From 5a88f78df753993469dab4d1831f8fb4256a9468 Mon Sep 17 00:00:00 2001 From: "Cen Zhang (Microsoft)" Date: Fri, 14 Aug 2026 00:09:44 -0400 Subject: 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: 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) --- kernel/reboot.c | 19 +++++++++++++++---- kernel/signal.c | 12 ++++++++++++ 2 files changed, 27 insertions(+), 4 deletions(-) (limited to 'kernel') diff --git a/kernel/reboot.c b/kernel/reboot.c index f070c5c1103a..d177d89fcc33 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 @@ -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; } diff --git a/kernel/signal.c b/kernel/signal.c index bbc0fd4cc4d7..2162fad7b940 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -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 -- cgit v1.2.3 From cdd812d0683dee14ead02c9eded568685e61b23f Mon Sep 17 00:00:00 2001 From: Daehyeon Ko <4ncienth@gmail.com> Date: Mon, 31 Aug 2026 09:12:21 +0900 Subject: exit: hold a reference to thread_pid across proc_flush_pid Commit 0a36bad01731 ("release_task: kill the no longer needed get/put_pid(thread_pid)") removed the reference around proc_flush_pid(). It assumed that free_pids(post.pids) at the end of release_task() would keep thread_pid alive until then. That assumption is wrong. __change_pid() only records a detached PID in post.pids when pid_has_task() is false for every PIDTYPE. If another task still uses the exiting task's PID as its process group or session ID, __unhash_process() removes the exiting task's PIDTYPE_PID link but leaves the PID out of post.pids. release_task() therefore holds no reference to it after dropping tasklist_lock. The other task can then remove the remaining PIDTYPE links. Its free_pids() call schedules delayed_put_pid(), and the RCU callback can free the PID before the first release_task() reaches proc_flush_pid(). An unprivileged reproducer races wait4(-1) against setsid() to trigger this ordering. Three of three fresh v7.2 KASAN boots reported: BUG: KASAN: slab-use-after-free in proc_invalidate_siblings_dcache+0x3e2/0x3f0 Read of size 8 by task h7_pid_reaper/1921 Call Trace: proc_invalidate_siblings_dcache release_task wait_consider_task __do_wait do_wait kernel_wait4 Freed by task 0: kmem_cache_free put_pid delayed_put_pid rcu_core Last potentially related work creation: __call_rcu_common free_pids ksys_setsid KASAN identified a 144-byte object from the pid cache and located the bad read 80 bytes into the freed object, matching pid->inodes. With an explicit reference, three of three fresh boots completed without a KASAN report. The concurrent RCU callback dropped its reference while proc_flush_pid() was protected, and the balancing put_pid() performed the final free afterward. Take a reference before __unhash_process() clears p->thread_pid and release it after proc_flush_pid() completes. A tested source reproducer is available privately on request. No controlled read or write, information leak, or privilege escalation is claimed. The mainline patch applies directly to v6.19.y and newer; v6.16.y through v6.18.y need a context-adjusted backport. Fixes: 0a36bad01731 ("release_task: kill the no longer needed get/put_pid(thread_pid)") Reported-by: syzbot+0aee5e8066eddbbe7397@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=0aee5e8066eddbbe7397 Reported-by: syzbot+e8b3520b53e78e90034e@syzkaller.appspotmail.com Link: https://syzkaller.appspot.com/bug?extid=e8b3520b53e78e90034e Cc: stable@vger.kernel.org # see patch description, needs adjustments for 6.16.y-6.18.y Signed-off-by: Daehyeon Ko <4ncienth@gmail.com> Link: https://patch.msgid.link/20260831001221.3755948-1-4ncienth@gmail.com Acked-by: Oleg Nesterov Reviewed-by: Bradley Morgan Signed-off-by: Christian Brauner (Amutable) --- kernel/exit.