On 7/18/2019 7:30 AM, Aaron Goidel wrote: > As of now, setting watches on filesystem objects has, at most, applied a > check for read access to the inode, and in the case of fanotify, requires > CAP_SYS_ADMIN. No specific security hook or permission check has been > provided to control the setting of watches. Using any of inotify, dnotify, > or fanotify, it is possible to observe, not only write-like operations, but > even read access to a file. Modeling the watch as being merely a read from > the file is insufficient for the needs of SELinux. This is due to the fact > that read access should not necessarily imply access to information about > when another process reads from a file. Furthermore, fanotify watches grant > more power to an application in the form of permission events. While > notification events are solely, unidirectional (i.e. they only pass > information to the receiving application), permission events are blocking. > Permission events make a request to the receiving application which will > then reply with a decision as to whether or not that action may be > completed. This causes the issue of the watching application having the > ability to exercise control over the triggering process. Without drawing a > distinction within the permission check, the ability to read would imply > the greater ability to control an application. Additionally, mount and > superblock watches apply to all files within the same mount or superblock. > Read access to one file should not necessarily imply the ability to watch > all files accessed within a given mount or superblock. > > In order to solve these issues, a new LSM hook is implemented and has been > placed within the system calls for marking filesystem objects with inotify, > fanotify, and dnotify watches. These calls to the hook are placed at the > point at which the target inode has been resolved and are provided with the > inode, the mask of requested notification events, and the type of object on > which the mark is being set (inode, superblock, or mount). The mask and > mark_type have already been translated into common FS_* values shared by > the entirety of the fs notification infrastructure. > > This only provides a hook at the point of setting a watch, and presumes > that permission to set a particular watch implies the ability to receive > all notification about that object which match the mask. This is all that > is required for SELinux. If other security modules require additional hooks > or infrastructure to control delivery of notification, these can be added > by them. It does not make sense for us to propose hooks for which we have > no implementation. The understanding that all notifications received by the > requesting application are all strictly of a type for which the application > has been granted permission shows that this implementation is sufficient in > its coverage. > > Security modules wishing to provide complete control over fanotify must > also implement a security_file_open hook that validates that the access > requested by the watching application is authorized. Fanotify has the issue > that it returns a file descriptor with the file mode specified during > fanotify_init() to the watching process on event. This is already covered > by the LSM security_file_open hook if the security module implements > checking of the requested file mode there. Otherwise, a watching process > can obtain escalated access to a file for which it has not been authorized. > > The selinux_inode_notify hook implementation works by adding five new file > permissions: watch, watch_mount, watch_sb, watch_reads, and watch_with_perm > (descriptions about which will follow), and one new filesystem permission: > watch (which is applied to superblock checks). The hook then decides which > subset of these permissions must be held by the requesting application > based on the contents of the provided mask and the mark_type. The > selinux_file_open hook already checks the requested file mode and therefore > ensures that a watching process cannot escalate its access through > fanotify. > > The watch, watch_mount, and watch_sb permissions are the baseline > permissions for setting a watch on an object and each are a requirement for > any watch to be set on a file, mount, or superblock respectively. It should > be noted that having either of the other two permissions (watch_reads and > watch_with_perm) does not imply the watch, watch_mount, or watch_sb > permission. Superblock watches further require the filesystem watch > permission to the superblock. As there is no labeled object in view for > mounts, there is no specific check for mount watches beyond watch_mount to > the inode. Such a check could be added in the future, if a suitable labeled > object existed representing the mount. > > The watch_reads permission is required to receive notifications from > read-exclusive events on filesystem objects. These events include accessing > a file for the purpose of reading and closing a file which has been opened > read-only. This distinction has been drawn in order to provide a direct > indication in the policy for this otherwise not obvious capability. Read > access to a file should not necessarily imply the ability to observe read > events on a file. > > Finally, watch_with_perm only applies to fanotify masks since it is the > only way to set a mask which allows for the blocking, permission event. > This permission is needed for any watch which is of this type. Though > fanotify requires CAP_SYS_ADMIN, this is insufficient as it gives implicit > trust to root, which we do not do, and does not support least privilege. It looks as if there may be overlap between this work and [RFC PATCH] security, capability: pass object information to security_capable https://www.spinics.net/lists/selinux/msg28312.html I say this because your patch looks an awful lot like what I suggested as the alternative to passing object information to security_capable(). It's possible that I've muddled the discussions in my brain, and that there isn't any way to use either to do both jobs. But it would be worth a look, and a single new hook or change to existing hook would be a Good Thing. > > Signed-off-by: Aaron Goidel <acgoide@xxxxxxxxxxxxx> > --- > v2: > - Adds support for mark_type > - Adds watch_sb and watch_mount file permissions > - Adds watch as new filesystem permission > - LSM hook now recieves mark_type argument > - Changed LSM hook logic to implement checks for corresponding mark_types > - Adds missing hook description comment > - Fixes extrainous whitespace > - Updates patch description based on feedback > > fs/notify/dnotify/dnotify.c | 14 +++++++-- > fs/notify/fanotify/fanotify_user.c | 27 +++++++++++++++-- > fs/notify/inotify/inotify_user.c | 13 ++++++-- > include/linux/lsm_hooks.h | 6 ++++ > include/linux/security.h | 9 +++++- > security/security.c | 5 +++ > security/selinux/hooks.c | 47 +++++++++++++++++++++++++++++ > security/selinux/include/classmap.h | 5 +-- > 8 files changed, 116 insertions(+), 10 deletions(-) > > diff --git a/fs/notify/dnotify/dnotify.c b/fs/notify/dnotify/dnotify.c > index 250369d6901d..4690d8a66035 100644 > --- a/fs/notify/dnotify/dnotify.c > +++ b/fs/notify/dnotify/dnotify.c > @@ -22,6 +22,7 @@ > #include <linux/sched/signal.h> > #include <linux/dnotify.h> > #include <linux/init.h> > +#include <linux/security.h> > #include <linux/spinlock.h> > #include <linux/slab.h> > #include <linux/fdtable.h> > @@ -288,6 +289,16 @@ int fcntl_dirnotify(int fd, struct file *filp, unsigned long arg) > goto out_err; > } > > + /* > + * convert the userspace DN_* "arg" to the internal FS_* > + * defined in fsnotify > + */ > + mask = convert_arg(arg); > + > + error = security_inode_notify(inode, mask, FSNOTIFY_OBJ_TYPE_INODE); > + if (error) > + goto out_err; > + > /* expect most fcntl to add new rather than augment old */ > dn = kmem_cache_alloc(dnotify_struct_cache, GFP_KERNEL); > if (!dn) { > @@ -302,9 +313,6 @@ int fcntl_dirnotify(int fd, struct file *filp, unsigned long arg) > goto out_err; > } > > - /* convert the userspace DN_* "arg" to the internal FS_* defines in fsnotify */ > - mask = convert_arg(arg); > - > /* set up the new_fsn_mark and new_dn_mark */ > new_fsn_mark = &new_dn_mark->fsn_mark; > fsnotify_init_mark(new_fsn_mark, dnotify_group); > diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c > index a90bb19dcfa2..9e3137badb6b 100644 > --- a/fs/notify/fanotify/fanotify_user.c > +++ b/fs/notify/fanotify/fanotify_user.c > @@ -528,9 +528,10 @@ static const struct file_operations fanotify_fops = { > }; > > static int fanotify_find_path(int dfd, const char __user *filename, > - struct path *path, unsigned int flags) > + struct path *path, unsigned int flags, __u64 mask) > { > int ret; > + unsigned int mark_type; > > pr_debug("%s: dfd=%d filename=%p flags=%x\n", __func__, > dfd, filename, flags); > @@ -567,8 +568,30 @@ static int fanotify_find_path(int dfd, const char __user *filename, > > /* you can only watch an inode if you have read permissions on it */ > ret = inode_permission(path->dentry->d_inode, MAY_READ); > + if (ret) { > + path_put(path); > + goto out; > + } > + > + switch (flags & FANOTIFY_MARK_TYPE_BITS) { > + case FAN_MARK_MOUNT: > + mark_type = FSNOTIFY_OBJ_TYPE_VFSMOUNT; > + break; > + case FAN_MARK_FILESYSTEM: > + mark_type = FSNOTIFY_OBJ_TYPE_SB; > + break; > + case FAN_MARK_INODE: > + mark_type = FSNOTIFY_OBJ_TYPE_INODE; > + break; > + default: > + ret = -EINVAL; > + goto out; > + } > + > + ret = security_inode_notify(path->dentry->d_inode, mask, mark_type); > if (ret) > path_put(path); > + > out: > return ret; > } > @@ -1014,7 +1037,7 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask, > goto fput_and_out; > } > > - ret = fanotify_find_path(dfd, pathname, &path, flags); > + ret = fanotify_find_path(dfd, pathname, &path, flags, mask); > if (ret) > goto fput_and_out; > > diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c > index 7b53598c8804..73b321a30bbc 100644 > --- a/fs/notify/inotify/inotify_user.c > +++ b/fs/notify/inotify/inotify_user.c > @@ -39,6 +39,7 @@ > #include <linux/poll.h> > #include <linux/wait.h> > #include <linux/memcontrol.h> > +#include <linux/security.h> > > #include "inotify.h" > #include "../fdinfo.h" > @@ -342,7 +343,8 @@ static const struct file_operations inotify_fops = { > /* > * find_inode - resolve a user-given path to a specific inode > */ > -static int inotify_find_inode(const char __user *dirname, struct path *path, unsigned flags) > +static int inotify_find_inode(const char __user *dirname, struct path *path, > + unsigned int flags, __u64 mask) > { > int error; > > @@ -351,8 +353,15 @@ static int inotify_find_inode(const char __user *dirname, struct path *path, uns > return error; > /* you can only watch an inode if you have read permissions on it */ > error = inode_permission(path->dentry->d_inode, MAY_READ); > + if (error) { > + path_put(path); > + return error; > + } > + error = security_inode_notify(path->dentry->d_inode, mask, > + FSNOTIFY_OBJ_TYPE_INODE); > if (error) > path_put(path); > + > return error; > } > > @@ -744,7 +753,7 @@ SYSCALL_DEFINE3(inotify_add_watch, int, fd, const char __user *, pathname, > if (mask & IN_ONLYDIR) > flags |= LOOKUP_DIRECTORY; > > - ret = inotify_find_inode(pathname, &path, flags); > + ret = inotify_find_inode(pathname, &path, flags, mask); > if (ret) > goto fput_and_out; > > diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h > index 47f58cfb6a19..9b3f5a5f3246 100644 > --- a/include/linux/lsm_hooks.h > +++ b/include/linux/lsm_hooks.h > @@ -394,6 +394,9 @@ > * Check permission before removing the extended attribute > * identified by @name for @dentry. > * Return 0 if permission is granted. > + * @inode_notify: > + * Check permissions before setting a watch on events as defined by @mask, > + * on an object @inode, whose type is defined by @mark_type. > * @inode_getsecurity: > * Retrieve a copy of the extended attribute representation of the > * security label associated with @name for @inode via @buffer. Note that > @@ -1571,6 +1574,8 @@ union security_list_options { > int (*inode_getxattr)(struct dentry *dentry, const char *name); > int (*inode_listxattr)(struct dentry *dentry); > int (*inode_removexattr)(struct dentry *dentry, const char *name); > + int (*inode_notify)(struct inode *inode, u64 mask, > + unsigned int mark_type); > int (*inode_need_killpriv)(struct dentry *dentry); > int (*inode_killpriv)(struct dentry *dentry); > int (*inode_getsecurity)(struct inode *inode, const char *name, > @@ -1881,6 +1886,7 @@ struct security_hook_heads { > struct hlist_head inode_getxattr; > struct hlist_head inode_listxattr; > struct hlist_head inode_removexattr; > + struct hlist_head inode_notify; > struct hlist_head inode_need_killpriv; > struct hlist_head inode_killpriv; > struct hlist_head inode_getsecurity; > diff --git a/include/linux/security.h b/include/linux/security.h > index 659071c2e57c..b12666513138 100644 > --- a/include/linux/security.h > +++ b/include/linux/security.h > @@ -301,6 +301,8 @@ int security_inode_listsecurity(struct inode *inode, char *buffer, size_t buffer > void security_inode_getsecid(struct inode *inode, u32 *secid); > int security_inode_copy_up(struct dentry *src, struct cred **new); > int security_inode_copy_up_xattr(const char *name); > +int security_inode_notify(struct inode *inode, u64 mask, > + unsigned int mark_type); > int security_kernfs_init_security(struct kernfs_node *kn_dir, > struct kernfs_node *kn); > int security_file_permission(struct file *file, int mask); > @@ -387,7 +389,6 @@ int