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Tue, 18 Nov 2025 11:07:19 -0800 (PST) Received: from localhost ([116.86.198.140]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2985c242003sm179764525ad.24.2025.11.18.11.07.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 18 Nov 2025 11:07:18 -0800 (PST) Date: Wed, 19 Nov 2025 03:07:17 +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 v7 05/13] printk: Add synchronisation for concurrent console state changes Message-ID: <9ffc0801a64e5f7d0685c263c1908a34cf89d1a5.1763492585.git.chris@chrisdown.name> 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. Signed-off-by: Chris Down Reviewed-by: Petr Mladek --- kernel/printk/printk.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 928d77c56c77..b8679b0da42f 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1903,16 +1903,20 @@ 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