Re: [PATCH v4] evm: Correct inode_init_security hooks behaviors

[Nicolas Bouchinet <nicolas.bouchinet@clip-os.org>]

Hi Roberto, sorry for the late reply.

On Thu, Nov 03, 2022 at 04:27:51PM +0100, Roberto Sassu wrote:
> On Wed, 2022-10-26 at 16:30 +0200, Nicolas Bouchinet wrote:
> > From: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
> > 
> > Fixes a NULL pointer dereference occurring in the
> > `evm_protected_xattr_common` function of the EVM LSM. The bug is
> > triggered if a `inode_init_security` hook returns 0 without initializing
> > the given `struct xattr` fields (which is the case of BPF) and if no
> > other LSM overrides thoses fields after. This also leads to memory
> > leaks.
> > 
> > The `call_int_hook_xattr` macro has been inlined into the
> > `security_inode_init_security` hook in order to check hooks return
> > values and skip ones who doesn't init `xattrs`.
> > 
> > Modify `evm_init_hmac` function to init the EVM hmac using every
> > entry of the given xattr array.
> > 
> > The `MAX_LSM_EVM_XATTR` value is now based on the security modules
> > compiled in, which gives room for SMACK, SELinux, Apparmor, BPF and
> > IMA/EVM security attributes.
> > 
> > Changes the default return value of the `inode_init_security` hook
> > definition to `-EOPNOTSUPP`.
> > 
> > Changes the hook documentation to match the behavior of the LSMs using
> > it (only xattr->value is initialised with kmalloc and thus is the only
> > one that should be kfreed by the caller).
> > 
> > Cc: roberto.sassu@huaweicloud.com
> > Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
> > ---
> > Changes since v3:
> > https://lore.kernel.org/linux-integrity/Y1fu4jofqLHVDprT@archlinux/
> > 
> > * Fixes compilation error reported by the kernel test robot.
> > ---
> >  include/linux/lsm_hook_defs.h       |  2 +-
> >  include/linux/lsm_hooks.h           |  4 ++--
> >  security/integrity/evm/evm.h        |  1 +
> >  security/integrity/evm/evm_crypto.c |  9 +++++++--
> >  security/integrity/evm/evm_main.c   |  7 ++++---
> >  security/security.c                 | 31 ++++++++++++++++++++++-------
> >  6 files changed, 39 insertions(+), 15 deletions(-)
> > 
> > diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> > index 806448173033..e5dd0c0f6345 100644
> > --- a/include/linux/lsm_hook_defs.h
> > +++ b/include/linux/lsm_hook_defs.h
> > @@ -111,7 +111,7 @@ LSM_HOOK(int, 0, path_notify, const struct path *path, u64 mask,
> >  	 unsigned int obj_type)
> >  LSM_HOOK(int, 0, inode_alloc_security, struct inode *inode)
> >  LSM_HOOK(void, LSM_RET_VOID, inode_free_security, struct inode *inode)
> > -LSM_HOOK(int, 0, inode_init_security, struct inode *inode,
> > +LSM_HOOK(int, -EOPNOTSUPP, inode_init_security, struct inode *inode,
> >  	 struct inode *dir, const struct qstr *qstr, const char **name,
> >  	 void **value, size_t *len)
> >  LSM_HOOK(int, 0, inode_init_security_anon, struct inode *inode,
> > diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> > index 84a0d7e02176..95aff9383de1 100644
> > --- a/include/linux/lsm_hooks.h
> > +++ b/include/linux/lsm_hooks.h
> > @@ -229,8 +229,8 @@
> >   *	This hook is called by the fs code as part of the inode creation
> >   *	transaction and provides for atomic labeling of the inode, unlike
> >   *	the post_create/mkdir/... hooks called by the VFS.  The hook function
> > - *	is expected to allocate the name and value via kmalloc, with the caller
> > - *	being responsible for calling kfree after using them.
> > + *	is expected to allocate the value via kmalloc, with the caller
> > + *	being responsible for calling kfree after using it.
> 
> Please also update the description of @name as well (remove allocated).
ACK, will do it.
> 
> >   *	If the security module does not use security attributes or does
> >   *	not wish to put a security attribute on this particular inode,
> >   *	then it should return -EOPNOTSUPP to skip this processing.
> > diff --git a/security/integrity/evm/evm.h b/security/integrity/evm/evm.h
> > index f8b8c5004fc7..6d9628ca7c24 100644
> > --- a/security/integrity/evm/evm.h
> > +++ b/security/integrity/evm/evm.h
> > @@ -61,5 +61,6 @@ int evm_calc_hash(struct dentry *dentry, const char *req_xattr_name,
> >  int evm_init_hmac(struct inode *inode, const struct xattr *xattr,
> >  		  char *hmac_val);
> >  int evm_init_secfs(void);
> > +int evm_protected_xattr(const char *req_xattr_name);
> >  
> >  #endif
> > diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
