Re: [PATCH] integrity: evm: Fix RCU list related warnings.

[Mimi Zohar]

Hi Stephen,

On Fri, 2020-05-08 at 10:14 +1000, Stephen Rothwell wrote:
> Hi all,
> 
> On Thu, 30 Apr 2020 21:32:05 +0530 madhuparnabhowmik10@xxxxxxxxx wrote:
> >
> > From: Madhuparna Bhowmik <madhuparnabhowmik10@xxxxxxxxx>
> > 
> > This patch fixes the following warning and few other
> > instances of traversal of evm_config_xattrnames list:
> > 
> > [   32.848432] =============================
> > [   32.848707] WARNING: suspicious RCU usage
> > [   32.848966] 5.7.0-rc1-00006-ga8d5875ce5f0b #1 Not tainted
> > [   32.849308] -----------------------------
> > [   32.849567] security/integrity/evm/evm_main.c:231 RCU-list traversed in non-reader section!!
> > 
> > Since entries are only added to the list and never deleted,
> > use list_For_each_entry_lockless() instead of
> > list_for_each_entry_rcu() for traversing the list.
> > Also, add a relevant comment in evm_secfs.c to indicate this fact.
> > 
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@xxxxxxxxx>
> > Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > ---
> >  security/integrity/evm/evm_crypto.c | 2 +-
> >  security/integrity/evm/evm_main.c   | 4 ++--
> >  security/integrity/evm/evm_secfs.c  | 9 ++++++++-
> >  3 files changed, 11 insertions(+), 4 deletions(-)
> > 
> > diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
> > index 35682852ddea..b2dc87da5f50 100644
> > --- a/security/integrity/evm/evm_crypto.c
> > +++ b/security/integrity/evm/evm_crypto.c
> > @@ -207,7 +207,7 @@ static int evm_calc_hmac_or_hash(struct dentry *dentry,
> >  	data->hdr.length = crypto_shash_digestsize(desc->tfm);
> >  
> >  	error = -ENODATA;
> > -	list_for_each_entry_rcu(xattr, &evm_config_xattrnames, list) {
> > +	list_for_each_entry_lockless(xattr, &evm_config_xattrnames, list) {
> >  		bool is_ima = false;
> >  
> >  		if (strcmp(xattr->name, XATTR_NAME_IMA) == 0)
> > diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> > index d361d7fdafc4..0d36259b690d 100644
> > --- a/security/integrity/evm/evm_main.c
> > +++ b/security/integrity/evm/evm_main.c
> > @@ -97,7 +97,7 @@ static int evm_find_protected_xattrs(struct dentry *dentry)
> >  	if (!(inode->i_opflags & IOP_XATTR))
> >  		return -EOPNOTSUPP;
> >  
> > -	list_for_each_entry_rcu(xattr, &evm_config_xattrnames, list) {
> > +	list_for_each_entry_lockless(xattr, &evm_config_xattrnames, list) {
> >  		error = __vfs_getxattr(dentry, inode, xattr->name, NULL, 0);
> >  		if (error < 0) {
> >  			if (error == -ENODATA)
> > @@ -228,7 +228,7 @@ static int evm_protected_xattr(const char *req_xattr_name)
> >  	struct xattr_list *xattr;
> >  
> >  	namelen = strlen(req_xattr_name);
> > -	list_for_each_entry_rcu(xattr, &evm_config_xattrnames, list) {
> > +	list_for_each_entry_lockless(xattr, &evm_config_xattrnames, list) {
> >  		if ((strlen(xattr->name) == namelen)
> >  		    && (strncmp(req_xattr_name, xattr->name, namelen) == 0)) {
> >  			found = 1;
> > diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c
> > index 39ad1038d45d..cfc3075769bb 100644
> > --- a/security/integrity/evm/evm_secfs.c
> > +++ b/security/integrity/evm/evm_secfs.c
> > @@ -232,7 +232,14 @@ static ssize_t evm_write_xattrs(struct file *file, const char __user *buf,
> >  		goto out;
> >  	}
> >  
> > -	/* Guard against races in evm_read_xattrs */
> > +	/*
> > +	 * xattr_list_mutex guards against races in evm_read_xattrs().
> > +	 * Entries are only added to the evm_config_xattrnames list
> > +	 * and never deleted. Therefore, the list is traversed
> > +	 * using list_for_each_entry_lockless() without holding
> > +	 * the mutex in evm_calc_hmac_or_hash(), evm_find_protected_xattrs()
> > +	 * and evm_protected_xattr().
> > +	 */
> >  	mutex_lock(&xattr_list_mutex);
> >  	list_for_each_entry(tmp, &evm_config_xattrnames, list) {
> >  		if (strcmp(xattr->name, tmp->name) == 0) {
> 
> I will apply this to linux-next today.
> 
> Is there something stopping this being applied to a tree?  And does it
> need to be sent to Linus soon?

Sorry for the delay in pushing this and other fixes to the next-
integrity branch. ÂIt's in my next-integrity-testing branch.

This isn't a bug per-se, just annotating the reason for the lack of
locking.

thanks,

Mimi