[PATCH] evm: Correct inode_init_security hooks behaviors
Nicolas Bouchinet
nicolas.bouchinet at clip-os.org
Mon Oct 24 12:50:55 UTC 2022
Hi Roberto,
On Fri, Oct 21, 2022 at 04:02:11PM +0200, Roberto Sassu wrote:
> On Thu, 2022-10-20 at 15:55 +0200, Nicolas Bouchinet wrote:
> > From: Nicolas Bouchinet <nicolas.bouchinet at ssi.gouv.fr>
> >
> > Fixes a NULL pointer dereference occuring 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.
>
> + eBPF mailing list, KP
>
> Looking at include/linux/lsm_hooks.h:
>
> * @inode_init_security:
>
> [...]
>
> * 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.
>
> [...]
>
> * Returns 0 if @name and @value have been successfully set,
> * -EOPNOTSUPP if no security attribute is needed, or
>
> In my opinion, it should be responsibility of the eBPF infrastructure
> to ensure that this is true (meaning that it cannot let security
> modules attach to that hook without an additional check).
>
> What do you think?
>
> Nicolas, in the past I addressed the same issue of lacking support of
> multiple LSMs providing an xattr at inode creation time.
>
> Would this patch set be fine for you, or you would still do
> differently?
IMHO, changing the default return value of the hook to `-EOPNOTSUPP`
in the lsm_hook_defs.h should be enough, the LSM implementation however
does not follows the current documentation as returning `-EOPNOTSUPP`
skips the whole processing of LSMs `xattrs`. This is why I check the
return value of each LSM `inode_init_security()` functions in the patch.
>
> https://lore.kernel.org/all/20210427113732.471066-1-roberto.sassu@huawei.com/
>
> At the end of the cover letter, you can find also a TestLSM I developed
> to ensure that support for multiple LSMs works correctly. I also tested
> the patch set with the SELinux and SMACK test suites.
Thanks, I'll give it a look !
>
> Thanks
>
> Roberto
>
> > Adds a `call_int_hook_xattr` macro that fetches and feed the
> > `new_xattrs` array with every called hook xattr values.
> >
> > Adds a `evm_init_hmacs` function which init the EVM hmac using every
> > entry of the array contrary to `evm_init_hmac`.
> >
> > Fixes the `evm_inode_init_security` function to use `evm_init_hmacs`.
> >
> > The `MAX_LSM_EVM_XATTR` value has been raised to 5 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: stable at vger.kernel.org
> > Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet at ssi.gouv.fr>
> > ---
> > include/linux/lsm_hook_defs.h | 2 +-
> > include/linux/lsm_hooks.h | 4 ++--
> > security/integrity/evm/evm.h | 2 ++
> > security/integrity/evm/evm_crypto.c | 23 ++++++++++++++++++++++-
> > security/integrity/evm/evm_main.c | 11 ++++++-----
> > security/security.c | 29 ++++++++++++++++++++++++++-
> > --
> > 6 files changed, 59 insertions(+), 12 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.
