Re: [PATCH v3 2/2] pid: fix cad_pid use-after-free race

[Pavel Tikhomirov]

Reviewed-by: Pavel Tikhomirov <ptikhomirov@xxxxxxxxxxxxx>

On 7/19/26 17:58, Cen Zhang (Microsoft) wrote:
> proc_do_cad_pid() reads the global cad_pid pointer 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.
> 
> The sysctl is mode 0600, but access is checked against the owning user
> namespace, so an unprivileged user can reach it via userns, pidns, and a
> proc mount.
> 
> 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.
> 
> KASAN crash stack:
>   kernel/pid.c:545 pid_nr_ns()        # reads freed pid->level
>   kernel/pid.c:556 pid_vnr()
>   kernel/pid.c:775 proc_do_cad_pid()
>   fs/proc/proc_sysctl.c proc_sys_call_handler()
>   fs/read_write.c vfs_read()
>   fs/read_write.c __x64_sys_pread64()
> 
> Fixes: 9ec52099e4b8 ("[PATCH] replace cad_pid by a struct pid")
> Reported-by: AutonomousCodeSecurity@xxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/20260717210143.4734-1-blbllhy@xxxxxxxxx/
> Suggested-by: Mateusz Guzik <mjguzik@xxxxxxxxx>
> Suggested-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> Reviewed-by: Bradley Morgan <include@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Cen Zhang (Microsoft) <blbllhy@xxxxxxxxx>
> ---
> v3:
>   - Keep kill_cad_pid() inside the RCU read-side critical section
>     instead of taking a pid reference, as suggested by Oleg.
> 
> v2:
>   - Split out kill_cad_pid() deinline into a preparatory patch.
>   - Annotate cad_pid as __rcu and use rcu_dereference().
>   - Protect kill_cad_pid() by taking a pid reference under RCU.
>   - Add a comment explaining why synchronize_rcu() is used instead of
>     call_rcu().
> 
>  include/linux/sched.h |  2 +-
>  init/main.c           |  2 +-
>  kernel/pid.c          | 22 +++++++++++++++++++---
>  kernel/reboot.c       |  2 +-
>  4 files changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 373bcc0598d1..31ce72b1233c 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1767,7 +1767,7 @@ static inline bool is_lazy_mmu_mode_active(void)
>  }
>  #endif
>  
> -extern struct pid *cad_pid;
> +extern struct pid __rcu *cad_pid;
>  
>  /*
>   * Per process flags
> diff --git a/init/main.c b/init/main.c
> index e363232b428b..19a10d0c2760 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1636,7 +1636,7 @@ 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/pid.c b/kernel/pid.c
> index 234ebee29375..fb7c0d18efa9 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -559,7 +559,13 @@ EXPORT_SYMBOL_GPL(pid_vnr);
>  
>  int kill_cad_pid(int sig, int priv)
>  {
> -	return kill_pid(cad_pid, sig, priv);
> +	int ret;
> +
> +	rcu_read_lock();
> +	ret = kill_pid(rcu_dereference(cad_pid), sig, priv);
> +	rcu_read_unlock();
> +
> +	return ret;
>  }
>  
>  pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
> @@ -773,11 +779,15 @@ static int proc_do_cad_pid(const struct ctl_table *table, int write, void *buffe
>  		size_t *lenp, loff_t *ppos)
>  {
>  	struct pid *new_pid;
> +	struct pid *old_pid;
>  	pid_t tmp_pid;
>  	int r;
>  	struct ctl_table tmp_table = *table;
>  
> -	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);
> @@ -788,7 +798,13 @@ 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/reboot.c b/kernel/reboot.c
> index 695c33e75efd..fc191a48c0e9 100644
> --- a/kernel/reboot.c
> +++ b/kernel/reboot.c
> @@ -24,7 +24,7 @@
>   */
>  
>  static int C_A_D = 1;
> -struct pid *cad_pid;
> +struct pid __rcu *cad_pid;
>  EXPORT_SYMBOL(cad_pid);
>  
>  #if defined(CONFIG_ARM)

-- 
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.