From nobody Mon Dec 1 22:02:39 2025 Received: from mail-pj1-f53.google.com (mail-pj1-f53.google.com [209.85.216.53]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5ABDE301011 for ; Thu, 27 Nov 2025 19:43:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.53 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764272635; cv=none; b=rQ4lQj/5wl2p1IG2qW8uick1T9db+VFtEiVriIagW9FaZgBdX9AG534fVeSMfSi0HpEUV+yyfADqlDtvBqZKOQmQ95erxfO2l6idi+GdggRT8RlmOAoE75DDkdFXEldJr3dr79Cb2EDSqndXOyb65I9k8mszv4qZ9S8DEYw74Yg= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764272635; c=relaxed/simple; bh=GYpL1D27YyA+yv9/IVHWUMwrW1V91WDGgRexOvFe/sM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ky9o4ATT5rQgyAGmf9QLxx3S0UbfpKFIP6Vo1Ta4YU2j3nC5Y97tNnJOapDNt5UHyP5xqwvhdfnhox8EuI2yf16uxVZBHdQozhMBKUg0VkV+fFAdE1rW7LKyRVr3samL+6m2Uv2EEoJizL1mqCTwXYrU1tCOeHNmjJr6D7psGU4= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=chrisdown.name; spf=pass smtp.mailfrom=chrisdown.name; dkim=pass (1024-bit key) header.d=chrisdown.name header.i=@chrisdown.name header.b=VeCHi97D; arc=none smtp.client-ip=209.85.216.53 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=chrisdown.name Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=chrisdown.name Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=chrisdown.name header.i=@chrisdown.name header.b="VeCHi97D" Received: by mail-pj1-f53.google.com with SMTP id 98e67ed59e1d1-3418ad69672so698873a91.3 for ; Thu, 27 Nov 2025 11:43:51 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chrisdown.name; s=google; t=1764272629; x=1764877429; darn=vger.kernel.org; h=user-agent:in-reply-to:content-disposition:mime-version:references :message-id:subject:cc:to:from:date:from:to:cc:subject:date :message-id:reply-to; bh=OreMNRvlDDWDO6K1IY1zHwGuV3qnhUW9gqvQBT+IdRs=; b=VeCHi97DZzqePzAy14XQkxz6Ai1rHA5RE5MRCxvyHH4Igz9wO6vq2NOMubyuv9T0Ac QLOc8fImxhZwymMLpklpS5LyH6NhzErZJxylSEgaaDwkwxwImqTy8TqCYI+j+6PuXklj MT368XQOYPJGgjMXut72UHtIFUAbMrdBVy1eA= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1764272629; x=1764877429; h=user-agent:in-reply-to:content-disposition:mime-version:references :message-id:subject:cc:to:from:date:x-gm-gg:x-gm-message-state:from :to:cc:subject:date:message-id:reply-to; bh=OreMNRvlDDWDO6K1IY1zHwGuV3qnhUW9gqvQBT+IdRs=; b=TuuLBF+bgOteKbZnzrGapHQgxf3BrlZxdb8kVq5J79sYqaWKGLYNG2zL2OOj8+NAwJ 84JojPno5D4nab9LatgvSRd3HOg3qtfzHF5DnYWbNAK+0hn3QjpuXyYFOfnTgfyizDWq XMBK1JxjjgRz+pl7vzK7bl0bQYh5x/Et/GNEiBgws1nX8SUrws8qfvs2WsoAbjTqnpcu W60CervJW/gesfsYkVOj0O1/BKRqAdO3lbu/0yfTJYkRa1uQjYiUdcrJyJLHYg6ceeop CU1ifeDQqqrzKjXk4ovJ0xyu6qIlujPcFsW6IoQD9+6HSmSI8Y2PHi1kIBaKdqb/Gwrh n1sQ== X-Gm-Message-State: AOJu0YwD63Fh2V6Yq5ICmJu/TgiIwGL5xk9fAyXNEtbxLsRWPBjGQy02 wI0IYThJ6gepiYvyBY92iyPQvorrz57E+cVgUniS9+d1zv07MXfaCoF3Z8cCdL2WJJdIj7fLBj3 +VJRIATHY3A== X-Gm-Gg: ASbGncuFXEsQYR0iLg7UORUFLNLJrORQS8Byq9DPd987ASaY7buluCyyodW/tXbaC0a yLBTKsJqipWhTu4ZlcIMj7Gr6Vo9vLoS2eqU+DhUqERwzt/i+49YbC/wFSpWFXUI3o9AGI3Mtmu doT07nKnZ0xYVrS6nssaPsr+FaCa2V6PLan84lnm5ymp775Sb4ug2yNcjiJtRqXsDQxjmwOV4Qy G6tjkTDRD5Fea0Yea1t/ZUtEnRs89vZRy8U2oPevTi8BnCFdvTBb79hjhcTjI+/MnXY8xw0Wu7R G62dNTyKT4iHOcVcAuVSEfQYkr8Barh5oWPfZrgabGpn/VKy1iJN6xCbceeYxU5BPVgNrbBS1vT Cnsxn/5nJxhiYSPDdKUhNIdm7TEM2l+pp4yrM2dm8lXp+p5lurIY1R1miHQ2JElGTOUkEMnDtVA g14nK8AL5Z X-Google-Smtp-Source: AGHT+IFGS74MB0q/5+rS/gDJlLeLqmDEuTD0HPyf4i8c8TTbvs070mgYY9vCiYPTHYVsJ5hDNELb3w== X-Received: by 2002:a17:90b:5544:b0:32e:2059:ee83 with SMTP id 98e67ed59e1d1-34733e4c8efmr27395707a91.7.1764272629216; Thu, 27 Nov 2025 11:43:49 -0800 (PST) Received: from localhost ([116.86.198.140]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3475e9304a8sm4819825a91.1.2025.11.27.11.43.