Re: [PATCH v10 2/4] security: Allow all LSMs to provide xattrs for inode_init_security hook

From: Roberto Sassu <roberto.sassu@huaweicloud.com>
To: Casey Schaufler <casey@schaufler-ca.com>,
	Paul Moore <paul@paul-moore.com>
Cc: zohar@linux.ibm.com, dmitry.kasatkin@gmail.com,
	jmorris@namei.org, serge@hallyn.com,
	stephen.smalley.work@gmail.com, eparis@parisplace.org,
	reiserfs-devel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-integrity@vger.kernel.org,
	linux-security-module@vger.kernel.org, selinux@vger.kernel.org,
	bpf@vger.kernel.org, kpsingh@kernel.org, keescook@chromium.org,
	nicolas.bouchinet@clip-os.org,
	Roberto Sassu <roberto.sassu@huawei.com>
Subject: Re: [PATCH v10 2/4] security: Allow all LSMs to provide xattrs for inode_init_security hook
Date: Wed, 5 Apr 2023 11:43:53 +0200	[thread overview]
Message-ID: <7549b624-421e-30b9-ca99-de42929354c7@huaweicloud.com> (raw)
In-Reply-To: <5dbb9430-1e26-ec12-26a2-3718c84e33c2@schaufler-ca.com>

On 4/5/2023 4:08 AM, Casey Schaufler wrote:
> On 4/4/2023 11:54 AM, Paul Moore wrote:
>> On Fri, Mar 31, 2023 at 8:33 AM Roberto Sassu
>> <roberto.sassu@huaweicloud.com> wrote:
>>> From: Roberto Sassu <roberto.sassu@huawei.com>
>>>
>>> Currently, security_inode_init_security() supports only one LSM providing
>>> an xattr and EVM calculating the HMAC on that xattr, plus other inode
>>> metadata.
>>>
>>> Allow all LSMs to provide one or multiple xattrs, by extending the security
>>> blob reservation mechanism. Introduce the new lbs_xattr_count field of the
>>> lsm_blob_sizes structure, so that each LSM can specify how many xattrs it
>>> needs, and the LSM infrastructure knows how many xattr slots it should
>>> allocate.
>>>
>>> Dynamically allocate the new_xattrs array to be populated by LSMs with the
>>> inode_init_security hook, and pass it to the latter instead of the
>>> name/value/len triple. Unify the !initxattrs and initxattrs case, simply
>>> don't allocate the new_xattrs array in the former.
>>>
>>> Also, pass to the hook the number of xattrs filled by each LSM, so that
>>> there are no gaps when the next LSM fills the array. Gaps might occur
>>> because an LSM can legitimately request xattrs to the LSM infrastructure,
>>> but not fill the reserved slots, if it was not initialized.
>>>
>>> Update the documentation of security_inode_init_security() to reflect the
>>> changes, and fix the description of the xattr name, as it is not allocated
>>> anymore.
>>>
>>> Finally, adapt both SELinux and Smack to use the new definition of the
>>> inode_init_security hook, and to fill the reserved slots in the xattr
>>> array. Introduce the lsm_get_xattr_slot() helper to retrieve an available
>>> slot to fill, and to increment the number of filled slots.
>>>
>>> Move the xattr->name assignment after the xattr->value one, so that it is
>>> done only in case of successful memory allocation. For Smack, also reserve
>>> space for the other defined xattrs although they are not set yet in
>>> smack_inode_init_security().
>>>
>>> Reported-by: Nicolas Bouchinet <nicolas.bouchinet@clip-os.org> (EVM crash)
>>> Link: https://lore.kernel.org/linux-integrity/Y1FTSIo+1x+4X0LS@archlinux/
>>> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
>>> ---
>>>   include/linux/lsm_hook_defs.h |  6 +--
>>>   include/linux/lsm_hooks.h     | 20 ++++++++++
>>>   security/security.c           | 71 +++++++++++++++++++++++------------
>>>   security/selinux/hooks.c      | 17 +++++----
>>>   security/smack/smack_lsm.c    | 32 ++++++++++------
>>>   5 files changed, 99 insertions(+), 47 deletions(-)
>> This looks good aside from a few small things (below).  From what I
>> can see, there are only two outstanding issues to answer: the number
>> of Smack xattrs, sign-off from Casey for the Smack bits.
