linux-kernel - Re: [PATCH v2 1/1] psi: Fix uaf issue when psi trigger is destroyed while being polled

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Message-ID: <YeZlCfZZkU5jyFS+@gondor.apana.org.au>
Date:   Tue, 18 Jan 2022 17:58:17 +1100
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     ebiggers@...nel.org, surenb@...gle.com, hannes@...xchg.org,
        tj@...nel.org, lizefan.x@...edance.com, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, corbet@....net, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        stable@...r.kernel.org, kernel-team@...roid.com,
        syzbot+cdb5dd11c97cc532efad@...kaller.appspotmail.com
Subject: Re: [PATCH v2 1/1] psi: Fix uaf issue when psi trigger is destroyed
 while being polled

Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>
> Of course, in practice, for pointers, the whole "dereference off a
> pointer" on the read side *does* imply a barrier in all relevant
> situations. So yes, a smp_store_release() -> READ_ONCE() does work in
> practice, although it's technically wrong (in particular, it's wrong
> on alpha, because of the completely broken memory ordering that alpha
> has that doesn't even honor data dependencies as read-side orderings)

READ_ONCE has contained the alpha barrier since 2017:

commit 76ebbe78f7390aee075a7f3768af197ded1bdfbb
Author: Will Deacon <will@...nel.org>
Date:   Tue Oct 24 11:22:47 2017 +0100

    locking/barriers: Add implicit smp_read_barrier_depends() to READ_ONCE()

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt