From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 409891F0E36; Tue, 21 Jan 2025 18:07:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737482829; cv=none; b=jauBAin5iPdC8ErZM8OuMMttXVQd4vaNUWivZoTDxgcYokgdd2hHewL5IHhKjrNEo7spd2sQ7NLdnPPv7Yf6ifXmTCBcsvJX4ADgrJzsIcIQQe1NTypqVAtSI9Fy0PeVmgfSF+C8EjIHx7sOY1U8zrTnWkimwwmeFLsiJ5Gt7qs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737482829; c=relaxed/simple; bh=CF6n6S1VzZFw4QnXlsXDk6XQMpYx6karaszNOsWmubM=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=Sm5wxKzMeSuD8NiNhAxIyev6Ma/kS0Cz95u0LKBQ30hbuy0JB2gZ/4anx70HZYcX4PSpxkHWO1dEsKbiGXbYcxaR0hgyLlsaPSB7c4vTo77mmws58oQPBbaNi8Gmso69m34xX0O9Hoo3O8DwjpUXHz2YWf+WEh8z5iG9z5/qGHs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=T0HGJqpZ; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="T0HGJqpZ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C1C06C4CEDF; Tue, 21 Jan 2025 18:07:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1737482829; bh=CF6n6S1VzZFw4QnXlsXDk6XQMpYx6karaszNOsWmubM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=T0HGJqpZEQ4el2GBOVfYIvpwiifdoBM3wPG12C5dmjfkDLqZJrIU6DWkStw9tcUP9 kJXl2UzTTOGwBoSSIZRL3ixWuWdInw1OjMcNDxL0PPtSeV3C45yy46oPYajS1L54Xs +3hNjyYxPjIaciBm1wtRpgjgUlFLsfQj0lmXzfaE= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Krister Johansen , Ming-Hung Tsai , Mikulas Patocka Subject: [PATCH 5.15 022/127] dm thin: make get_first_thin use rcu-safe list first function Date: Tue, 21 Jan 2025 18:51:34 +0100 Message-ID: <20250121174530.530966680@linuxfoundation.org> X-Mailer: git-send-email 2.48.1 In-Reply-To: <20250121174529.674452028@linuxfoundation.org> References: <20250121174529.674452028@linuxfoundation.org> User-Agent: quilt/0.68 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 5.15-stable review patch. If anyone has any objections, please let me know. ------------------ From: Krister Johansen commit 80f130bfad1dab93b95683fc39b87235682b8f72 upstream. The documentation in rculist.h explains the absence of list_empty_rcu() and cautions programmers against relying on a list_empty() -> list_first() sequence in RCU safe code. This is because each of these functions performs its own READ_ONCE() of the list head. This can lead to a situation where the list_empty() sees a valid list entry, but the subsequent list_first() sees a different view of list head state after a modification. In the case of dm-thin, this author had a production box crash from a GP fault in the process_deferred_bios path. This function saw a valid list head in get_first_thin() but when it subsequently dereferenced that and turned it into a thin_c, it got the inside of the struct pool, since the list was now empty and referring to itself. The kernel on which this occurred printed both a warning about a refcount_t being saturated, and a UBSAN error for an out-of-bounds cpuid access in the queued spinlock, prior to the fault itself. When the resulting kdump was examined, it was possible to see another thread patiently waiting in thin_dtr's synchronize_rcu. The thin_dtr call managed to pull the thin_c out of the active thins list (and have it be the last entry in the active_thins list) at just the wrong moment which lead to this crash. Fortunately, the fix here is straight forward. Switch get_first_thin() function to use list_first_or_null_rcu() which performs just a single READ_ONCE() and returns NULL if the list is already empty. This was run against the devicemapper test suite's thin-provisioning suites for delete and suspend and no regressions were observed. Signed-off-by: Krister Johansen Fixes: b10ebd34ccca ("dm thin: fix rcu_read_lock being held in code that can sleep") Cc: stable@vger.kernel.org Acked-by: Ming-Hung Tsai Signed-off-by: Mikulas Patocka Signed-off-by: Greg Kroah-Hartman --- drivers/md/dm-thin.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) --- a/drivers/md/dm-thin.c +++ b/drivers/md/dm-thin.c @@ -2317,10 +2317,9 @@ static struct thin_c *get_first_thin(str struct thin_c *tc = NULL; rcu_read_lock(); - if (!list_empty(&pool->active_thins)) { - tc = list_entry_rcu(pool->active_thins.next, struct thin_c, list); + tc = list_first_or_null_rcu(&pool->active_thins, struct thin_c, list); + if (tc) thin_get(tc); - } rcu_read_unlock(); return tc;