From e61b135f7bfe47f547fb566328a97ca8baa3548c Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Thu, 22 Sep 2022 17:17:10 +0200 Subject: integrity: implement get and set acl hook The current way of setting and getting posix acls through the generic xattr interface is error prone and type unsafe. The vfs needs to interpret and fixup posix acls before storing or reporting it to userspace. Various hacks exist to make this work. The code is hard to understand and difficult to maintain in it's current form. Instead of making this work by hacking posix acls through xattr handlers we are building a dedicated posix acl api around the get and set inode operations. This removes a lot of hackiness and makes the codepaths easier to maintain. A lot of background can be found in [1]. So far posix acls were passed as a void blob to the security and integrity modules. Some of them like evm then proceed to interpret the void pointer and convert it into the kernel internal struct posix acl representation to perform their integrity checking magic. This is obviously pretty problematic as that requires knowledge that only the vfs is guaranteed to have and has lead to various bugs. Add a proper security hook for setting posix acls and pass down the posix acls in their appropriate vfs format instead of hacking it through a void pointer stored in the uapi format. I spent considerate time in the security module and integrity infrastructure and audited all codepaths. EVM is the only part that really has restrictions based on the actual posix acl values passed through it (e.g., i_mode). Before this dedicated hook EVM used to translate from the uapi posix acl format sent to it in the form of a void pointer into the vfs format. This is not a good thing. Instead of hacking around in the uapi struct give EVM the posix acls in the appropriate vfs format and perform sane permissions checks that mirror what it used to to in the generic xattr hook. IMA doesn't have any restrictions on posix acls. When posix acls are changed it just wants to update its appraisal status to trigger an EVM revalidation. The removal of posix acls is equivalent to passing NULL to the posix set acl hooks. This is the same as before through the generic xattr api. Link: https://lore.kernel.org/all/20220801145520.1532837-1-brauner@kernel.org [1] Acked-by: Paul Moore (LSM) Signed-off-by: Christian Brauner (Microsoft) --- security/integrity/evm/evm_main.c | 83 ++++++++++++++++++++++++++++++++++++++- 1 file changed, 82 insertions(+), 1 deletion(-) (limited to 'security/integrity/evm/evm_main.c') diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c index 23d484e05e6f..dcc5e704ef70 100644 --- a/security/integrity/evm/evm_main.c +++ b/security/integrity/evm/evm_main.c @@ -8,7 +8,7 @@ * * File: evm_main.c * implements evm_inode_setxattr, evm_inode_post_setxattr, - * evm_inode_removexattr, and evm_verifyxattr + * evm_inode_removexattr, evm_verifyxattr, and evm_inode_set_acl. */ #define pr_fmt(fmt) "EVM: "fmt @@ -670,6 +670,87 @@ int evm_inode_removexattr(struct user_namespace *mnt_userns, return evm_protect_xattr(mnt_userns, dentry, xattr_name, NULL, 0); } +#ifdef CONFIG_FS_POSIX_ACL +static int evm_inode_set_acl_change(struct user_namespace *mnt_userns, + struct dentry *dentry, const char *name, + struct posix_acl *kacl) +{ + int rc; + + umode_t mode; + struct inode *inode = d_backing_inode(dentry); + + if (!kacl) + return 1; + + rc = posix_acl_update_mode(mnt_userns, inode, &mode, &kacl); + if (rc || (inode->i_mode != mode)) + return 1; + + return 0; +} +#else +static inline int evm_inode_set_acl_change(struct user_namespace *mnt_userns, + struct dentry *dentry, + const char *name, + struct posix_acl *kacl) +{ + return 0; +} +#endif + +/** + * evm_inode_set_acl - protect the EVM extended attribute from posix acls + * @mnt_userns: user namespace of the idmapped mount + * @dentry: pointer to the affected dentry + * @acl_name: name of the posix acl + * @kacl: pointer to the posix acls + * + * Prevent modifying posix acls causing the EVM HMAC to be re-calculated + * and 'security.evm' xattr updated, unless the existing 'security.evm' is + * valid. + */ +int evm_inode_set_acl(struct user_namespace *mnt_userns, struct dentry *dentry, + const char *acl_name, struct posix_acl *kacl) +{ + enum integrity_status evm_status; + + /* Policy permits modification of the protected xattrs even though + * there's no HMAC key loaded + */ + if (evm_initialized & EVM_ALLOW_METADATA_WRITES) + return 0; + + evm_status = evm_verify_current_integrity(dentry); + if ((evm_status == INTEGRITY_PASS) || + (evm_status == INTEGRITY_NOXATTRS)) + return 