gh-143637: Fix re-entrant mutation of ancillary data in socket.sendmsg() #143892

priyanshu2282-cyber · 2026-01-15T20:43:38Z

Fix a crash in socket.sendmsg() caused by re-entrant mutation of ancillary data during argument parsing.
Hold a strong reference to each ancillary item while parsing to avoid use-after-free, and added a regression test.

Issue: Heap out-of-bound read in socket.sendmsg ancillary parser after re-entrant __index__ clears the control list #143637

priyanshu2282-cyber · 2026-01-15T21:43:05Z

Hi, this PR fixes an internal correctness issue in socket.sendmsg().
Would it be appropriate to add the skip news label?

picnixz · 2026-01-17T06:03:39Z

No. It remains uaerfacing. In general, every bug fix should be announced if it can be seen only with pure python code and the public API

picnixz · 2026-01-17T06:04:33Z

Modules/socketmodule.c

-#else
-        if (!get_CMSG_LEN(bufsize, &space)) {
-#endif
+        if(!get_CMSG_SPACE(bufsize, &space)){


Why this change??

This value is only used to decide how much memory to allocate for msg.msg_control.
get_CMSG_LEN() can give a smaller size on some systems because it doesnt include extra padding that the OS needs which means the buffer can be too small.
get_CMSG_SPACE() includes this padding, so it correctly calculates how much space is needed. This function already uses get_CMSG_SPACE() elsewhere for the same reason, so using it here keeps the logic consistent and safe.

The function is not necessarily present though?

get_CMSG_SPACE() here is not the platform macro. It’s a CPython helper defined in socketmodule.c itself, so it is always available when this code is compiled. Internally, that helper already handles platform differences and falls back appropriately when CMSG_SPACE is not provided by the system. So using get_CMSG_SPACE() unconditionally here doesn’t introduce a new dependency, it just reuses the same helper that this file already relies on in other places.

I still do not understand why this was modified in this PR? is it affecting the issue? has it been the LLM who decided on it? never use LLM for writing your code if you do not understand why it wrote this or that.

after revisiting the issue, I realizd it doesn’t actually relate to the re-entrant mutation bug. I’ve reverted it and kept the PR focused. Thanks, this helped me understand the problem much better.

Can you answer questions when I ask them? I asked whether it was an LLM who did this. If so, stop using an LLM without understanding its changes as it is against our rules. I will also close PRs that are LLM-aided but not understood.

I should have answered that directly I am sorry about that, yes I used LLM, without fully understanding that was a mistake on my part.
I understand your concern and won’t include changes I can’t clearly justify going forward.

picnixz · 2026-01-17T06:04:40Z

Modules/socketmodule.c

            goto finally;
        }
-        controllen += space;
+        controllen+=space;


Why this change?

Good catch thanks, this line change is incidental and not required for the fix.
I’ll revert it.

picnixz · 2026-01-17T06:05:01Z

Lib/test/test_socket.py

    # involve recvmsg() or recvmsg_into().
+    @unittest.skipUnless(hasattr(socket.socket, "sendmsg"),
+                     "sendmsg not supported")
+    def _test_sendmsg_reentrant_ancillary_mutation(self):


Why a private method?

This is a helper method rather than a standalone test because it depends on the surrounding test class setup and socket lifecycle, and is not meant to be picked up directly by unittest discovery.
The skipUnless(sendmsg) decorator is placed here so the helper only runs on platforms that support sendmsg(),with execution still controlled by the enclosing test logic.
I can add a small public test_* wrapper calling this helper if that’s prefferable.

But where is it tested then?

Sorry for the confusion, you are right, I have missed it- I have added a public test method that invokes _test_sendmsg_reentrant_ancillary_mutation in my updated PR is it correct?, Thanks for the clarification.

Just rename _test_sendmsg_reentrant_ancillary_mutation() to test_sendmsg_reentrant_ancillary_mutation().

bedevere-app · 2026-01-17T06:05:14Z

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

priyanshu2282-cyber · 2026-01-17T08:46:14Z

Thanks for guiding, I have added the news entry.

priyanshu2282-cyber · 2026-01-17T09:40:48Z

I have made the requested changes; please review again.

bedevere-app · 2026-01-17T09:40:53Z

Thanks for making the requested changes!

@picnixz: please review the changes made to this pull request.

…nshu2282-cyber/cpython into fix-sendmsg-reentrant-cmsg

vstinner · 2026-01-19T13:06:03Z

Lib/test/test_socket.py

    # involve recvmsg() or recvmsg_into().
+    @unittest.skipUnless(hasattr(socket.socket, "sendmsg"),
+                     "sendmsg not supported")
+    def _test_sendmsg_reentrant_ancillary_mutation(self):


Just rename _test_sendmsg_reentrant_ancillary_mutation() to test_sendmsg_reentrant_ancillary_mutation().

vstinner · 2026-01-19T13:09:54Z

Modules/socketmodule.c

+    }
+    ncmsgs = PyTuple_GET_SIZE(cmsg_fast);


The indentation looks wrong.

vstinner · 2026-01-19T13:10:15Z

Modules/socketmodule.c

+                        "(iiy*):[sendmsg() ancillary data items]",
+                        &cmsgs[ncmsgbufs].level,
+                        &cmsgs[ncmsgbufs].type,
+                        &cmsgs[ncmsgbufs].data)){


