[PATCH v3 11/34] lsm: get rid of the lsm_names list and do some cleanup

[Roberto Sassu]

On Tue, 2025-09-02 at 10:20 -0700, John Johansen wrote:
> On 8/14/25 15:50, Paul Moore wrote:
> > The LSM currently has a lot of code to maintain a list of the currently
> > active LSMs in a human readable string, with the only user being the
> > "/sys/kernel/security/lsm" code.  Let's drop all of that code and
> > generate the string on first use and then cache it for subsequent use.
> > 
> > Signed-off-by: Paul Moore <paul at paul-moore.com>
> > ---
> >   include/linux/lsm_hooks.h |  1 -
> >   security/inode.c          | 59 +++++++++++++++++++++++++++++++++++++--
> >   security/lsm_init.c       | 49 --------------------------------
> >   3 files changed, 57 insertions(+), 52 deletions(-)
> > 
> > diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> > index 7343dd60b1d5..65a8227bece7 100644
> > --- a/include/linux/lsm_hooks.h
> > +++ b/include/linux/lsm_hooks.h
> > @@ -172,7 +172,6 @@ struct lsm_info {
> >   
> >   
> >   /* DO NOT tamper with these variables outside of the LSM framework */
> > -extern char *lsm_names;
> >   extern struct lsm_static_calls_table static_calls_table __ro_after_init;
> >   
> >   /**
> > diff --git a/security/inode.c b/security/inode.c
> > index 43382ef8896e..a5e7a073e672 100644
> > --- a/security/inode.c
> > +++ b/security/inode.c
> > @@ -22,6 +22,8 @@
> >   #include <linux/lsm_hooks.h>
> >   #include <linux/magic.h>
> >   
> > +#include "lsm.h"
> > +
> >   static struct vfsmount *mount;
> >   static int mount_count;
> >   
> > @@ -315,12 +317,65 @@ void securityfs_remove(struct dentry *dentry)
> >   EXPORT_SYMBOL_GPL(securityfs_remove);
> >   
> >   #ifdef CONFIG_SECURITY
> > +#include <linux/spinlock.h>
> > +
> >   static struct dentry *lsm_dentry;
> > +
> > +/* NOTE: we never free the string below once it is set. */
> > +static DEFINE_SPINLOCK(lsm_read_lock);
> 
> nit, this is only used on the write side, so not the best name
> 
> > +static char *lsm_read_str = NULL;
> > +static ssize_t lsm_read_len = 0;
> > +
> >   static ssize_t lsm_read(struct file *filp, char __user *buf, size_t count,
> >   			loff_t *ppos)
> >   {
> > -	return simple_read_from_buffer(buf, count, ppos, lsm_names,
> > -		strlen(lsm_names));
> > +	int i;
> > +	char *str;
> > +	ssize_t len;
> > +
> > +restart:
> > +
> > +	rcu_read_lock();

Uhm, it seems we cannot use plain RCU here, simple_read_from_buffer()
can sleep.

