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smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=QXf+KT54; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="QXf+KT54" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 75EADC4CEEB; Mon, 9 Jun 2025 07:48:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1749455339; bh=MfXFjxrBzTof+d+yK1Q+a+zQOzKcP3+f+P3hYnAry5I=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=QXf+KT54DdqtqutcXoYOkkEka5sL25QPGDie958NNcHENr0/TGOIU5tH5GUfx8fUM +UgAJcP60JL3GKCfYcN3P8yUCLg6aH5TlcjizxdlToSGXZ/O87c2+WZF6W7ERNW08p vvGgSB58mwIxImvFZF6f6Cz69jwDlsucuAjzBziA03HivxAz1ZIbn9G8q3HBzUGqUc p+WBIP2TW4MpRE/ZaPMVBlz9E8dl217FuU5U1GzoyIipjVsW41ZFa2cg6QpNhPGRUx vNiH0v2xaz60T9g0tht7nPJmcWgtyTg4vBCSvvugDKSphQN0CZUvMz/g2YbsSid/ll wzkrrHLS+DA4g== Message-ID: Date: Mon, 9 Jun 2025 09:48:56 +0200 Precedence: bulk X-Mailing-List: linux-serial@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] tty: sysrq: Introduce compile-time crash-only mode To: Marwan Seliem , gregkh@linuxfoundation.org Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org References: <20250607151957.222347-1-marwanmhks@gmail.com> Content-Language: en-US From: Jiri Slaby Autocrypt: addr=jirislaby@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 07. 06. 25, 17:19, Marwan Seliem wrote: > This commit introduces a new Kconfig option, CONFIG_MAGIC_SYSRQ_CRASH_ONLY, > which allows for a significant hardening of the system by restricting > the Magic SysRq functionality at compile time. > > Security Impact: > - Reduces attack surface by disabling non-essential SysRq commands > - Maintains critical crash-dump capability required for debugging > - Eliminates runtime configuration vulnerabilities > > When CONFIG_MAGIC_SYSRQ_CRASH_ONLY is enabled: > > 1. Restricted Commands: Only the 'c' (trigger a system crash/dump) > SysRq command remains operational. All other built-in SysRq commands > (e.g., reboot, sync, show-memory, SAK) are disabled. I must admit I don't much understand the purpose of this. It can be spelled as: you can crash the system only by sysrq-c from now on. Don't use sysrq-r or others. Who did ask for this? ... > --- a/drivers/tty/sysrq.c > +++ b/drivers/tty/sysrq.c > @@ -584,7 +620,6 @@ void __handle_sysrq(u8 key, bool check_mask) > { > const struct sysrq_key_op *op_p; > int orig_suppress_printk; > - int i; > > orig_suppress_printk = suppress_printk; > suppress_printk = 0; > @@ -599,7 +634,15 @@ void __handle_sysrq(u8 key, bool check_mask) > */ > printk_force_console_enter(); > > +#ifdef CONFIG_MAGIC_SYSRQ_CRASH_ONLY > + if (key != 'c') { /* In CRASH_ONLY mode, only 'c' is considered */ > + op_p = NULL; > + } else { > + op_p = __sysrq_get_key_op(key); > + } > +#else > op_p = __sysrq_get_key_op(key); > +#endif These inline #ifdefs are horrid. > if (op_p) { > /* > * Should we check for enabled operations (/proc/sysrq-trigger ... > @@ -1104,6 +1157,10 @@ static inline void sysrq_unregister_handler(void) > > int sysrq_toggle_support(int enable_mask) > { > +#ifdef CONFIG_MAGIC_SYSRQ_CRASH_ONLY > + pr_warn("SysRq: CONFIG_MAGIC_SYSRQ_CRASH_ONLY is set. Runtime toggle is not allowed.\n"); This can be invoked from userspace. So you can nicely DoS the machine by the added warn, right? Hint: use ratelimiting. > + return -EPERM; > +#else > bool was_enabled = sysrq_on(); > > sysrq_enabled = enable_mask; ... > @@ -1145,12 +1203,30 @@ static int __sysrq_swap_key_ops(u8 key, const struct sysrq_key_op *insert_op_p, > > int register_sysrq_key(u8 key, const struct sysrq_key_op *op_p) > { > +#ifdef CONFIG_MAGIC_SYSRQ_CRASH_ONLY > + /* > + * In CRASH_ONLY mode, do not allow registering new SysRq ops. > + */ > + pr_warn("SysRq: CONFIG_MAGIC_SYSRQ_CRASH_ONLY is set. Cannot register new SysRq key '%c'.\n", key); > + return -EPERM; > +#endif > return __sysrq_swap_key_ops(key, op_p, NULL); > } > EXPORT_SYMBOL(register_sysrq_key); > > int unregister_sysrq_key(u8 key, const struct sysrq_key_op *op_p) > { > +#ifdef CONFIG_MAGIC_SYSRQ_CRASH_ONLY > + /* > + * In CRASH_ONLY mode, do not allow unregistering the crash op. > + * Other ops should be NULL anyway due to sysrq_init_crash_only_table. > + */ > + if (op_p == &sysrq_crash_op) { > + pr_warn("SysRq: CONFIG_MAGIC_SYSRQ_CRASH_ONLY is set. Cannot unregister the crash SysRq key '%c'.\n", key); > + return -EPERM; No need for this return ^^. > + } > + return -EPERM; /* Attempt to unregister anything else is also an error */ > +#endif > return __sysrq_swap_key_ops(key, NULL, op_p); > } > EXPORT_SYMBOL(unregister_sysrq_key); > @@ -1209,6 +1285,7 @@ static inline void sysrq_init_procfs(void) > static int __init sysrq_init(void) > { > sysrq_init_procfs(); > + sysrq_init_crash_only_table(); > > if (sysrq_on()) > sysrq_register_handler(); > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug > index ebe33181b6e6..c05b80cfb8aa 100644 > --- a/lib/Kconfig.debug > +++ b/lib/Kconfig.debug > @@ -640,6 +640,19 @@ config MAGIC_SYSRQ_DEFAULT_ENABLE > This may be set to 1 or 0 to enable or disable them all, or > to a bitmask as described in Documentation/admin-guide/sysrq.rst. > > +config MAGIC_SYSRQ_CRASH_ONLY > + bool "Restrict Magic SysRq to crash command only" > + depends on MAGIC_SYSRQ > + default n > + help > + If you say Y here, the Magic SysRq key functionality will be > + severely restricted at compile time. Only the 'c' command (trigger > + a system crash) will be available. All other SysRq commands will be > + disabled, and no new SysRq commands can be registered at runtime. > + The /proc/sys/kernel/sysrq setting will be ineffective for > + non-crash commands, and attempts to change it may be blocked. > + This is a security hardening option. Is it for real? thanks, -- js suse labs