Re: keys: request_key_auth shows a global pid in /proc/keys across pid namespaces

[James Bottomley]

On Sun, 2026-08-09 at 19:02 +0800, Maoyi Xie wrote:
> Hi all,
> 
> I think request_key_auth leaks a global pid into /proc/keys. I would
> appreciate it if you could take a look.
> 
> request_key_auth_describe() prints the requestor's pid:
> 
> 	if (key_is_positive(key))
> 		seq_printf(m, " pid:%d ci:%zu", rka->pid, rka-
> >callout_len);
> 
> rka->pid is current->pid, captured in request_key_auth_new() when the
> auth key is made. It is a raw pid_t in the init pid namespace. But
> /proc/keys is read by whoever opens it, and their pid namespace can
> differ. The uid and gid on that line already go through
> seq_user_ns(m).
> The pid does not. So a reader in another pid namespace gets a number
> that means nothing there, or worse lands on a different task.
> 
> Not everyone can read the line. The owner uid must map into the
> reader's
> user namespace, and the reader needs VIEW on the key. The auth key
> gives
> VIEW to the requestor's own uid and to a possessor. So the reader has
> to
> be the requestor's uid, in a different pid namespace. That fits a
> sandbox at the same uid, made with unshare, systemd-nspawn or
> bubblewrap. A default Docker container does not hit it, since it
> hides
> /proc/keys.
> 
> I found this with a static check for ids that reach userspace with no
> namespace translation. I reproduced it in a qemu VM on 7.1-rc1 as a
> normal user, no hardware or kernel changes needed. The reproducer
> calls
> request_key, then reads /proc/keys from a sibling pid namespace.
> Before
> the patch it reads the requestor's global pid, which does not exist
> in
> that namespace. After the patch it reads pid 0, since the requestor
> has
> no pid there. There is no keyrings selftest in the tree, so this is
> the
> only test.
> 
> I am happy to send a patch. The raw pid_t cannot be translated later.
> So
> I stored a struct pid and print it with pid_nr_ns, the way the uid is
> already scoped.
> 
> Thanks,
> Maoyi
> https://maoyixie.com/
> 
> ---
> 
> diff --git a/include/keys/request_key_auth-type.h
> b/include/keys/request_key_auth-type.h
> index 01e42ee5f4099..464636278c4f8 100644
> --- a/include/keys/request_key_auth-type.h
> +++ b/include/keys/request_key_auth-type.h
> @@ -22,7 +22,7 @@ struct request_key_auth {
>  	const struct cred	*cred;
>  	void			*callout_info;
>  	size_t			callout_len;
> -	pid_t			pid;
> +	struct pid		*pid;
>  	char			op[8];
>  } __randomize_layout;
>  
> diff --git a/security/keys/request_key_auth.c
> b/security/keys/request_key_auth.c
> index 282e09d8fa46c..e8d1037526b19 100644
> --- a/security/keys/request_key_auth.c
> +++ b/security/keys/request_key_auth.c
> @@ -13,6 +13,8 @@
>  #include <linux/slab.h>
>  #include <linux/uaccess.h>
>  #include "internal.h"
> +#include <linux/pid_namespace.h>
> +#include <linux/proc_fs.h>
>  #include <keys/request_key_auth-type.h>
>  
>  static int request_key_auth_preparse(struct key_preparsed_payload
> *);
> @@ -73,7 +75,10 @@ static void request_key_auth_describe(const struct
> key *key,
>  	seq_puts(m, "key:");
>  	seq_puts(m, key->description);
>  	if (key_is_positive(key))
> -		seq_printf(m, " pid:%d ci:%zu", rka->pid, rka-
> >callout_len);
> +		seq_printf(m, " pid:%d ci:%zu",
> +			   pid_nr_ns(rka->pid,
> +				     proc_pid_ns(file_inode(m-
> >file)->i_sb)),
> +			   rka->callout_len);
>  }
>  
>  /*
> @@ -113,6 +118,7 @@ static void free_request_key_auth(struct
> request_key_auth *rka)
>  	if (rka->cred)
>  		put_cred(rka->cred);
>  	kfree(rka->callout_info);
> +	put_pid(rka->pid);
>  	kfree(rka);
>  }
>  
> @@ -226,14 +232,14 @@ struct key *request_key_auth_new(struct key
> *target, const char *op,
>  
>  		irka = cred->request_key_auth->payload.data[0];
>  		rka->cred = get_cred(irka->cred);
> -		rka->pid = irka->pid;
> +		rka->pid = get_pid(irka->pid);
>  
>  		up_read(&cred->request_key_auth->sem);
>  	}
>  	else {
>  		/* it isn't - use this process as the context */
>  		rka->cred = get_cred(cred);
> -		rka->pid = current->pid;
> +		rka->pid = get_pid(task_pid(current));

I'm afraid this get_pid/put_pid doesn't look right because what it will
do is pin the pid in the creator pid namespace.  Unfortunately, some
keys and keyrings aren't bound to a pid namespace, so if I create a key
in my user keyring and then enter a new pid namespace, with this change
it will still show up as the pid number of the init namespace (pinning
the struct pid there) and vice versa, which still sounds like the wrong
behaviour.

I think if you really want to fix this, you have to keep the pid as is,
but make sure it always records the pid in the init_pid_ns (i.e. always
translate back to init_ns) and then translate to the current pid_ns in
the output generator of /proc/keys.

Regards,

James