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) (limited to 'kernel') diff --git a/kernel/exit.c b/kernel/exit.c index 182c06671c78..e12072f1507c 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -261,8 +261,11 @@ repeat: pidfs_exit(p); cgroup_task_release(p); - /* Retrieve @thread_pid before __unhash_process() may set it to NULL. */ - thread_pid = task_pid(p); + /* + * Pin @thread_pid before __unhash_process() clears it. The last + * PIDTYPE detach can otherwise free it before proc_flush_pid(). + */ + thread_pid = get_pid(task_pid(p)); write_lock_irq(&tasklist_lock); ptrace_release_task(p); @@ -291,8 +294,8 @@ repeat: } write_unlock_irq(&tasklist_lock); - /* @thread_pid can't go away until free_pids() below */ proc_flush_pid(thread_pid); + put_pid(thread_pid); exit_cred_namespaces(p); add_device_randomness(&p->se.sum_exec_runtime, sizeof(p->se.sum_exec_runtime)); -- cgit v1.2.3 From 56ea4e86832d8abe8930394473566c194d189f85 Mon Sep 17 00:00:00 2001 From: Norbert Szetei Date: Mon, 7 Sep 2026 16:22:17 +0200 Subject: nstree: check listing permission before taking a namespace reference legitimize_ns() takes a reference on the candidate namespace before may_list_ns() has decided whether the caller may see it. The __free(ns_put) cleanup on the denied path can drop the last reference to a mount namespace while we still hold the rcu read lock, and put_mnt_ns() may sleep there. This is the same problem commit 2ec2aff3c8e2 ("ns: make sure reference are dropped outside of rcu lock") fixed for the put_user() path. Neither ns_requested() nor may_list_ns() needs a reference, both only look at the namespace type and at the caller's own namespaces, so do the checks first and take the reference last. Splat: Voluntary context switch within RCU read-side critical section! WARNING: kernel/rcu/tree_plugin.h:332 at rcu_note_context_switch+0x238/0x2a0, CPU#5: a/3442 CPU: 5 UID: 1000 PID: 3442 Comm: a Not tainted 7.0.0-30-generic #30-Ubuntu PREEMPT(lazy) RIP: 0010:rcu_note_context_switch+0x238/0x2a0 Call Trace: __schedule+0xcf/0x650 schedule+0x27/0x90 schedule_preempt_disabled+0x15/0x30 __mutex_lock.constprop.0+0x550/0xaf0 __mutex_lock_slowpath+0x13/0x20 mutex_lock+0x3b/0x50 exp_funnel_lock+0xb2/0x260 synchronize_rcu_expedited+0xe7/0x220 namespace_unlock+0x26a/0x320 put_mnt_ns+0xd3/0x120 mntns_put+0xe/0x20 do_listns+0x13e/0x560 __do_sys_listns+0x126/0x2d0 __x64_sys_listns+0x20/0x30 x64_sys_call+0x2366/0x2390 do_syscall_64+0x105/0x5a0 entry_SYSCALL_64_after_hwframe+0x76/0x7e Fixes: 76b6f5dfb3fd ("nstree: add listns()") Signed-off-by: Norbert Szetei Link: https://patch.msgid.link/ABA32239-733B-438C-B95A-B13ED69FF0F3@doyensec.com Reviewed-by: Bradley Morgan Signed-off-by: Christian Brauner (Amutable) --- kernel/nstree.c | 10 ++-------- 1 file changed, 2 insertions(+), 8 deletions(-) (limited to 'kernel') diff --git a/kernel/nstree.c b/kernel/nstree.c index 6d12e5900ac0..831f279d174a 100644 --- a/kernel/nstree.c +++ b/kernel/nstree.c @@ -533,19 +533,13 @@ DEFINE_FREE(ns_put, struct ns_common *, if (!IS_ERR_OR_NULL(_T)) ns_put(_T)) static inline struct ns_common *__must_check legitimize_ns(const struct klistns *kls, struct ns_common *candidate) { - struct ns_common *ns __free(ns_put) = NULL; - if (!ns_requested(kls, candidate)) return NULL; - ns = ns_get_unless_inactive(candidate); - if (!ns) - return NULL; - - if (!may_list_ns(kls, ns)) + if (!may_list_ns(kls, candidate)) return NULL; - return no_free_ptr(ns); + return ns_get_unless_inactive(candidate); } static ssize_t do_listns_userns(struct klistns *kls) -- cgit v1.2.3