From 61df7b82820494368bd46071ca97e43a3dfc3b11 Mon Sep 17 00:00:00 2001 From: Paul Moore Date: Thu, 2 May 2024 17:57:51 -0400 Subject: [PATCH 1/2] lsm: fixup the inode xattr capability handling The current security_inode_setxattr() and security_inode_removexattr() hooks rely on individual LSMs to either call into the associated capability hooks (cap_inode_setxattr() or cap_inode_removexattr()), or return a magic value of 1 to indicate that the LSM layer itself should perform the capability checks. Unfortunately, with the default return value for these LSM hooks being 0, an individual LSM hook returning a 1 will cause the LSM hook processing to exit early, potentially skipping a LSM. Thankfully, with the exception of the BPF LSM, none of the LSMs which currently register inode xattr hooks should end up returning a value of 1, and in the BPF LSM case, with the BPF LSM hooks executing last there should be no real harm in stopping processing of the LSM hooks. However, the reliance on the individual LSMs to either call the capability hooks themselves, or signal the LSM with a return value of 1, is fragile and relies on a specific set of LSMs being enabled. This patch is an effort to resolve, or minimize, these issues. Before we discuss the solution, there are a few observations and considerations that we need to take into account: * BPF LSM registers an implementation for every LSM hook, and that implementation simply returns the hook's default return value, a 0 in this case. We want to ensure that the default BPF LSM behavior results in the capability checks being called. * SELinux and Smack do not expect the traditional capability checks to be applied to the xattrs that they "own". * SELinux and Smack are currently written in such a way that the xattr capability checks happen before any additional LSM specific access control checks. SELinux does apply SELinux specific access controls to all xattrs, even those not "owned" by SELinux. * IMA and EVM also register xattr hooks but assume that the LSM layer and specific LSMs have already authorized the basic xattr operation. In order to ensure we perform the capability based access controls before the individual LSM access controls, perform only one capability access control check for each operation, and clarify the logic around applying the capability controls, we need a mechanism to determine if any of the enabled LSMs "own" a particular xattr and want to take responsibility for controlling access to that xattr. The solution in this patch is to create a new LSM hook, 'inode_xattr_skipcap', that is not exported to the rest of the kernel via a security_XXX() function, but is used by the LSM layer to determine if a LSM wants to control access to a given xattr and avoid the traditional capability controls. Registering an inode_xattr_skipcap hook is optional, if a LSM declines to register an implementation, or uses an implementation that simply returns the default value (0), there is no effect as the LSM continues to enforce the capability based controls (unless another LSM takes ownership of the xattr). If none of the LSMs signal that the capability checks should be skipped, the capability check is performed and if access is granted the individual LSM xattr access control hooks are executed, keeping with the DAC-before-LSM convention. Cc: stable@vger.kernel.org Acked-by: Casey Schaufler Signed-off-by: Paul Moore --- include/linux/lsm_hook_defs.h | 1 + security/security.c | 70 ++++++++++++++++++++++++----------- security/selinux/hooks.c | 28 ++++++++++---- security/smack/smack_lsm.c | 31 +++++++++++++++- 4 files changed, 98 insertions(+), 32 deletions(-) diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h index