This script segfaults:
##
import _pickle
obj = _pickle.Pickler(open("/bin/ls")) #can be open(__file__) for scripts
try: obj.__init__('pouet', 87)
except Exception as err: pass

obj.dump(0)
###


[Switching to Thread -1210775360 (LWP 19096)]
0xb79fbf91 in pickler_write (self=0xb7a2fe4c, s=0xbff441a1 "...", n=2)
    at /home/ajaksu/py3k/Modules/_pickle.c:442
442                 memcpy(self->write_buf + self->buf_size, s, n);
(gdb) backtrace
#0  0xb79fbf91 in pickler_write (self=0xb7a2fe4c, s=0xbff441a1 "...",
    n=2) at /home/ajaksu/py3k/Modules/_pickle.c:442
#1  0xb7a00a8c in dump (self=0xb7a2fe4c, obj=0x81fbd78) at
/home/ajaksu/py3k/Modules/_pickle.c:2288
#2  0xb7a00bb8 in Pickler_dump (self=0xb7a2fe4c, args=0xb7b30034) at
/home/ajaksu/py3k/Modules/_pickle.c:2328
#3  0x081626b1 in PyCFunction_Call (func=0xb796c3ec, arg=0xb7b30034,
kw=0x0) at Objects/methodobject.c:81
#4  0x080b378f in call_function (pp_stack=0xbff442e4, oparg=1) at
Python/ceval.c:3403
#5  0x080ae8d2 in PyEval_EvalFrameEx (f=0x829bafc, throwflag=0) at
Python/ceval.c:2205
#6  0x080b1c24 in PyEval_EvalCodeEx (co=0xb7acf2c8, globals=0xb7a9a8f4,
locals=0xb7a9a8f4, args=0x0, argcount=0, kws=0x0,
    kwcount=0, defs=0x0, defcount=0, kwdefs=0x0, closure=0x0) at
Python/ceval.c:2840

Found using Fusil.

pickler_write() has no check for self->write_buf == NULL

Suggested patch:
===================================================================
--- Modules/_pickle.c   (Revision 66010)
+++ Modules/_pickle.c   (Arbeitskopie)
@@ -421,6 +421,10 @@
 {
     PyObject *data, *result;

+    if (self->write_buf == NULL) {
+        PyErr_SetString(PyExc_SystemError, "Invalid write buffer");
+        return -1;
+    }
     if (s == NULL) {
         if (!(self->buf_size))
             return 0;

Oh, that's nasty. Recalling __init__ with bad arguments breaks the
internal invariants as it clears the Pickler's content before parsing
the arguments. I suspect that Unpickler is vulnerable too.

Adding a NULL check in pickler_write will only fix this particular
example. I could probably find another crash example that doesn't use
pickler_write.

Unpickler looks safe as Unpickler_load() checks if Unpickler was
properly initialized. And only Pickler_dump is vulnerable right now (new
methods, if any, exposed for issue3385 will have to take into account
this vulnerability).

I will try to find time next weekend to fix this (and other pickle 
blockers).

Here's the fix. The added check in Pickler_dump should prevent any
segfaults due to __init__() errors. 

I also added the check proposed by Christian as a safe-guard in case a
core developer adds a new method that doesn't check if the object was
properly initialized.

Rather than attach a full _pickle.c file, please generate a unified diff
with just your changes.  The patch should include a test for the
crashing condition.  If you can upload that I'll try to accept it for
rc3.  Deferring for now.

Oops. I must have been quite tired when I submitted that.

Here's the patch for the fix and the test case.

The patch is fine.

Amaury, please apply the patch and close the issue.  Thanks!

Committed r66963.