Re: [PATCH] lsm: copy comm before calling audit_log to avoid race in string printing

From: James Morris
Date: Tue Apr 14 2015 - 19:55:50 EST

On Tue, 14 Apr 2015, Richard Guy Briggs wrote:

> When task->comm is passed directly to audit_log_untrustedstring() without
> getting a copy or using the task_lock, there is a race that could happen that
> would output a NULL (\0) in the middle of the output string that would
> effectively truncate the rest of the report text after the comm= field in the
> audit log message, losing fields.
> Using get_task_comm() to get a copy while acquiring the task_lock to prevent
> this and to prevent the result from being a mixture of old and new values of
> comm would incur potentially unacceptable overhead, considering that the value
> can be influenced by userspace and therefore untrusted anyways.
> Copy the value before passing it to audit_log_untrustedstring() ensures that a
> local copy is used to calculate the length *and* subsequently printed. Even if
> this value contains a mix of old and new values, it will only calculate and
> copy up to the first NULL, preventing the rest of the audit log message being
> truncated.
> Use a second local copy of comm to avoid a race between the first and second
> calls to audit_log_untrustedstring() with comm.
> Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Richard Guy Briggs <rgb@xxxxxxxxxx>


James Morris

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