f804b76cde44..8fd87f823d3a 100644 --- a/include/linux/lsm_hook_defs.h +++ b/include/linux/lsm_hook_defs.h @@ -144,6 +144,7 @@ LSM_HOOK(int, 0, inode_setattr, struct mnt_idmap *idmap, struct dentry *dentry, LSM_HOOK(void, LSM_RET_VOID, inode_post_setattr, struct mnt_idmap *idmap, struct dentry *dentry, int ia_valid) LSM_HOOK(int, 0, inode_getattr, const struct path *path) +LSM_HOOK(int, 0, inode_xattr_skipcap, const char *name) LSM_HOOK(int, 0, inode_setxattr, struct mnt_idmap *idmap, struct dentry *dentry, const char *name, const void *value, size_t size, int flags) diff --git a/security/security.c b/security/security.c index e5da848c50b9..b52e81ac5526 100644 --- a/security/security.c +++ b/security/security.c @@ -2278,7 +2278,20 @@ int security_inode_getattr(const struct path *path) * @size: size of xattr value * @flags: flags * - * Check permission before setting the extended attributes. + * This hook performs the desired permission checks before setting the extended + * attributes (xattrs) on @dentry. It is important to note that we have some + * additional logic before the main LSM implementation calls to detect if we + * need to perform an additional capability check at the LSM layer. + * + * Normally we enforce a capability check prior to executing the various LSM + * hook implementations, but if a LSM wants to avoid this capability check, + * it can register a 'inode_xattr_skipcap' hook and return a value of 1 for + * xattrs that it wants to avoid the capability check, leaving the LSM fully + * responsible for enforcing the access control for the specific xattr. If all + * of the enabled LSMs refrain from registering a 'inode_xattr_skipcap' hook, + * or return a 0 (the default return value), the capability check is still + * performed. If no 'inode_xattr_skipcap' hooks are registered the capability + * check is performed. * * Return: Returns 0 if permission is granted. */ @@ -2286,20 +2299,20 @@ int security_inode_setxattr(struct mnt_idmap *idmap, struct dentry *dentry, const char *name, const void *value, size_t size, int flags) { - int ret; + int rc; if (unlikely(IS_PRIVATE(d_backing_inode(dentry)))) return 0; - /* - * SELinux and Smack integrate the cap call, - * so assume that all LSMs supplying this call do so. - */ - ret = call_int_hook(inode_setxattr, idmap, dentry, name, value, size, - flags); - if (ret == 1) - ret = cap_inode_setxattr(dentry, name, value, size, flags); - return ret; + /* enforce the capability checks at the lsm layer, if needed */ + if (!call_int_hook(inode_xattr_skipcap, name)) { + rc = cap_inode_setxattr(dentry, name, value, size, flags); + if (rc) + return rc; + } + + return call_int_hook(inode_setxattr, idmap, dentry, name, value, size, + flags); } /** @@ -2452,26 +2465,39 @@ int security_inode_listxattr(struct dentry *dentry) * @dentry: file * @name: xattr name * - * Check permission before removing the extended attribute identified by @name - * for @dentry. + * This hook performs the desired permission checks before setting the extended + * attributes (xattrs) on @dentry. It is important to note that we have some + * additional logic before the main LSM implementation calls to detect if we + * need to perform an additional capability check at the LSM layer. + * + * Normally we enforce a capability check prior to executing the various LSM + * hook implementations, but if a LSM wants to avoid this capability check, + * it can register a 'inode_xattr_skipcap' hook and return a value of 1 for + * xattrs that it wants to avoid the capability check, leaving the LSM fully + * responsible for enforcing the access control for the specific xattr. If all + * of the enabled LSMs refrain from registering a 'inode_xattr_skipcap' hook, + * or