[PATCH v2] evm: Correct inode_init_security hooks behaviors
Casey Schaufler
casey at schaufler-ca.com
Fri Oct 21 15:42:20 UTC 2022
On 10/21/2022 8:14 AM, Nicolas Bouchinet wrote:
> From: Nicolas Bouchinet <nicolas.bouchinet at 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.
>
> 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.
This does not match the code, which sets the value based on the
security modules compiled in rather than the constant 5.
>
> 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>
> ---
>
> Changes since v1:
> https://lore.kernel.org/linux-integrity/Y1FTSIo+1x+4X0LS@archlinux/
> * The `call_int_hook_xattr` macro has been inlined into the
> `security_inode_init_security` hook (as suggested by Paul Moor,
> Mickaël Salaün and Casey Schaufler).
> * The MAX_LSM_EVM_XATTR value is processed based on compiled LSMs (as
> proposed by Casey Schaufler).
> * Various typos and cosmetic changes has been fixed (as suggested by
> Mickaël Salaün)
> * A MAX_LSM_EVM_XATTR test has been wrapped in a WAN_ON_ONCE macro (as
> suggested by Mickaël Salaün).
> ---
> 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 | 22 ++++++++++++++++++++++
> security/integrity/evm/evm_main.c | 9 +++++----
> security/security.c | 28 +++++++++++++++++++++-------
> 6 files changed, 53 insertions(+), 14 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..db6868875e09 100644
> --- a/security/integrity/evm/evm_crypto.c
> +++ b/security/integrity/evm/evm_crypto.c
> @@ -385,6 +385,28 @@ 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..4f89b9eaf028 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);
> }
> @@ -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);
> @@ -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_xattr, xattr_data->digest);
> if (rc < 0)
> goto out;
>
> diff --git a/security/security.c b/security/security.c
> index 14d30fec8a00..cf11bd7d6a1b 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 +1109,23 @@ 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;
> + if (WARN_ON_ONCE(i >= MAX_LSM_EVM_XATTR))
> + ret = -ENOMEM;
> + if (ret != 0)
> + break;
> + lsm_xattr++;
> + i++;
> + }
> if (ret)
> 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);
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