> > * 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..a2f9886e924d 100644
> > --- a/security/integrity/evm/evm.h
> > +++ b/security/integrity/evm/evm.h
> > @@ -60,6 +60,8 @@ int evm_calc_hash(struct dentry *dentry, const char
> > *req_xattr_name,
> > struct evm_digest *data);
> > int evm_init_hmac(struct inode *inode, const struct xattr *xattr,
> > char *hmac_val);
> > +int evm_init_hmacs(struct inode *inode, const struct xattr *xattrs,
> > + char *hmac_val);
> > int evm_init_secfs(void);
> >
> > #endif
> > diff --git a/security/integrity/evm/evm_crypto.c
> > b/security/integrity/evm/evm_crypto.c
> > index 708de9656bbd..e5a34306cab6 100644
> > --- a/security/integrity/evm/evm_crypto.c
> > +++ b/security/integrity/evm/evm_crypto.c
> > @@ -347,7 +347,6 @@ static int evm_is_immutable(struct dentry
> > *dentry, struct inode *inode)
> > return rc;
> > }
> >
> > -
> > /*
> > * Calculate the hmac and update security.evm xattr
> > *
> > @@ -385,6 +384,28 @@ int evm_update_evmxattr(struct dentry *dentry,
> > const char *xattr_name,
> > return rc;
> > }
> >
> > +int evm_protected_xattr(const char *req_xattr_name);
> > +
> > +int evm_init_hmacs(struct inode *inode, const struct xattr
> > *lsm_xattrs,
> > + char *hmac_val)
> > +{
> > + struct shash_desc *desc;
> > +
> > + desc = init_desc(EVM_XATTR_HMAC, HASH_ALGO_SHA1);
> > + if (IS_ERR(desc)) {
> > + pr_info("init_desc failed\n");
> > + return PTR_ERR(desc);
> > + }
> > +
> > + 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;
> > +}
> > +
> > int evm_init_hmac(struct inode *inode, const struct xattr
> > *lsm_xattr,
> > char *hmac_val)
> > {
> > diff --git a/security/integrity/evm/evm_main.c
> > b/security/integrity/evm/evm_main.c
> > index 2e6fb6e2ffd2..bb071c55d656 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;
> > 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);
> > }
> > @@ -835,14 +837,13 @@ void evm_inode_post_setattr(struct dentry
> > *dentry, int ia_valid)
> > * evm_inode_init_security - initializes security.evm HMAC value
> > */
> > int evm_inode_init_security(struct inode *inode,
> > - const struct xattr *lsm_xattr,
> > + const struct xattr *lsm_xattrs,
> > struct xattr *evm_xattr)
> > {
> > 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);
> > @@ -850,7 +851,7 @@ int evm_inode_init_security(struct inode *inode,
> > return -ENOMEM;
> >
> > xattr_data->data.type = EVM_XATTR_HMAC;
> > - rc = evm_init_hmac(inode, lsm_xattr, xattr_data->digest);
> > + rc = evm_init_hmacs(inode, lsm_xattrs, xattr_data->digest);
> > if (rc < 0)
> > goto out;
> >
> > diff --git a/security/security.c b/security/security.c
> > index 14d30fec8a00..47012c118536 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -30,7 +30,7 @@
> > #include <linux/msg.h>
> > #include <net/flow.h>
> >
> > -#define MAX_LSM_EVM_XATTR 2
> > +#define MAX_LSM_EVM_XATTR 5
> >
> > /* How many LSMs were built into the kernel? */
> > #define LSM_COUNT (__end_lsm_info - __start_lsm_info)
> > @@ -746,6 +746,29 @@ static int lsm_superblock_alloc(struct
> > super_block *sb)
> > RC; \
> > })
> >
> > +#define call_int_hook_xattr(XATTRS, FUNC, IRC, ...) ({
> > \
> > + int RC = IRC; \
> > + int i = 0; \
> > + do { \
> > + struct security_hook_list *P; \
> > + \
> > + hlist_for_each_entry(P, &security_hook_heads.FUNC,
> > list) { \
> > + RC = P->hook.FUNC(__VA_ARGS__); \
> > + if (RC == -EOPNOTSUPP) \
> > + continue; \
> > + if (RC != 0 && RC != IRC) \
> > + break; \
> > + if (i >= MAX_LSM_EVM_XATTR) { \
> > + RC = -ENOMEM; \
> > + break; \
> > + } \
> > + XATTRS++; \
> > + i++; \
> > + } \
> > + } while (0); \
> > + RC; \
> > +})
> > +
> > /* Security operations */
> >
> > int security_binder_set_context_mgr(const struct cred *mgr)
> > @@ -1103,7 +1126,7 @@ 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,
> > + ret = call_int_hook_xattr(lsm_xattr, inode_init_security,
> > -EOPNOTSUPP, inode, dir, qstr,
> > &lsm_xattr->name,
> > &lsm_xattr->value,
> > &lsm_xattr->value_len);
> > @@ -1111,7 +1134,7 @@ int security_inode_init_security(struct inode
> > *inode, struct inode *dir,
> > 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, evm_xattr);
> > if (ret)
> > goto out;
> > ret = initxattrs(inode, new_xattrs, fs_data);
>
Best regards,
Nicolas Bouchinet
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