security_ismaclabel(const char *name); > int security_secid_to_secctx(u32 secid, char **secdata, u32 *seclen); > int security_secctx_to_secid(const char *secdata, u32 seclen, u32 *secid); > void security_release_secctx(char *secdata, u32 seclen); > - > void security_inode_invalidate_secctx(struct inode *inode); > int security_inode_notifysecctx(struct inode *inode, void *ctx, u32 ctxlen); > int security_inode_setsecctx(struct dentry *dentry, void *ctx, u32 ctxlen); > @@ -776,6 +777,12 @@ static inline int security_inode_removexattr(struct dentry *dentry, > return cap_inode_removexattr(dentry, name); > } > > +static inline int security_inode_notify(struct inode *inode, u64 mask, > + unsigned int mark_type) > +{ > + return 0; > +} > + > static inline int security_inode_need_killpriv(struct dentry *dentry) > { > return cap_inode_need_killpriv(dentry); > diff --git a/security/security.c b/security/security.c > index 613a5c00e602..bc30e201c137 100644 > --- a/security/security.c > +++ b/security/security.c > @@ -1251,6 +1251,11 @@ int security_inode_removexattr(struct dentry *dentry, const char *name) > return evm_inode_removexattr(dentry, name); > } > > +int security_inode_notify(struct inode *inode, u64 mask, unsigned int mark_type) > +{ > + return call_int_hook(inode_notify, 0, inode, mask, mark_type); > +} > + > int security_inode_need_killpriv(struct dentry *dentry) > { > return call_int_hook(inode_need_killpriv, 0, dentry); > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > index c61787b15f27..c967e46a34ea 100644 > --- a/security/selinux/hooks.c > +++ b/security/selinux/hooks.c > @@ -92,6 +92,8 @@ > #include <linux/kernfs.h> > #include <linux/stringhash.h> /* for hashlen_string() */ > #include <uapi/linux/mount.h> > +#include <linux/fsnotify.h> > +#include <linux/fanotify.h> > > #include "avc.h" > #include "objsec.h" > @@ -3261,6 +3263,50 @@ static int selinux_inode_removexattr(struct dentry *dentry, const char *name) > return -EACCES; > } > > +static int selinux_inode_notify(struct inode *inode, u64 mask, > + unsigned int mark_type) > +{ > + int ret; > + u32 perm; > + > + struct common_audit_data ad; > + > + ad.type = LSM_AUDIT_DATA_INODE; > + ad.u.inode = inode; > + > + /* > + * Set permission needed based on the type of mark being set. > + * Performs an additional check for sb watches. > + */ > + switch (mark_type) { > + case FSNOTIFY_OBJ_TYPE_VFSMOUNT: > + perm = FILE__WATCH_MOUNT; > + break; > + case FSNOTIFY_OBJ_TYPE_SB: > + perm = FILE__WATCH_SB; > + ret = superblock_has_perm(current_cred(), inode->i_sb, > + FILESYSTEM__WATCH, &ad); > + if (ret) > + return ret; > + break; > + case FSNOTIFY_OBJ_TYPE_INODE: > + perm = FILE__WATCH; > + break; > + default: > + return -EINVAL; > + } > + > + // check if the mask is requesting ability to set a blocking watch > + if (mask & (ALL_FSNOTIFY_PERM_EVENTS)) > + perm |= FILE__WATCH_WITH_PERM; // if so, check that permission > + > + // is the mask asking to watch file reads? > + if (mask & (FS_ACCESS | FS_ACCESS_PERM | FS_CLOSE_NOWRITE)) > + perm |= FILE__WATCH_READS; // check that permission as well > + > + return inode_has_perm(current_cred(), inode, perm, &ad); > +} > + > /* > * Copy the inode security context value to the user. > * > @@ -6797,6 +6843,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = { > LSM_HOOK_INIT(inode_getsecid, selinux_inode_getsecid), > LSM_HOOK_INIT(inode_copy_up, selinux_inode_copy_up), > LSM_HOOK_INIT(inode_copy_up_xattr, selinux_inode_copy_up_xattr), > + LSM_HOOK_INIT(inode_notify, selinux_inode_notify), > > LSM_HOOK_INIT(kernfs_init_security, selinux_kernfs_init_security), > > diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h > index 201f7e588a29..32e9b03be3dd 100644 > --- a/security/selinux/include/classmap.h > +++ b/security/selinux/include/classmap.h > @@ -7,7 +7,8 @@ > > #define COMMON_FILE_PERMS COMMON_FILE_SOCK_PERMS, "unlink", "link", \ > "rename", "execute", "quotaon", "mounton", "audit_access", \ > - "open", "execmod" > + "open", "execmod", "watch", "watch_mount", "watch_sb", \ > + "watch_with_perm", "watch_reads" > > #define COMMON_SOCK_PERMS COMMON_FILE_SOCK_PERMS, "bind", "connect", \ > "listen", "accept", "getopt", "setopt", "shutdown", "recvfrom", \ > @@ -60,7 +61,7 @@ struct security_class_mapping secclass_map[] = { > { "filesystem", > { "mount", "remount", "unmount", "getattr", > "relabelfrom", "relabelto", "associate", "quotamod", > - "quotaget", NULL } }, > + "quotaget", "watch", NULL } }, > { "file", > { COMMON_FILE_PERMS, > "execute_no_trans", "entrypoint", NULL } },
