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Date: Wed, 1 Nov 2017 09:05:42 +0100 From: Ingo Molnar <mingo@...nel.org> To: Thomas Gleixner <tglx@...utronix.de> Cc: dvyukov@...gle.com, hpa@...or.com, linux-kernel@...r.kernel.org, bot+2af19c9e1ffe4d4ee1d16c56ae7580feaee75765@...kaller.appspotmail.com, torvalds@...ux-foundation.org, peterz@...radead.org, gratian.crisan@...com, linux-tip-commits@...r.kernel.org Subject: Re: [tip:core/urgent] futex: Fix more put_pi_state() vs. exit_pi_state_list() races * Thomas Gleixner <tglx@...utronix.de> wrote: > On Tue, 31 Oct 2017, tip-bot for Peter Zijlstra wrote: > > Commit-ID: d5f6ac33189af48a0dc011190af5144947a30a76 > > Gitweb: https://git.kernel.org/tip/d5f6ac33189af48a0dc011190af5144947a30a76 > > Author: Peter Zijlstra <peterz@...radead.org> > > AuthorDate: Tue, 31 Oct 2017 11:18:53 +0100 > > Committer: Ingo Molnar <mingo@...nel.org> > > CommitDate: Tue, 31 Oct 2017 11:54:52 +0100 > > > > futex: Fix more put_pi_state() vs. exit_pi_state_list() races > > > > Dmitry (through syzbot) reported being able to trigger the WARN in > > get_pi_state() and a use-after-free on: > > > > raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock); > > > > Both are due to this race: > > > > exit_pi_state_list() put_pi_state() > > > > lock(&curr->pi_lock) > > while() { > > pi_state = list_first_entry(head); > > hb = hash_futex(&pi_state->key); > > unlock(&curr->pi_lock); > > > > dec_and_test(&pi_state->refcount); > > > > lock(&hb->lock) > > lock(&pi_state->pi_mutex.wait_lock) // uaf if pi_state free'd > > lock(&curr->pi_lock); > > > > .... > > > > unlock(&curr->pi_lock); > > get_pi_state(); // WARN; refcount==0 > > > > The problem is we take the reference count too late, and don't allow it > > being 0. Fix it by using inc_not_zero() and simply retrying the loop > > when we fail to get a refcount. In that case put_pi_state() should > > remove the entry from the list. > > > > Reported-by: Dmitry Vyukov <dvyukov@...gle.com> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org> > > Cc: Gratian Crisan <gratian.crisan@...com> > > Cc: Linus Torvalds <torvalds@...ux-foundation.org> > > Cc: Peter Zijlstra <peterz@...radead.org> > > Cc: Thomas Gleixner <tglx@...utronix.de> > > Cc: dvhart@...radead.org > > Cc: syzbot <bot+2af19c9e1ffe4d4ee1d16c56ae7580feaee75765@...kaller.appspotmail.com> > > Cc: syzkaller-bugs@...glegroups.com > > Fixes: c74aef2d06a9 ("futex: Fix pi_state->owner serialization") > > Link: http://lkml.kernel.org/r/20171031101853.xpfh72y643kdfhjs@hirez.programming.kicks-ass.net > > Signed-off-by: Ingo Molnar <mingo@...nel.org> > > That lacks a stable tag. > > Other than that a late: > > Reviewed-by: Thomas Gleixner <tglx@...utronix.de> Ok, I've amended the commit with these two changes. Thanks, Ingo
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