The indentation looks wrong, it was correct in the existing code.

vstinner · 2026-01-19T13:13:50Z

Lib/test/test_socket.py

+        self._test_sendmsg_reentrant_ancillary_mutation()
+
+    def _test_sendmsg_reentrant_ancillary_mutation(self):
+        import socket


it's already imported at the module top level.

vstinner · 2026-01-19T13:14:13Z

Lib/test/test_socket.py

+    def _test_sendmsg_reentrant_ancillary_mutation(self):
+        import socket
+
+        seq = []


no need to declare it before Mut, you can declare after Mut declaration.

vstinner · 2026-01-19T13:14:51Z

Lib/test/test_socket.py

+
+        class Mut:
+            def __init__(self):
+                self.tripped = False


I don't think that we need this tripped flag machinery, just always call seq.clear() in __index__(), no?

vstinner · 2026-01-19T15:47:58Z

Lib/test/test_socket.py

            pass


+class SendmsgReentrancyTests(unittest.TestCase):


Why not adding the test to SendmsgStreamTests?

I have tried it but the test are failing so I decided it move it into a seperate class.

What was the error?

AttributeError: 'SendmsgUnixStreamTest' object has no attribute '_test_sendmsg_reentrant_ancillary_mutation'. Did you mean: 'test_sendmsg_reentrant_ancillary_mutation'?

AttributeError: 'SendmsgTCPTest' object has no attribute '_test_sendmsg_reentrant_ancillary_mutation'. Did you mean: 'test_sendmsg_reentrant_ancillary_mutation'?

got these two errors instead removing private method completely.

According to what I got is SendmsgStreamTests the class adds _ prefix internally before the method name, just to confirm. I mean by looking the class, Is this class expects _test_* implementation for each test_* method.

Ah yes, these tests use one function for the server, and one function for the client. Since you expect an error before actually sending data to the peer, I suggest moving this test to the generic GeneralModuleTests class.

Thanks for the clarification, I have made the test available in GeneralModuleTests class. Happy to make any further adjustments as needed.

Modules/socketmodule.c

Co-authored-by: Victor Stinner <[email protected]>

Modules/socketmodule.c

Co-authored-by: Victor Stinner <[email protected]>

priyanshu2282-cyber added 2 commits January 16, 2026 02:06

socket: fix re-entrant mutation in sendmsg ancillary data

597c672

test_socket: add regression test for sendmsg re-entrancy

7e57097

bedevere-app bot added the awaiting review label Jan 15, 2026

bedevere-app bot mentioned this pull request Jan 15, 2026

Heap out-of-bound read in socket.sendmsg ancillary parser after re-entrant __index__ clears the control list #143637

Open

pythongh-143637: Fix reentrant mutation crash in socket.sendmsg

a6fbd4b

chore: rerun CI

be7371d

picnixz requested changes Jan 17, 2026

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bedevere-app bot removed the awaiting review label Jan 17, 2026

bedevere-app bot added the awaiting changes label Jan 17, 2026

priyanshu2282-cyber and others added 2 commits January 17, 2026 14:06

restore spacing style for controllen calculation

4703c2b

📜🤖 Added by blurb_it.

2bd7e14

bedevere-app bot added awaiting change review and removed awaiting changes labels Jan 17, 2026

bedevere-app bot requested a review from picnixz January 17, 2026 09:40

priyanshu2282-cyber added 5 commits January 17, 2026 21:27

add public wrapper for sendmsg re-entrant ancillary mutation test

710daaa

Merge branch 'fix-sendmsg-reentrant-cmsg' of https://github.com/priya…

a2992a8

…nshu2282-cyber/cpython into fix-sendmsg-reentrant-cmsg

fixed trailing whitespace

75d8ae9

PySequence_Tuple instead of PySequence_Fast and removed INCREF, DECREF

6f0ddf4

Fix trailing whitespace

ff8af98

tonghuaroot mentioned this pull request Jan 18, 2026

gh-143988: tation crashes in socket sendmsg/recvmsg_into via __buffer__ #143987

Open

Fix sendmsg re-entrant ancillary mutation crash

5f0a05a

vstinner reviewed Jan 19, 2026

View reviewed changes

Corrected Indentation and moved test to seperate class

082e6b7

vstinner reviewed Jan 19, 2026

View reviewed changes

Modules/socketmodule.c Outdated Show resolved Hide resolved

Update Modules/socketmodule.c

fb99129

Co-authored-by: Victor Stinner <[email protected]>

priyanshu2282-cyber requested a review from vstinner January 20, 2026 10:20

vstinner reviewed Jan 20, 2026

View reviewed changes

Modules/socketmodule.c Outdated Show resolved Hide resolved

priyanshu2282-cyber and others added 2 commits January 20, 2026 17:43

Update Modules/socketmodule.c

7ec2dd6

Co-authored-by: Victor Stinner <[email protected]>

Move test to GeneralModuleTests

57b33f1

priyanshu2282-cyber requested a review from vstinner January 20, 2026 13:35

Uh oh!

gh-143637: Fix re-entrant mutation of ancillary data in socket.sendmsg() #143892

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gh-143637: Fix re-entrant mutation of ancillary data in socket.sendmsg() #143892

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