Messages in this thread Patch in this message | | | Date | Wed, 1 Dec 2010 16:30:01 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH v2] do_exit(): Make sure we run with get_fs() == USER_DS. |
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On Wed, 1 Dec 2010 11:50:32 +0900 (JST) KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
> > If a user manages to trigger an oops with fs set to KERNEL_DS, fs is not > > otherwise reset before do_exit(). do_exit may later (via mm_release in fork.c) > > do a put_user to a user-controlled address, potentially allowing a user to > > leverage an oops into a controlled write into kernel memory. > > > > A more logical place to put this might be when we know an oops has occurred, > > before we call do_exit(), but that would involve changing every architecture, in > > multiple places. Let's just stick it in do_exit instead. > > > > Signed-off-by: Nelson Elhage <nelhage@ksplice.com> > > --- > > kernel/exit.c | 8 ++++++++ > > 1 files changed, 8 insertions(+), 0 deletions(-) > > > > diff --git a/kernel/exit.c b/kernel/exit.c > > index 21aa7b3..68899b3 100644 > > --- a/kernel/exit.c > > +++ b/kernel/exit.c > > @@ -914,6 +914,14 @@ NORET_TYPE void do_exit(long code) > > if (unlikely(!tsk->pid)) > > panic("Attempted to kill the idle task!"); > > > > + /* > > + * If do_exit is called because this processes oopsed, it's possible > > + * that get_fs() was left as KERNEL_DS, so reset it to USER_DS before > > + * continuing. This is relevant at least for clearing clear_child_tid in > > + * mm_release. > > + */ > > + set_fs(USER_DS); > > "This is relevant" is no good explanation ;) > Please recognize this is tricky code and Please consider to write more > careful and looooong comments.
I've seen worse comments. And occasionally none at all :)
Is this better?
--- a/kernel/exit.c~do_exit-make-sure-we-run-with-get_fs-==-user_ds-fix +++ a/kernel/exit.c @@ -917,8 +917,9 @@ NORET_TYPE void do_exit(long code) /* * If do_exit is called because this processes oopsed, it's possible * that get_fs() was left as KERNEL_DS, so reset it to USER_DS before - * continuing. This is relevant at least for clearing clear_child_tid in - * mm_release. + * continuing. Amongst other possible reasons, this is to prevent + * mm_release()->clear_child_tid() from writing to a user-controlled + * kernel address. */ set_fs(USER_DS); _
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