FAILED: patch "[PATCH] reboot: fix cad_pid use-after-free race" failed to apply to 5.15-stable tree

PATCH New [email protected] 2 participants 2 messages Sep 16

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2026-09-16 08:13 UTC #0 lore raw

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 5a88f78df753993469dab4d1831f8fb4256a9468
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026091656-cabbie-washing-ded5@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 5a88f78df753993469dab4d1831f8fb4256a9468 Mon Sep 17 00:00:00 2001
From: "Cen Zhang (Microsoft)" <[email protected]>
Date: Fri, 14 Aug 2026 00:09:44 -0400
Subject: [PATCH] reboot: fix cad_pid use-after-free race

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 [email protected]

Closes https://lore.kernel.org/all/[email protected]/ open here

Link https://lore.kernel.org/all/[email protected]/ open here

Cc [email protected]

Signed-off-by Cen Zhang (Microsoft) <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Reviewed-by Bradley Morgan <[email protected]>

Reviewed-by Oleg Nesterov <[email protected]>

Reviewed-by Pavel Tikhomirov <[email protected]>

Signed-off-by Christian Brauner (Amutable) <[email protected]>

include/linux/sched.h +1 -1

index 3f100d69b053..fdafd164a42d 100644
@@ -1778,7 +1778,7 @@ static inline bool is_lazy_mmu_mode_active(void)
 }
 #endif
 
-extern struct pid *cad_pid;
+extern struct pid __rcu *cad_pid;
 
 /*
  * Per process flags

include/linux/sched/signal.h +1 -4

index 584ae88b435e..d45a5476b97d 100644
@@ -562,10 +562,7 @@ 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)

init/main.c +1 -1

index 92d34e496a33..f46f3a8b3efd 100644
@@ -1644,7 +1644,7 @@ 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);

kernel/reboot.c +15 -4

index f070c5c1103a..d177d89fcc33 100644
@@ -13,7 +13,9 @@
 #include <linux/kexec.h>
 #include <linux/kmod.h>
 #include <linux/kmsg_dump.h>
+#include <linux/rcupdate.h>
 #include <linux/reboot.h>
+#include <linux/sched/signal.h>
 #include <linux/suspend.h>
 #include <linux/syscalls.h>
 #include <linux/syscore_ops.h>

@@ -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

@@ -1371,10 +1372,14 @@ 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);

@@ -1385,7 +1390,13 @@ 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;
 }

kernel/signal.c +12 -0

index bbc0fd4cc4d7..2162fad7b940 100644
@@ -1899,6 +1899,18 @@ 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

Sasha Levin

maintainer

2026-09-16 19:07 UTC #1 lore raw

PATCH 1/1 reboot: fix cad_pid use-after-free race

From: "Cen Zhang (Microsoft)" <[email protected]>

[ Upstream commit 5a88f78df753993469dab4d1831f8fb4256a9468 ]