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oss-security: [oss-security] Interesting kernel bug

[oss-security] Interesting kernel bug

From: Dan Rosenberg <dan.j.rosenberg_at_nospam>
Date: Fri Sep 24 2010 - 20:05:44 GMT
To: oss-security@lists.openwall.com

A bug I found was just fixed upstream:

Disregard the commit statement's mention of a reliable trigger, since
none exists - a result of a combination of miscommunication and
careless reporting on my part.

The bug was introduced in May 2010, and affects >=, so no
distros would appear to be affected. In 32-bit compatibility mode,
when invoking the readv() or writev() syscalls, if the provided user
pointer and length result in an access_ok() check failing, then an
uninitialized pointer on the stack will be kfree()'d. This is likely
to be an exploitable condition (for example, via pre-initializing the
stack with other carefully chosen syscalls, allowing control of the

It came up during discussion that on x86-64, the access_ok() will
never fail, because there's no way for a user running in 32-bit mode
to supply an address that's outside of userspace address range.
However, it's possible that this may be triggerable on other
architectures that I know less about. S390 was mentioned at one

Anyone who knows more about miscellaneous architectures and their
address space segmentations? Perhaps it affects someone after all.
As of now, I don't think this could be considered a security issue
since it appears to be completely not exploitable, but maybe someone
more knowledgeable could shed more light on the issue.