>>
>>> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
>>> index 6bb55e61e8e..a1896f90089 100644
>>> --- a/include/linux/lsm_hook_defs.h
>>> +++ b/include/linux/lsm_hook_defs.h
>>> @@ -111,9 +111,9 @@ 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,
>>> -        struct inode *dir, const struct qstr *qstr, const char **name,
>>> -        void **value, size_t *len)
>>> +LSM_HOOK(int, -EOPNOTSUPP, inode_init_security, struct inode *inode,
>>> +        struct inode *dir, const struct qstr *qstr, struct xattr *xattrs,
>>> +        int *xattr_count)
>>>   LSM_HOOK(int, 0, inode_init_security_anon, struct inode *inode,
>>>           const struct qstr *name, const struct inode *context_inode)
>>>   LSM_HOOK(int, 0, inode_create, struct inode *dir, struct dentry *dentry,
>>> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
>>> index c2be66c669a..9eb9b686493 100644
>>> --- a/include/linux/lsm_hooks.h
>>> +++ b/include/linux/lsm_hooks.h
>>> @@ -28,6 +28,7 @@
>>>   #include <linux/security.h>
>>>   #include <linux/init.h>
>>>   #include <linux/rculist.h>
>>> +#include <linux/xattr.h>
>>>
>>>   union security_list_options {
>>>          #define LSM_HOOK(RET, DEFAULT, NAME, ...) RET (*NAME)(__VA_ARGS__);
>>> @@ -63,8 +64,27 @@ struct lsm_blob_sizes {
>>>          int     lbs_ipc;
>>>          int     lbs_msg_msg;
>>>          int     lbs_task;
>>> +       int     lbs_xattr_count; /* number of xattr slots in new_xattrs array */
>>>   };
>>>
>>> +/**
>>> + * lsm_get_xattr_slot - Return the next available slot and increment the index
>>> + * @xattrs: array storing LSM-provided xattrs
>>> + * @xattr_count: number of already stored xattrs (updated)
>>> + *
>>> + * Retrieve the first available slot in the @xattrs array to fill with an xattr,
>>> + * and increment @xattr_count.
>>> + *
>>> + * Return: The slot to fill in @xattrs if non-NULL, NULL otherwise.
>>> + */
>>> +static inline struct xattr *lsm_get_xattr_slot(struct xattr *xattrs,
>>> +                                              int *xattr_count)
>>> +{
>>> +       if (unlikely(!xattrs))
>>> +               return NULL;
>>> +       return xattrs + (*xattr_count)++;
>>> +}
>>> +
>>>   /*
>>>    * LSM_RET_VOID is used as the default value in LSM_HOOK definitions for void
>>>    * LSM hooks (in include/linux/lsm_hook_defs.h).
>>> diff --git a/security/security.c b/security/security.c
>>> index f4170efcddd..1aeaa8ce449 100644
>>> --- a/security/security.c
>>> +++ b/security/security.c
>>> @@ -31,8 +31,6 @@
>>>   #include <linux/msg.h>
>>>   #include <net/flow.h>
>>>
>>> -#define MAX_LSM_EVM_XATTR      2
>>> -
>>>   /* How many LSMs were built into the kernel? */
>>>   #define LSM_COUNT (__end_lsm_info - __start_lsm_info)
>>>
>>> @@ -212,6 +210,8 @@ static void __init lsm_set_blob_sizes(struct lsm_blob_sizes *needed)
>>>          lsm_set_blob_size(&needed->lbs_msg_msg, &blob_sizes.lbs_msg_msg);
>>>          lsm_set_blob_size(&needed->lbs_superblock, &blob_sizes.lbs_superblock);
>>>          lsm_set_blob_size(&needed->lbs_task, &blob_sizes.lbs_task);
>>> +       lsm_set_blob_size(&needed->lbs_xattr_count,
>>> +                         &blob_sizes.lbs_xattr_count);
>>>   }
>>>
>>>   /* Prepare LSM for initialization. */
>>> @@ -378,6 +378,7 @@ static void __init ordered_lsm_init(void)
>>>          init_debug("msg_msg blob size    = %d\n", blob_sizes.lbs_msg_msg);
>>>          init_debug("superblock blob size = %d\n", blob_sizes.lbs_superblock);
>>>          init_debug("task blob size       = %d\n", blob_sizes.lbs_task);
>>> +       init_debug("xattr slots          = %d\n", blob_sizes.lbs_xattr_count);
>>>
>>>          /*
>>>           * Create any kmem_caches needed for blobs
>>> @@ -1591,11 +1592,15 @@ EXPORT_SYMBOL(security_dentry_create_files_as);
>>>    * created inode and set up the incore security field for the new inode.  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.  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.