return a 0 (the default return value), the capability check is still + * performed. If no 'inode_xattr_skipcap' hooks are registered the capability + * check is performed. * * Return: Returns 0 if permission is granted. */ int security_inode_removexattr(struct mnt_idmap *idmap, struct dentry *dentry, const char *name) { - int ret; + int rc; if (unlikely(IS_PRIVATE(d_backing_inode(dentry)))) return 0; - /* - * SELinux and Smack integrate the cap call, - * so assume that all LSMs supplying this call do so. - */ - ret = call_int_hook(inode_removexattr, idmap, dentry, name); - if (ret == 1) - ret = cap_inode_removexattr(idmap, dentry, name); - return ret; + + /* enforce the capability checks at the lsm layer, if needed */ + if (!call_int_hook(inode_xattr_skipcap, name)) { + rc = cap_inode_removexattr(idmap, dentry, name); + if (rc) + return rc; + } + + return call_int_hook(inode_removexattr, idmap, dentry, name); } /** diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c index 7eed331e90f0..17e6640bbf5b 100644 --- a/security/selinux/hooks.c +++ b/security/selinux/hooks.c @@ -3177,6 +3177,23 @@ static bool has_cap_mac_admin(bool audit) return true; } +/** + * selinux_inode_xattr_skipcap - Skip the xattr capability checks? + * @name: name of the xattr + * + * Returns 1 to indicate that SELinux "owns" the access control rights to xattrs + * named @name; the LSM layer should avoid enforcing any traditional + * capability based access controls on this xattr. Returns 0 to indicate that + * SELinux does not "own" the access control rights to xattrs named @name and is + * deferring to the LSM layer for further access controls, including capability + * based controls. + */ +static int selinux_inode_xattr_skipcap(const char *name) +{ + /* require capability check if not a selinux xattr */ + return !strcmp(name, XATTR_NAME_SELINUX); +} + static int selinux_inode_setxattr(struct mnt_idmap *idmap, struct dentry *dentry, const char *name, const void *value, size_t size, int flags) @@ -3188,15 +3205,9 @@ static int selinux_inode_setxattr(struct mnt_idmap *idmap, u32 newsid, sid = current_sid(); int rc = 0; - if (strcmp(name, XATTR_NAME_SELINUX)) { - rc = cap_inode_setxattr(dentry, name, value, size, flags); - if (rc) - return rc; - - /* Not an attribute we recognize, so just check the - ordinary setattr permission. */ + /* if not a selinux xattr, only check the ordinary setattr perm */ + if (strcmp(name, XATTR_NAME_SELINUX)) return dentry_has_perm(current_cred(), dentry, FILE__SETATTR); - } if (!selinux_initialized()) return (inode_owner_or_capable(idmap, inode) ? 0 : -EPERM); @@ -7175,6 +7186,7 @@ static struct security_hook_list selinux_hooks[] __ro_after_init = { LSM_HOOK_INIT(inode_permission, selinux_inode_permission), LSM_HOOK_INIT(inode_setattr, selinux_inode_setattr), LSM_HOOK_INIT(inode_getattr, selinux_inode_getattr), + LSM_HOOK_INIT(inode_xattr_skipcap, selinux_inode_xattr_skipcap), LSM_HOOK_INIT(inode_setxattr, selinux_inode_setxattr), LSM_HOOK_INIT(inode_post_setxattr, selinux_inode_post_setxattr), LSM_HOOK_INIT(inode_getxattr, selinux_inode_getxattr), diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c index 70ba2841e181..1833bd50d97a 100644 --- a/security/smack/smack_lsm.c +++ b/security/smack/smack_lsm.c @@ -1282,6 +1282,33 @@ static int smack_inode_getattr(const struct path *path) return rc; } +/** + * smack_inode_xattr_skipcap - Skip the xattr capability checks? + * @name: name of the xattr + * + * Returns 1 to indicate that Smack "owns" the access control rights to xattrs + * named @name; the LSM layer should avoid enforcing any traditional + * capability based access controls on this xattr. Returns 0 to indicate that + * Smack