0; + + /* Exception if the HMAC is not going to be calculated. */ + if (evm_hmac_disabled() && (evm_status == INTEGRITY_NOLABEL || + evm_status == INTEGRITY_UNKNOWN)) + return 0; + + /* + * Writing other xattrs is safe for portable signatures, as portable + * signatures are immutable and can never be updated. + */ + if (evm_status == INTEGRITY_FAIL_IMMUTABLE) + return 0; + + if (evm_status == INTEGRITY_PASS_IMMUTABLE && + !evm_inode_set_acl_change(mnt_userns, dentry, acl_name, kacl)) + return 0; + + if (evm_status != INTEGRITY_PASS && + evm_status != INTEGRITY_PASS_IMMUTABLE) + integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry), + dentry->d_name.name, "appraise_metadata", + integrity_status_msg[evm_status], + -EPERM, 0); + return evm_status == INTEGRITY_PASS ? 0 : -EPERM; +} + static void evm_reset_status(struct inode *inode) { struct integrity_iint_cache *iint; -- cgit v1.2.3-58-ga151 From e10796b87e1d1bad593c89c5e8cc2e0992bd1335 Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Thu, 22 Sep 2022 17:17:15 +0200 Subject: evm: remove evm_xattr_acl_change() The security and integrity infrastructure has dedicated hooks now so evm_xattr_acl_change() is dead code. Before this commit the callchain was: evm_protect_xattr() -> evm_xattr_change() -> evm_xattr_acl_change() where evm_protect_xattr() was hit from evm_inode_setxattr() and evm_inode_removexattr(). But now we have evm_inode_set_acl() and evm_inode_remove_acl() and have switched over the vfs to rely on the posix acl api so the code isn't hit anymore. Suggested-by: Paul Moore Signed-off-by: Christian Brauner (Microsoft) --- security/integrity/evm/evm_main.c | 64 --------------------------------------- 1 file changed, 64 deletions(-) (limited to 'security/integrity/evm/evm_main.c') diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c index dcc5e704ef70..e074c2b4d499 100644 --- a/security/integrity/evm/evm_main.c +++ b/security/integrity/evm/evm_main.c @@ -434,66 +434,6 @@ static enum integrity_status evm_verify_current_integrity(struct dentry *dentry) return evm_verify_hmac(dentry, NULL, NULL, 0, NULL); } -/* - * evm_xattr_acl_change - check if passed ACL changes the inode mode - * @mnt_userns: user namespace of the idmapped mount - * @dentry: pointer to the affected dentry - * @xattr_name: requested xattr - * @xattr_value: requested xattr value - * @xattr_value_len: requested xattr value length - * - * Check if passed ACL changes the inode mode, which is protected by EVM. - * - * Returns 1 if passed ACL causes inode mode change, 0 otherwise. - */ -static int evm_xattr_acl_change(struct user_namespace *mnt_userns, - struct dentry *dentry, const char *xattr_name, - const void *xattr_value, size_t xattr_value_len) -{ -#ifdef CONFIG_FS_POSIX_ACL - umode_t mode; - struct posix_acl *acl = NULL, *acl_res; - struct inode *inode = d_backing_inode(dentry); - int rc; - - /* - * An earlier comment here mentioned that the idmappings for - * ACL_{GROUP,USER} don't matter since EVM is only interested in the - * mode stored as part of POSIX ACLs. Nonetheless, if it must translate - * from the uapi POSIX ACL representation to the VFS internal POSIX ACL - * representation it should do so correctly. There's no guarantee that - * we won't change POSIX ACLs in a way that ACL_{GROUP,USER} matters - * for the mode at some point and it's difficult to keep track of all - * the LSM and integrity modules and what they do to POSIX ACLs. - * - * Frankly, EVM shouldn't try to interpret the uapi struct for POSIX - * ACLs it received. It requires knowledge that only the VFS is - * guaranteed to have. - */ - acl = vfs_set_acl_prepare(mnt_userns, i_user_ns(inode), - xattr_value, xattr_value_len); - if (IS_ERR_OR_NULL(acl)) - return 1; - - acl_res = acl; - /* - * Passing mnt_userns is necessary to correctly determine the GID in - * an idmapped mount, as the GID is used to clear the setgid bit in - * the inode mode. - */ - rc = posix_acl_update_mode(mnt_userns, inode, &mode, &acl_res); - - posix_acl_release(acl); - - if (rc) - return 1; - - if (inode->i_mode != mode) - return 1; -#endif - return 0; -} - /* * evm_xattr_change - check if passed xattr value differs from current value * @mnt_userns: user namespace of the idmapped mount @@ -513,10 +453,6 @@ static int evm_xattr_change(struct user_namespace *mnt_userns, char *xattr_data = NULL; int rc = 0; - if (posix_xattr_acl(xattr_name)) - return evm_xattr_acl_change(mnt_userns, dentry, xattr_name, - xattr_value, xattr_value_len); - rc = vfs_getxattr_alloc(&init_user_ns, dentry, xattr_name, &xattr_data, 0, GFP_NOFS); if (rc < 0) -- cgit v1.2.3-58-ga151