linux-kernel - [PATCH] Add a read memory barrier to wait_on

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Date:   Sun, 31 Jul 2022 07:43:34 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
cc:     Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: [PATCH] Add a read memory barrier to wait_on_buffer

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 a memory barrier.
Consequently, if someone calls sb_bread and then reads the buffer data,
the read of buffer data may be speculatively executed before
wait_on_buffer(bh) and it may return invalid data.

Also, there is this pattern present several times:
	wait_on_buffer(bh);
	if (!buffer_uptodate(bh))
		err = -EIO;
It may be possible that buffer_uptodate is executed before wait_on_buffer
and it may return spurious error.

Fix these bugs by adding a read memory barrier to wait_on_buffer().

Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
Cc: stable@...r.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
@@ -353,6 +353,11 @@ static inline void wait_on_buffer(struct
 	might_sleep();
 	if (buffer_locked(bh))
 		__wait_on_buffer(bh);
+	/*
+	 * Make sure that the following accesses to buffer state or buffer data
+	 * are not reordered with buffer_locked(bh).
+	 */
+	smp_rmb();
 }
 
 static inline int trylock_buffer(struct buffer_head *bh)