>>> + * hooks called by the VFS.  The hook function is expected to populate the
>>> + * @xattrs array, by calling lsm_get_xattr_slot() to retrieve the slots
>>> + * reserved by the security module with the lbs_xattr_count field of the
>>> + * lsm_blob_sizes structure.  For each slot, the hook function should set ->name
>>> + * to the attribute name suffix (e.g. selinux), to allocate ->value (will be
>>> + * freed by the caller) and set it to the attribute value, to set ->value_len to
>>> + * the length of the value.  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.
>>>    *
>>>    * Return: Returns 0 on success, -EOPNOTSUPP if no security attribute is
>>>    * needed, or -ENOMEM on memory allocation failure.
>>> @@ -1604,33 +1609,51 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
>>>                                   const struct qstr *qstr,
>>>                                   const initxattrs initxattrs, void *fs_data)
>>>   {
>>> -       struct xattr new_xattrs[MAX_LSM_EVM_XATTR + 1];
>>> -       struct xattr *lsm_xattr, *evm_xattr, *xattr;
>>> -       int ret;
>>> +       struct security_hook_list *P;
>>> +       struct xattr *new_xattrs = NULL;
>>> +       int ret = -EOPNOTSUPP, xattr_count = 0;
>>>
>>>          if (unlikely(IS_PRIVATE(inode)))
>>>                  return 0;
>>>
>>> -       if (!initxattrs)
>>> -               return call_int_hook(inode_init_security, -EOPNOTSUPP, inode,
>>> -                                    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);
>>> -       if (ret)
>>> +       if (!blob_sizes.lbs_xattr_count)
>>> +               return 0;
>>> +
>>> +       if (initxattrs) {
>>> +               /* Allocate +1 for EVM and +1 as terminator. */
>>> +               new_xattrs = kcalloc(blob_sizes.lbs_xattr_count + 2,
>>> +                                    sizeof(*new_xattrs), GFP_NOFS);
>>> +               if (!new_xattrs)
>>> +                       return -ENOMEM;
>>> +       }
>>> +
>>> +       hlist_for_each_entry(P, &security_hook_heads.inode_init_security,
>>> +                            list) {
>>> +               ret = P->hook.inode_init_security(inode, dir, qstr, new_xattrs,
>>> +                                                 &xattr_count);
>>> +               if (ret && ret != -EOPNOTSUPP)
>>> +                       goto out;
>>> +               /*
>>> +                * As documented in lsm_hooks.h, -EOPNOTSUPP in this context
>>> +                * means that the LSM is not willing to provide an xattr, not
>>> +                * that it wants to signal an error. Thus, continue to invoke
>>> +                * the remaining LSMs.
>>> +                */
>>> +       }
>>> +
>>> +       /* If initxattrs() is NULL, xattr_count is zero, skip the call. */
>>> +       if (!xattr_count)
>>>                  goto out;
>>>
>>> -       evm_xattr = lsm_xattr + 1;
>>> -       ret = evm_inode_init_security(inode, lsm_xattr, evm_xattr);
>>> +       ret = evm_inode_init_security(inode, new_xattrs,
>>> +                                     new_xattrs + xattr_count);
>> I think it's cleaner to write '&new_xattrs[xattr_count]' for the third
>> parameter above (no concerns around pointer math), and stylistically
>> it matches better with the for-kfree loop below.
>>
>>>          if (ret)
>>>                  goto out;
>>>          ret = initxattrs(inode, new_xattrs, fs_data);
>>>   out:
>>> -       for (xattr = new_xattrs; xattr->value != NULL; xattr++)
>>> -               kfree(xattr->value);
>>> +       for (; xattr_count > 0; xattr_count--)
>>> +               kfree(new_xattrs[xattr_count - 1].value);
>>> +       kfree(new_xattrs);
>>>          return (ret == -EOPNOTSUPP) ? 0 : ret;
>>>   }
>>>   EXPORT_SYMBOL(security_inode_init_security);
>> ..
>>
>>> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
>>> index cfcbb748da2..8392983334b 100644
>>> --- a/security/smack/smack_lsm.c
>>> +++ b/security/smack/smack_lsm.c
>>> @@ -52,6 +52,15 @@
>>>   #define SMK_RECEIVING  1
>>>   #define SMK_SENDING    2
>>>
>>> +/*
>>> + * Smack uses multiple xattrs.