> > index 708de9656bbd..06639f3cfb38 100644
> > --- a/security/integrity/evm/evm_crypto.c
> > +++ b/security/integrity/evm/evm_crypto.c
> > @@ -385,7 +385,7 @@ int evm_update_evmxattr(struct dentry *dentry, const char *xattr_name,
> >  	return rc;
> >  }
> >  
> > -int evm_init_hmac(struct inode *inode, const struct xattr *lsm_xattr,
> > +int evm_init_hmac(struct inode *inode, const struct xattr *lsm_xattrs,
> >  		  char *hmac_val)
> >  {
> >  	struct shash_desc *desc;
> > @@ -396,7 +396,12 @@ int evm_init_hmac(struct inode *inode, const struct xattr *lsm_xattr,
> >  		return PTR_ERR(desc);
> >  	}
> >  
> > -	crypto_shash_update(desc, lsm_xattr->value, lsm_xattr->value_len);
> > +	for (int i = 0; lsm_xattrs[i].value != NULL; i++) {
> > +		if (evm_protected_xattr(lsm_xattrs[i].name))
> > +			crypto_shash_update(desc,
> > +					    lsm_xattrs[i].value,
> > +					    lsm_xattrs[i].value_len);
> > +	}
> >  	hmac_add_misc(desc, inode, EVM_XATTR_HMAC, hmac_val);
> >  	kfree(desc);
> >  	return 0;
> > diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> > index 2e6fb6e2ffd2..0420453a80e8 100644
> > --- a/security/integrity/evm/evm_main.c
> > +++ b/security/integrity/evm/evm_main.c
> > @@ -284,6 +284,8 @@ static int evm_protected_xattr_common(const char *req_xattr_name,
> >  	int found = 0;
> >  	struct xattr_list *xattr;
> >  
> > +	if (!req_xattr_name)
> > +		return found;
> 
> Remove, and use the check below.
If I understood it well, this patchset will not be backported. Wouldn't it be necessary to make this check here
for backports ?
> 
> >  	namelen = strlen(req_xattr_name);
> >  	list_for_each_entry_lockless(xattr, &evm_config_xattrnames, list) {
> >  		if (!all_xattrs && !xattr->enabled)
> > @@ -305,7 +307,7 @@ static int evm_protected_xattr_common(const char *req_xattr_name,
> >  	return found;
> >  }
> >  
> > -static int evm_protected_xattr(const char *req_xattr_name)
> > +int evm_protected_xattr(const char *req_xattr_name)
> >  {
> >  	return evm_protected_xattr_common(req_xattr_name, false);
> >  }
> > @@ -841,8 +843,7 @@ int evm_inode_init_security(struct inode *inode,
> >  	struct evm_xattr *xattr_data;
> >  	int rc;
> >  
> > -	if (!(evm_initialized & EVM_INIT_HMAC) ||
> > -	    !evm_protected_xattr(lsm_xattr->name))
> > +	if (!(evm_initialized & EVM_INIT_HMAC))
> >  		return 0;
> >  
> >  	xattr_data = kzalloc(sizeof(*xattr_data), GFP_NOFS);
> > diff --git a/security/security.c b/security/security.c
> > index 14d30fec8a00..79524f8734f1 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -30,7 +30,11 @@
> >  #include <linux/msg.h>
> >  #include <net/flow.h>
> >  
> > -#define MAX_LSM_EVM_XATTR	2
> > +#define MAX_LSM_EVM_XATTR                                \
> > +	((IS_ENABLED(CONFIG_EVM) ? 1 : 0) +              \
> > +	 (IS_ENABLED(CONFIG_SECURITY_SELINUX) ? 1 : 0) + \
> > +	 (IS_ENABLED(CONFIG_SECURITY_SMACK) ? 1 : 0) +   \
> > +	 (IS_ENABLED(CONFIG_BPF_LSM) ? 1 : 0))
> >  
> >  /* How many LSMs were built into the kernel? */
> >  #define LSM_COUNT (__end_lsm_info - __start_lsm_info)
> > @@ -1091,9 +1095,11 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
> >  				 const struct qstr *qstr,
> >  				 const initxattrs initxattrs, void *fs_data)
> >  {
> > +	int i = 0;
> > +	int ret = -EOPNOTSUPP;
> >  	struct xattr new_xattrs[MAX_LSM_EVM_XATTR + 1];
> >  	struct xattr *lsm_xattr, *evm_xattr, *xattr;
> > -	int ret;
> > +	struct security_hook_list *hook_ptr;
> >  
> >  	if (unlikely(IS_PRIVATE(inode)))
> >  		return 0;
> > @@ -1103,15 +1109,26 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
> >  				     dir, qstr, NULL, NULL, NULL);
> >  	memset(new_xattrs, 0, sizeof(new_xattrs));
> >  	lsm_xattr = new_xattrs;
> > -	ret = call_int_hook(inode_init_security, -EOPNOTSUPP, inode, dir, qstr,
> > -						&lsm_xattr->name,
> > -						&lsm_xattr->value,
> > -						&lsm_xattr->value_len);
> > +	hlist_for_each_entry(hook_ptr, &security_hook_heads.inode_init_security,
> > +			     list) {
> > +		ret = hook_ptr->hook.inode_init_security(inode, dir, qstr,
> > +				&lsm_xattr->name,
> > +				&lsm_xattr->value,
> > +				&lsm_xattr->value_len);
> > +		if (ret == -EOPNOTSUPP)
> > +			continue;
> 
> This does not work properly. Suppose that you have an LSM with xattr
> and another without. The final ret will be -EOPNOTSUPP. Instead declare
> new_xattrs_set boolean, and set to true if ret = 0. After the loop,
> check the boolean instead of ret. If ret != 0 goto out.
> 
Your right, will change it.

