Problem: no locking around IPv6 prefix structures in prelist_remove

Bjoern A. Zeeb bz at
Mon May 26 09:25:08 UTC 2014

On 26 May 2014, at 09:11 , Steve Read < at> wrote:

> I have recently encountered an interesting double-free crash in prelist_remove() (management of IPv6 prefixes used by interface addresses) using a modified version of 9.2.  We've seen this once.
> It appears that two userland threads tried simultaneously to remove the last interface address that referenced a particular prefix, and both, therefore, tried to remove it from the global list of prefixes.  (Feel free to correct my interpretation of the purpose of prelist_remove and how it is invoked.)  One of them succeeded, and the other was left holding a chunk of free()ed memory, and crashed when trying to delete it.
> I looked at the code surrounding this function, and I can find no sign of locking around the prefix list or, indeed, anywhere in the call-stack (sys_ioctl=>kern_ioctl=>soo_ioctl==>ifi_ioctl=>in6_control=>prelist_remove). I looked in HEAD, and this part of the code appears to be more or less the same, in particular the question of locking.
> Should I submit a PR (no, we can't retry with a generic kernel)?

No need to for either.

markj@ has a patch to fix a good deal of racy prefix list locking which needs review and testing.

Bjoern A. Zeeb             "Come on. Learn, goddamn it.", WarGames, 1983

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