>>> + * SMACK64 - for access control, SMACK64EXEC - label for the program,
>> I think it would be good to move SMACK64EXEC to its own line; it took
>> me a minute to figure out why SMACK_INODE_INIT_XATTRS was set to '4'
>> when I only say three comment lines ... ;)
>>
>>> + * SMACK64MMAP - controls library loading,
>>> + * SMACK64TRANSMUTE - label initialization,
>>> + * Not saved on files - SMACK64IPIN and SMACK64IPOUT
>>> + */
>>> +#define SMACK_INODE_INIT_XATTRS 4
>> If smack_inode_init_security() only ever populates a single xattr, and
>> that is the only current user of SMACK_INODE_INIT_XATTRS, can we make
>> this '1' and shrink the xattr allocation a bit?
> 
> If the parent directory is marked with SMACK64_TRANSMUTE, the access
> rule allowing the access has the "t" mode, and the object being initialized
> is a directory, the new inode should get the SMACK64_TRANSMUTE attribute.
> The callers of security_inode_init_security() don't seem to care.
> I can't say if the evm code is getting SMACK64_TRANSMUTE or, for that
> matter, SMACK64_EXEC and SMACK64_MMAP, some other way. The older system
> allowed for multiple Smack xattrs, but I'm not clear on exactly how.

If you like to set an additional xattr, that would be possible now. 
Since we reserve multiple xattrs, we can call lsm_get_xattr_slot() 
another time and set SMACK64_TRANSMUTE.

I think, if the kernel config has CONFIG_EVM_EXTRA_SMACK_XATTRS set,
EVM would protect SMACK64_TRANSMUTE too.

Roberto

>>>   #ifdef SMACK_IPV6_PORT_LABELING
>>>   static DEFINE_MUTEX(smack_ipv6_lock);
>>>   static LIST_HEAD(smk_ipv6_port_list);
>>> @@ -939,26 +948,23 @@ static int smack_inode_alloc_security(struct inode *inode)
>>>    * @inode: the newly created inode
>>>    * @dir: containing directory object
>>>    * @qstr: unused
>>> - * @name: where to put the attribute name
>>> - * @value: where to put the attribute value
>>> - * @len: where to put the length of the attribute
>>> + * @xattrs: where to put the attributes
>>> + * @xattr_count: current number of LSM-provided xattrs (updated)
>>>    *
>>>    * Returns 0 if it all works out, -ENOMEM if there's no memory
>>>    */
>>>   static int smack_inode_init_security(struct inode *inode, struct inode *dir,
>>> -                                    const struct qstr *qstr, const char **name,
>>> -                                    void **value, size_t *len)
>>> +                                    const struct qstr *qstr,
>>> +                                    struct xattr *xattrs, int *xattr_count)
>>>   {
>>>          struct inode_smack *issp = smack_inode(inode);
>>>          struct smack_known *skp = smk_of_current();
>>>          struct smack_known *isp = smk_of_inode(inode);
>>>          struct smack_known *dsp = smk_of_inode(dir);
>>> +       struct xattr *xattr = lsm_get_xattr_slot(xattrs, xattr_count);
>>>          int may;
>>>
>>> -       if (name)
>>> -               *name = XATTR_SMACK_SUFFIX;
>>> -
>>> -       if (value && len) {
>>> +       if (xattr) {
>>>                  rcu_read_lock();
>>>                  may = smk_access_entry(skp->smk_known, dsp->smk_known,
>>>                                         &skp->smk_rules);
>>> @@ -976,11 +982,12 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
>>>                          issp->smk_flags |= SMK_INODE_CHANGED;
>>>                  }
>>>
>>> -               *value = kstrdup(isp->smk_known, GFP_NOFS);
>>> -               if (*value == NULL)
>>> +               xattr->value = kstrdup(isp->smk_known, GFP_NOFS);
>>> +               if (xattr->value == NULL)
>>>                          return -ENOMEM;
>>>
>>> -               *len = strlen(isp->smk_known);
>>> +               xattr->value_len = strlen(isp->smk_known);
>>> +               xattr->name = XATTR_SMACK_SUFFIX;
>>>          }
>>>
>>>          return 0;
>>> @@ -4854,6 +4861,7 @@ struct lsm_blob_sizes smack_blob_sizes __lsm_ro_after_init = {
>>>          .lbs_ipc = sizeof(struct smack_known *),
>>>          .lbs_msg_msg = sizeof(struct smack_known *),
>>>          .lbs_superblock = sizeof(struct superblock_smack),
>>> +       .lbs_xattr_count = SMACK_INODE_INIT_XATTRS,
>>>   };
>>>
>>>   static struct security_hook_list smack_hooks[] __lsm_ro_after_init = {
>>> --
>>> 2.25.1