> > +		if (WARN_ON_ONCE(i >= MAX_LSM_EVM_XATTR))
> > +			ret = -ENOMEM;
> > +		if (ret != 0)
> > +			break;
> 
> We can check here if the LSM behaved properly, i.e. it set the xattr
> name and value:
> 
> if (WARN_ON_ONCE(!lsm_xattr->name || !lsm_xattr->value)) {
> 	ret = -ENOMEM;
> 	goto out;
I'm OK with this, thanks, will change it !

> }
> 
> > +		lsm_xattr++;
> > +		i++;
> > +	}
> >  	if (ret)
> >  		goto out;
> >  
> >  	evm_xattr = lsm_xattr + 1;
> 
> It should be:
> 
> 	evm_xattr = lsm_xattr;
> 
> You incremented lsm_xattr already, after the LSMs set their xattr.

`lsm_xattr` is indeed already incremented, will patch this.

> 
> Once you complete the changes, I will send a patch set including your
> patch with some more patches.
> 
> Roberto
> 
> > -	ret = evm_inode_init_security(inode, lsm_xattr, evm_xattr);
> > +	ret = evm_inode_init_security(inode, new_xattrs, evm_xattr);
> >  	if (ret)
> >  		goto out;
> >  	ret = initxattrs(inode, new_xattrs, fs_data);
> 

Thank's a lot for your review,

Nicolas Bouchinet