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 27 Nov 2025 11:43:48 -0800 (PST) Date: Fri, 28 Nov 2025 03:43:46 +0800 From: Chris Down To: Petr Mladek Cc: linux-kernel@vger.kernel.org, Greg Kroah-Hartman , Sergey Senozhatsky , Steven Rostedt , John Ogness , Geert Uytterhoeven , Tony Lindgren , kernel-team@fb.com Subject: [PATCH v8 10/21] printk: Add synchronisation for concurrent console state changes Message-ID: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/2.2.15 (2b349c5e) (2025-10-02) Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" The syslog actions SYSLOG_ACTION_CONSOLE_OFF and SYSLOG_ACTION_CONSOLE_ON currently run without any locking. This creates a race condition if two processes attempt to toggle the console state simultaneously. While this race has existed for donkey's years, it was previously somewhat tolerable because it involved only a single integer variable (saved_console_loglevel). However, upcoming changes will introduce a second piece of state (saved_ignore_per_console_loglevel) to be saved and restored. With two variables, this can result in saved state getting lost. Here is a demonstration: CPU 0 (SYSLOG_ACTION_CONSOLE_OFF) CPU 1 (SYSLOG_ACTION_CONSOLE_ON) --------------------------------- -------------------------------- // saved_console_loglevel is DEFAULT if (saved =3D=3D DEFAULT) (True) saved =3D console_loglevel (7) // Race triggers here if (saved !=3D DEFAULT) (True) loglevel =3D saved (7) saved =3D DEFAULT // CPU 0 continues, unaware saved // was just reset by CPU 1 loglevel =3D minimum_console_loglevel (1) The result is that the console is now set to the minimum loglevel, but saved_console_loglevel is LOGLEVEL_DEFAULT. A subsequent CONSOLE_ON call will see saved =3D=3D LOGLEVEL_DEFAULT and thus refuse to restore the original loglevel. The console is effectively stuck in quiet mode until manually reset via sysctl. Oh dear. The callers of do_syslog are syscalls (so user context) or procfs write handlers. These contexts are allowed to sleep, so acquiring a mutex is safe. We also already make use of syslog_lock in SYSLOG_ACTION_SIZE_UNREAD and SYSLOG_ACTION_READ, so this is consistent. Reviewed-by: Petr Mladek Signed-off-by: Chris Down --- kernel/printk/printk.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 13e17d892ec9..98d2f4466aad 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1894,16 +1894,20 @@ int do_syslog(int type, char __user *buf, int len, = int source) break; /* Disable logging to console */ case SYSLOG_ACTION_CONSOLE_OFF: + mutex_lock(&syslog_lock); if (saved_console_loglevel =3D=3D LOGLEVEL_DEFAULT) saved_console_loglevel =3D console_loglevel; console_loglevel =3D minimum_console_loglevel; + mutex_unlock(&syslog_lock); break; /* Enable logging to console */ case SYSLOG_ACTION_CONSOLE_ON: + mutex_lock(&syslog_lock); if (saved_console_loglevel !=3D LOGLEVEL_DEFAULT) { console_loglevel =3D saved_console_loglevel; saved_console_loglevel =3D LOGLEVEL_DEFAULT; } + mutex_unlock(&syslog_lock); break; /* Set level of messages printed to console */ case SYSLOG_ACTION_CONSOLE_LEVEL: --=20 2.51.2