Roberto

> > +	if (!lsm_read_str) {
> should probably be
> if (!rcu_access_pointer(lsm_read_str)) {
> 
> > +		/* we need to generate the string and try again */
> > +		rcu_read_unlock();
> > +		goto generate_string;
> > +	}
> > +	len = simple_read_from_buffer(buf, count, ppos,
> > +				      rcu_dereference(lsm_read_str),
> > +				      lsm_read_len);
> > +	rcu_read_unlock();
> > +	return len;
> > +
> > +generate_string:
> > +
> > +	for (i = 0; i < lsm_active_cnt; i++)
> > +		/* the '+ 1' accounts for either a comma or a NUL */
> > +		len += strlen(lsm_idlist[i]->name) + 1;
> > +
> > +	str = kmalloc(len, GFP_KERNEL);
> > +	if (!str)
> > +		return -ENOMEM;
> > +	str[0] = '\0';
> > +
> > +	for (i = 0; i < lsm_active_cnt; i++) {
> > +		if (i > 0)
> > +			strcat(str, ",");
> > +		strcat(str, lsm_idlist[i]->name);
> > +	}
> > +
> > +	spin_lock(&lsm_read_lock);
> > +	if (lsm_read_str) {
> > +		/* we raced and lost */
> > +		spin_unlock(&lsm_read_lock);
> > +		kfree(str);
> > +		goto restart;
> > +	}
> > +	lsm_read_str = str;
> > +	lsm_read_len = len - 1;
> > +	spin_unlock(&lsm_read_lock);
> > +
> > +	goto restart;
> >   }
> >   
> >   static const struct file_operations lsm_ops = {
> > diff --git a/security/lsm_init.c b/security/lsm_init.c
> > index 9e495a36a332..87e2147016b3 100644
> > --- a/security/lsm_init.c
> > +++ b/security/lsm_init.c
> > @@ -10,8 +10,6 @@
> >   
> >   #include "lsm.h"
> >   
> > -char *lsm_names;
> > -
> >   /* Pointers to LSM sections defined in include/asm-generic/vmlinux.lds.h */
> >   extern struct lsm_info __start_lsm_info[], __end_lsm_info[];
> >   extern struct lsm_info __start_early_lsm_info[], __end_early_lsm_info[];
> > @@ -371,42 +369,6 @@ static void __init lsm_init_ordered(void)
> >   	}
> >   }
> >   
> > -static bool match_last_lsm(const char *list, const char *lsm)
> > -{
> > -	const char *last;
> > -
> > -	if (WARN_ON(!list || !lsm))
> > -		return false;
> > -	last = strrchr(list, ',');
> > -	if (last)
> > -		/* Pass the comma, strcmp() will check for '\0' */
> > -		last++;
> > -	else
> > -		last = list;
> > -	return !strcmp(last, lsm);
> > -}
> > -
> > -static int lsm_append(const char *new, char **result)
> > -{
> > -	char *cp;
> > -
> > -	if (*result == NULL) {
> > -		*result = kstrdup(new, GFP_KERNEL);
> > -		if (*result == NULL)
> > -			return -ENOMEM;
> > -	} else {
> > -		/* Check if it is the last registered name */
> > -		if (match_last_lsm(*result, new))
> > -			return 0;
> > -		cp = kasprintf(GFP_KERNEL, "%s,%s", *result, new);
> > -		if (cp == NULL)
> > -			return -ENOMEM;
> > -		kfree(*result);
> > -		*result = cp;
> > -	}
> > -	return 0;
> > -}
> > -
> >   static void __init lsm_static_call_init(struct security_hook_list *hl)
> >   {
> >   	struct lsm_static_call *scall = hl->scalls;
> > @@ -443,15 +405,6 @@ void __init security_add_hooks(struct security_hook_list *hooks, int count,
> >   		hooks[i].lsmid = lsmid;
> >   		lsm_static_call_init(&hooks[i]);
> >   	}
> > -
> > -	/*
> > -	 * Don't try to append during early_security_init(), we'll come back
> > -	 * and fix this up afterwards.
> > -	 */
> > -	if (slab_is_available()) {
> > -		if (lsm_append(lsmid->name, &lsm_names) < 0)
> > -			panic("%s - Cannot get early memory.\n", __func__);
> > -	}
> >   }
> >   
> >   int __init early_security_init(void)
> > @@ -488,8 +441,6 @@ int __init security_init(void)
> >   	lsm_early_for_each_raw(lsm) {
> >   		init_debug("  early started: %s (%s)\n", lsm->id->name,
> >   			   is_enabled(lsm) ? "enabled" : "disabled");
> > -		if (lsm->enabled)
> > -			lsm_append(lsm->id->name, &lsm_names);
> >   	}
> >   
> >   	/* Load LSMs in specified order. */
>