Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 AE64D4A68A0 for ; Wed, 16 Sep 2026 14:11:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789567916; cv=none; b=txAp7/WJU1IoGuGmiuG83gr4t8u/POlxx6eDvo6IMf3fbpIhF1TWiy0eYZJWeljC9dfAlRKtIhpuizfQDZVUPC0e4FQuQ4AowMgXqGfugv0XXXZaDI/ko5La9KgufIjb9satJFdFn8kH2xes7eehVYSLYk4Uea9F7LlkAcnnpLo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789567916; c=relaxed/simple; bh=kF+K1Ph48imZtjHVRaB3TBReUFX2/XUSUQ+Zb4Fij18=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=TAzFwn2EGA2dZciqHz/4D2jRhwpTtIEAVYeiaPzERibypnhUAKfQoTuGLAo5z01kMQNTnkdu81k04HUHSwblpCkHCT8JvG6bjOwyggX6CVBNBqpScTN3PTKZW94bZMXE647Gmast6IcQ6f0XsoZsySOF7wIJxkZmr0V/d/k7b/U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=oM64M2Re; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="oM64M2Re" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 765341F00899; Wed, 16 Sep 2026 14:11:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789567914; bh=/JP0SonJouS1sdgGFCZYlppjPqUvVU76LklqKBqHeDg=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=oM64M2Ret4o2CUhwipoatJKo4QDFrF6M2+H/FWvrvzADEMNtv4U+0Ld6YPKeSsslp jylb5Eo2Hh6rty1SvuWr4BTYWIhIzjadoijASxjbCUj5gtFREi5uW6mvf006SokBiJ WZrP+spEz9LGWnCns6NIg7bcxdWNqs/l3+w+tOXnYyyJGEw6ISSqzx91enOf9yFlcH F6410ej2ZBpq2TuPWQQn7jHKS5zlXW7jVoKlQ0BxlcjUkXDXjMESOPO/VjOHAvCL+7 1+A+CYRI8v4MBHYlfodHlsiDecKfjIzXD8ZmiXgKq3RwwtjW85Hb49YMbpRyC7Ngv/ UxHi8vEGL8EFg== From: Sasha Levin To: stable@vger.kernel.org Cc: "Cen Zhang (Microsoft)" , AutonomousCodeSecurity@microsoft.com, Mateusz Guzik , Bradley Morgan , Oleg Nesterov , "Eric W. Biederman" , Pavel Tikhomirov , "Christian Brauner (Amutable)" , Sasha Levin Subject: [PATCH 6.12.y 2/2] reboot: fix cad_pid use-after-free race Date: Wed, 16 Sep 2026 10:11:51 -0400 Message-ID: <20260916141151.2995957-2-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260916141151.2995957-1-sashal@kernel.org> References: <2026091653-fester-trodden-026e@gregkh> <20260916141151.2995957-1-sashal@kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: "Cen Zhang (Microsoft)" [ Upstream commit 5a88f78df753993469dab4d1831f8fb4256a9468 ] cad_pid is a single kernel-wide struct pid pointer. proc_do_cad_pid() reads it and passes it to pid_vnr() without protecting the lifetime of the referenced struct pid. A concurrent writer can replace cad_pid and drop the final reference to the old struct pid after the reader has loaded the pointer but before pid_vnr() has finished dereferencing it, causing a use-after-free. kill_cad_pid() has the same lifetime race when it passes cad_pid to kill_pid(). At the time this issue was reported, an unprivileged user could reach the sysctl through user and PID namespaces because cad_pid was registered in pid_table[]. Moving cad_pid back to the global reboot sysctl table corrected that namespace and permission mismatch, but did not fix the underlying lifetime race. Fix this by treating cad_pid as an RCU-protected pointer at both read sites and by waiting for a grace period before dropping the old reference on the write side. call_rcu(&old_pid->rcu, ...) cannot be used here because free_pid() also queues pid->rcu; queueing the same rcu_head twice can corrupt the RCU callback list. Original KASAN crash stack: kernel/pid.c:545 pid_nr_ns() # reads freed pid->level kernel/pid.c:556 pid_vnr() # calls pid_nr_ns() kernel/pid.c:775 proc_do_cad_pid() # calls pid_vnr(cad_pid) Fixes: 9ec52099e4b8 ("[PATCH] replace cad_pid by a struct pid") Reported-by: AutonomousCodeSecurity@microsoft.com Closes: https://lore.kernel.org/all/20260717210143.4734-1-blbllhy@gmail.com/ Link: https://lore.kernel.org/all/alz5ZYLE4kaq_v2P@redhat.com/ Link: https://lore.kernel.org/all/al4ICz9biJKtdZc4@redhat.com/ Suggested-by: Mateusz Guzik Suggested-by: Bradley Morgan Suggested-by: Oleg Nesterov Suggested-by: Eric W. Biederman Suggested-by: Pavel Tikhomirov Cc: stable@vger.kernel.org Signed-off-by: Cen Zhang (Microsoft) Link: https://patch.msgid.link/20260814040944.16561-1-blbllhy@gmail.com Reviewed-by: Bradley Morgan Reviewed-by: Oleg Nesterov Reviewed-by: Pavel Tikhomirov Signed-off-by: Christian Brauner (Amutable) Signed-off-by: Sasha Levin --- include/linux/sched.h | 2 +- include/linux/sched/signal.h | 5 +---- init/main.c | 2 +- kernel/reboot.c | 19 +++++++++++++++---- kernel/signal.c | 12 ++++++++++++ 5 files changed, 30 insertions(+), 10 deletions(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index c7d475b0f3b47..0fb5bb6c30d3f 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1683,7 +1683,7 @@ static inline void kick_process(struct task_struct *tsk) } #endif -extern struct pid *cad_pid; +extern struct pid __rcu *cad_pid; /* * Per process flags diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h index c8ed09ac29ac5..21e496bb1c1bd 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -558,10 +558,7 @@ static inline sigset_t *sigmask_to_save(void) return res; } -static inline int kill_cad_pid(int sig, int priv) -{ - return kill_pid(cad_pid, sig, priv); -} +int kill_cad_pid(int sig, int priv); /* These can be the second arg to send_sig_info/send_group_sig_info. */ #define SEND_SIG_NOINFO ((struct kernel_siginfo *) 0) diff --git a/init/main.c b/init/main.c index df9db4d47868a..4872cc96174ad 100644 --- a/init/main.c +++ b/init/main.c @@ -1523,7 +1523,7 @@ static noinline void __init kernel_init_freeable(void) */ set_mems_allowed(node_states[N_MEMORY]); - cad_pid = get_pid(task_pid(current)); + rcu_assign_pointer(cad_pid, get_pid(task_pid(current))); smp_prepare_cpus(setup_max_cpus); diff --git a/kernel/reboot.c b/kernel/reboot.c index 8ca5590a6d601..24c5bc26385af 100644 --- a/kernel/reboot.c +++ b/kernel/reboot.c @@ -13,7 +13,9 @@ #include #include #include +#include #include +#include #include #include #include @@ -24,8 +26,7 @@ */ static int C_A_D = 1; -struct pid *cad_pid; -EXPORT_SYMBOL(cad_pid); +struct pid __rcu *cad_pid; #if defined(CONFIG_ARM) #define DEFAULT_REBOOT_MODE = REBOOT_HARD @@ -1288,10 +1289,14 @@ static int proc_do_cad_pid(const struct ctl_table *table, int write, void *buffe { struct ctl_table tmp_table = *table; struct pid *new_pid; + struct pid *old_pid; pid_t tmp_pid; int r; - tmp_pid = pid_vnr(cad_pid); + rcu_read_lock(); + tmp_pid = pid_vnr(rcu_dereference(cad_pid)); + rcu_read_unlock(); + tmp_table.data = &tmp_pid; r = proc_dointvec(&tmp_table, write, buffer, lenp, ppos); @@ -1302,7 +1307,13 @@ static int proc_do_cad_pid(const struct ctl_table *table, int write, void *buffe if (!new_pid) return -ESRCH; - put_pid(xchg(&cad_pid, new_pid)); + old_pid = unrcu_pointer(xchg(&cad_pid, RCU_INITIALIZER(new_pid))); + /* + * Wait for cad_pid readers before put_pid(). We cannot use + * call_rcu() here because free_pid() already owns pid->rcu. + */ + synchronize_rcu(); + put_pid(old_pid); return 0; } diff --git a/kernel/signal.c b/kernel/signal.c index a665afdccecdd..1a6cd0503beb5 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -1936,6 +1936,18 @@ int kill_pid(struct pid *pid, int sig, int priv) } EXPORT_SYMBOL(kill_pid); +int kill_cad_pid(int sig, int priv) +{ + int ret; + + rcu_read_lock(); + ret = kill_pid(rcu_dereference(cad_pid), sig, priv); + rcu_read_unlock(); + + return ret; +} +EXPORT_SYMBOL(kill_cad_pid); + #ifdef CONFIG_POSIX_TIMERS /* * These functions handle POSIX timer signals. POSIX timers use -- 2.53.0