Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 1 Aug 2022 06:43:55 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
cc:     Will Deacon <will@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Alan Stern <stern@rowland.harvard.edu>,
        Andrea Parri <parri.andrea@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Akira Yokosawa <akiyks@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org
Subject: [PATCH v4 2/2] change buffer_locked, so that it has acquire
 semantics
In-Reply-To: <alpine.LRH.2.02.2208010628510.22006@file01.intranet.prod.int.rdu2.redhat.com>
Message-ID: <alpine.LRH.2.02.2208010642220.22006@file01.intranet.prod.int.rdu2.redhat.com>
References: <alpine.LRH.2.02.2207310703170.14394@file01.intranet.prod.int.rdu2.redhat.com> <CAMj1kXFYRNrP2k8yppgfdKg+CxWeYfHTbzLBuyBqJ9UVAR_vaQ@mail.gmail.com> <alpine.LRH.2.02.2207310920390.6506@file01.intranet.prod.int.rdu2.redhat.com>
 <alpine.LRH.2.02.2207311104020.16444@file01.intranet.prod.int.rdu2.redhat.com> <CAHk-=wiC_oidYZeMD7p0E-=TAuLgrNQ86-sB99=hRqFM8fVLDQ@mail.gmail.com> <alpine.LRH.2.02.2207311542280.21273@file01.intranet.prod.int.rdu2.redhat.com>
 <alpine.LRH.2.02.2207311639360.21350@file01.intranet.prod.int.rdu2.redhat.com> <CAHk-=wjA8HBrVqAqAetUvwNr=hcvhfnO7oMrOAd4V8bbSqokNA@mail.gmail.com> <alpine.LRH.2.02.2208010628510.22006@file01.intranet.prod.int.rdu2.redhat.com>
User-Agent: Alpine 2.02 (LRH 1266 2009-07-14)
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Precedence: bulk

Let's have a look at this piece of code in __bread_slow:
	get_bh(bh);
	bh->b_end_io = end_buffer_read_sync;
	submit_bh(REQ_OP_READ, 0, bh);
	wait_on_buffer(bh);
	if (buffer_uptodate(bh))
		return bh;
Neither wait_on_buffer nor buffer_uptodate contain any memory barrier.
Consequently, if someone calls sb_bread and then reads the buffer data,
the read of buffer data may be executed before wait_on_buffer(bh) on
architectures with weak memory ordering and it may return invalid data.

Fix this bug by changing the function buffer_locked to have the acquire
semantics - so that code that follows buffer_locked cannot be moved before
it.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org

Index: linux-2.6/include/linux/buffer_head.h
===================================================================
--- linux-2.6.orig/include/linux/buffer_head.h
+++ linux-2.6/include/linux/buffer_head.h
@@ -120,7 +120,6 @@ static __always_inline int test_clear_bu
 BUFFER_FNS(Uptodate, uptodate)
 BUFFER_FNS(Dirty, dirty)
 TAS_BUFFER_FNS(Dirty, dirty)
-BUFFER_FNS(Lock, locked)
 BUFFER_FNS(Req, req)
 TAS_BUFFER_FNS(Req, req)
 BUFFER_FNS(Mapped, mapped)
@@ -135,6 +134,17 @@ BUFFER_FNS(Meta, meta)
 BUFFER_FNS(Prio, prio)
 BUFFER_FNS(Defer_Completion, defer_completion)
 
+static __always_inline void set_buffer_locked(struct buffer_head *bh)
+{
+	set_bit(BH_Lock, &bh->b_state);
+}
+
+static __always_inline int buffer_locked(const struct buffer_head *bh)
+{
+	/* pairs with smp_mb__after_atomic() in unlock_buffer() */
+	return test_bit_acquire(BH_Lock, &bh->b_state);
+}
+
 #define bh_offset(bh)		((unsigned long)(bh)->b_data & ~PAGE_MASK)
 
 /* If we *know* page->private refers to buffer_heads */