
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				|  | @ -2309,6 +2309,9 @@ SProcRenderCompositeGlyphs(ClientPtr client) | |||
| 
 | ||||
|         i = elt->len; | ||||
|         if (i == 0xff) { | ||||
|             if (buffer + 4 > end) { | ||||
|                 return BadLength; | ||||
|             } | ||||
|             swapl((int *) buffer); | ||||
|             buffer += 4; | ||||
|         } | ||||
|  | @ -2319,12 +2322,18 @@ SProcRenderCompositeGlyphs(ClientPtr client) | |||
|                 buffer += i; | ||||
|                 break; | ||||
|             case 2: | ||||
|                 if (buffer + i * 2 > end) { | ||||
|                     return BadLength; | ||||
|                 } | ||||
|                 while (i--) { | ||||
|                     swaps((short *) buffer); | ||||
|                     buffer += 2; | ||||
|                 } | ||||
|                 break; | ||||
|             case 4: | ||||
|                 if (buffer + i * 4 > end) { | ||||
|                     return BadLength; | ||||
|                 } | ||||
|                 while (i--) { | ||||
|                     swapl((int *) buffer); | ||||
|                     buffer += 4; | ||||
|  |  | |||
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