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Message-ID: <0caf1e58-1e2d-c781-3f71-18aaf55cada2@schaufler-ca.com>
Date: Wed, 21 Apr 2021 15:43:49 -0700
From: Casey Schaufler <casey@...aufler-ca.com>
To: Roberto Sassu <roberto.sassu@...wei.com>, zohar@...ux.ibm.com,
jmorris@...ei.org, paul@...l-moore.com
Cc: linux-integrity@...r.kernel.org,
linux-security-module@...r.kernel.org,
linux-kernel@...r.kernel.org, selinux@...r.kernel.org,
reiserfs-devel@...r.kernel.org,
Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH v2 3/6] security: Pass xattrs allocated by LSMs to the
inode_init_security hook
On 4/21/2021 9:19 AM, Roberto Sassu wrote:
> In preparation for moving EVM to the LSM infrastructure, this patch
> replaces the name, value, len triple with the xattr array pointer provided
> by security_inode_init_security(). LSMs are expected to call the new
> function lsm_find_xattr_slot() to find the first unused slot of the array
> where the xattr should be written.
>
> This patch modifies also SELinux and Smack to search for an unused slot, to
> have a consistent behavior across LSMs (the unmodified version would
> overwrite the xattr set by the first LSM in the chain). It is also
> desirable to have the modification in those LSMs, as they are likely used
> as a reference for the development of new LSMs.
This looks better than V1. One safety comment below.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
> ---
> include/linux/lsm_hook_defs.h | 4 ++--
> include/linux/lsm_hooks.h | 18 +++++++++++++++---
> security/security.c | 13 +++++++------
> security/selinux/hooks.c | 13 ++++++-------
> security/smack/smack_lsm.c | 20 +++++++++-----------
> 5 files changed, 39 insertions(+), 29 deletions(-)
>
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 477a597db013..afb9dd122f60 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -111,8 +111,8 @@ LSM_HOOK(int, 0, path_notify, const struct path *path, u64 mask,
> 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)
> + struct inode *dir, const struct qstr *qstr, struct xattr *xattrs,
> + void *fs_data)
> 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 c5498f5174ce..e8c9bac29b9d 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -27,6 +27,7 @@
>
> #include <linux/security.h>
> #include <linux/init.h>
> +#include <linux/xattr.h>
> #include <linux/rculist.h>
>
> /**
> @@ -227,9 +228,11 @@
> * @inode contains the inode structure of the newly created inode.
> * @dir contains the inode structure of the parent directory.
> * @qstr contains the last path component of the new object
> - * @name will be set to the allocated name suffix (e.g. selinux).
> - * @value will be set to the allocated attribute value.
> - * @len will be set to the length of the value.
> + * @xattrs contains the full array of xattrs allocated by LSMs where
> + * ->name will be set to the allocated name suffix (e.g. selinux).
> + * ->value will be set to the allocated attribute value.
> + * ->len will be set to the length of the value.
> + * @fs_data contains filesystem-specific data.
> * Returns 0 if @name and @value have been successfully set,
> * -EOPNOTSUPP if no security attribute is needed, or
> * -ENOMEM on memory allocation failure.
> @@ -1661,4 +1664,13 @@ static inline void security_delete_hooks(struct security_hook_list *hooks,
>
> extern int lsm_inode_alloc(struct inode *inode);
>
Some "security researcher" with a fuzz tester is going to manage to dump junk
into the slots and ruin your week. I suggest a simple change to make bounds checking
possible. It should never happen, but if that was sufficient people would
love C
string processing better.
> +static inline struct xattr *lsm_find_xattr_slot(struct xattr *xattrs)
+static inline struct xattr *lsm_find_xattr_slot(struct xattr *xattrs, int available)
> +{
> + struct xattr *slot;
> +
> + for (slot = xattrs; slot && slot->name != NULL; slot++)
+ for (slot = xattrs; slot && slot->name != NULL; slot++)
if (WARN_ON(slot > xattrs[available]))
return NULL;
> + ;
> +
> + return slot;
> +}
> #endif /* ! __LINUX_LSM_HOOKS_H */
> diff --git a/security/security.c b/security/security.c
> index 7f14e59c4f8e..2c1fe1496069 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1037,18 +1037,16 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
>
> if (!initxattrs)
> return call_int_hook(inode_init_security, -EOPNOTSUPP, inode,
> - dir, qstr, NULL, NULL, NULL);
> + dir, qstr, NULL, fs_data);
> 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);
> + lsm_xattr, fs_data);
> 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);
> @@ -1071,10 +1069,13 @@ int security_old_inode_init_security(struct inode *inode, struct inode *dir,
> const struct qstr *qstr, const char **name,
> void **value, size_t *len)
> {
> + struct xattr xattr = { .name = NULL, .value = NULL, .value_len = 0 };
> + struct xattr *lsm_xattr = (name && value && len) ? &xattr : NULL;
> +
> if (unlikely(IS_PRIVATE(inode)))
> return -EOPNOTSUPP;
> return call_int_hook(inode_init_security, -EOPNOTSUPP, inode, dir,
> - qstr, name, value, len);
> + qstr, lsm_xattr, NULL);
> }
> EXPORT_SYMBOL(security_old_inode_init_security);
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index ddd097790d47..806827eb132a 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2916,11 +2916,11 @@ static int selinux_dentry_create_files_as(struct dentry *dentry, int mode,
>
> static int selinux_inode_init_security(struct inode *inode, struct inode *dir,
> const struct qstr *qstr,
> - const char **name,
> - void **value, size_t *len)
> + struct xattr *xattrs, void *fs_data)
> {
> const struct task_security_struct *tsec = selinux_cred(current_cred());
> struct superblock_security_struct *sbsec;
> + struct xattr *xattr = lsm_find_xattr_slot(xattrs);
> u32 newsid, clen;
> int rc;
> char *context;
> @@ -2947,16 +2947,15 @@ static int selinux_inode_init_security(struct inode *inode, struct inode *dir,
> !(sbsec->flags & SBLABEL_MNT))
> return -EOPNOTSUPP;
>
> - if (name)
> - *name = XATTR_SELINUX_SUFFIX;
> + if (xattr) {
> + xattr->name = XATTR_SELINUX_SUFFIX;
>
> - if (value && len) {
> rc = security_sid_to_context_force(&selinux_state, newsid,
> &context, &clen);
> if (rc)
> return rc;
> - *value = context;
> - *len = clen;
> + xattr->value = context;
> + xattr->value_len = clen;
> }
>
> return 0;
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index 12a45e61c1a5..af7eee0fee52 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -962,26 +962,24 @@ 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 attribute
> *
> * 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, void *fs_data)
> {
> 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_find_xattr_slot(xattrs);
> int may;
>
> - if (name)
> - *name = XATTR_SMACK_SUFFIX;
> + if (xattr) {
> + xattr->name = XATTR_SMACK_SUFFIX;
>
> - if (value && len) {
> rcu_read_lock();
> may = smk_access_entry(skp->smk_known, dsp->smk_known,
> &skp->smk_rules);
> @@ -999,11 +997,11 @@ 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);
> }
>
> return 0;
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