does not "own" the access control rights to xattrs named @name and is + * deferring to the LSM layer for further access controls, including capability + * based controls. + */ +static int smack_inode_xattr_skipcap(const char *name) +{ + if (strncmp(name, XATTR_SMACK_SUFFIX, strlen(XATTR_SMACK_SUFFIX))) + return 0; + + if (strcmp(name, XATTR_NAME_SMACK) == 0 || + strcmp(name, XATTR_NAME_SMACKIPIN) == 0 || + strcmp(name, XATTR_NAME_SMACKIPOUT) == 0 || + strcmp(name, XATTR_NAME_SMACKEXEC) == 0 || + strcmp(name, XATTR_NAME_SMACKMMAP) == 0 || + strcmp(name, XATTR_NAME_SMACKTRANSMUTE) == 0) + return 1; + + return 0; +} + /** * smack_inode_setxattr - Smack check for setting xattrs * @idmap: idmap of the mount @@ -1325,8 +1352,7 @@ static int smack_inode_setxattr(struct mnt_idmap *idmap, size != TRANS_TRUE_SIZE || strncmp(value, TRANS_TRUE, TRANS_TRUE_SIZE) != 0) rc = -EINVAL; - } else - rc = cap_inode_setxattr(dentry, name, value, size, flags); + } if (check_priv && !smack_privileged(CAP_MAC_ADMIN)) rc = -EPERM; @@ -5051,6 +5077,7 @@ static struct security_hook_list smack_hooks[] __ro_after_init = { LSM_HOOK_INIT(inode_permission, smack_inode_permission), LSM_HOOK_INIT(inode_setattr, smack_inode_setattr), LSM_HOOK_INIT(inode_getattr, smack_inode_getattr), + LSM_HOOK_INIT(inode_xattr_skipcap, smack_inode_xattr_skipcap), LSM_HOOK_INIT(inode_setxattr, smack_inode_setxattr), LSM_HOOK_INIT(inode_post_setxattr, smack_inode_post_setxattr), LSM_HOOK_INIT(inode_getxattr, smack_inode_getxattr), From dd44477e7fa15ba3b100dfc67bf7cf083f3dccf6 Mon Sep 17 00:00:00 2001 From: Paul Moore Date: Wed, 3 Jul 2024 17:00:20 -0400 Subject: [PATCH 2/2] selinux,smack: remove the capability checks in the removexattr hooks Commit 61df7b828204 ("lsm: fixup the inode xattr capability handling") moved the responsibility of doing the inode xattr capability checking out of the individual LSMs and into the LSM framework itself. Unfortunately, while the original commit added the capability checks to both the setxattr and removexattr code in the LSM framework, it only removed the setxattr capability checks from the individual LSMs, leaving duplicated removexattr capability checks in both the SELinux and Smack code. This patch removes the duplicated code from SELinux and Smack. Fixes: 61df7b828204 ("lsm: fixup the inode xattr capability handling") Acked-by: Casey Schaufler Signed-off-by: Paul Moore --- security/selinux/hooks.c | 10 ++-------- security/smack/smack_lsm.c | 3 +-- 2 files changed, 3 insertions(+), 10 deletions(-) diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c index 17e6640bbf5b..55c78c318ccd 100644 --- a/security/selinux/hooks.c +++ b/security/selinux/hooks.c @@ -3356,15 +3356,9 @@ static int selinux_inode_listxattr(struct dentry *dentry) static int selinux_inode_removexattr(struct mnt_idmap *idmap, struct dentry *dentry, const char *name) { - if (strcmp(name, XATTR_NAME_SELINUX)) { - int rc = cap_inode_removexattr(idmap, dentry, name); - if (rc) - return rc; - - /* Not an attribute we recognize, so just check the - ordinary setattr permission. */ + /* if not a selinux xattr, only check the ordinary setattr perm */ + if (strcmp(name, XATTR_NAME_SELINUX)) return dentry_has_perm(current_cred(), dentry, FILE__SETATTR); - } if (!selinux_initialized()) return 0; diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c index 1833bd50d97a..7d6d58868775 100644 --- a/security/smack/smack_lsm.c +++ b/security/smack/smack_lsm.c @@ -1461,8 +1461,7 @@ static int smack_inode_removexattr(struct mnt_idmap *idmap, strcmp(name, XATTR_NAME_SMACKMMAP) == 0) { if (!smack_privileged(CAP_MAC_ADMIN)) rc = -EPERM; - } else - rc = cap_inode_removexattr(idmap, dentry, name); + } if (rc != 0) return rc;