fs: Pass AT_GETATTR_NOSEC flag to getattr interface function

When vfs_getattr_nosec() calls a filesystem's getattr interface function
then the 'nosec' should propagate into this function so that
vfs_getattr_nosec() can again be called from the filesystem's gettattr
rather than vfs_getattr(). The latter would add unnecessary security
checks that the initial vfs_getattr_nosec() call wanted to avoid.
Therefore, introduce the getattr flag GETATTR_NOSEC and allow to pass
with the new getattr_flags parameter to the getattr interface function.
In overlayfs and ecryptfs use this flag to determine which one of the
two functions to call.

In a recent code change introduced to IMA vfs_getattr_nosec() ended up
calling vfs_getattr() in overlayfs, which in turn called
security_inode_getattr() on an exiting process that did not have
current->fs set anymore, which then caused a kernel NULL pointer
dereference. With this change the call to security_inode_getattr() can
be avoided, thus avoiding the NULL pointer dereference.

Reported-by: <syzbot+a67fc5321ffb4b311c98@syzkaller.appspotmail.com>
Fixes: db1d1e8b98 ("IMA: use vfs_getattr_nosec to get the i_version")
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: <linux-fsdevel@vger.kernel.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Tyler Hicks <code@tyhicks.com>
Cc: Mimi Zohar <zohar@linux.ibm.com>
Suggested-by: Christian Brauner <brauner@kernel.org>
Co-developed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Link: https://lore.kernel.org/r/20231002125733.1251467-1-stefanb@linux.vnet.ibm.com
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/ecryptfs/inode.c

@@ -998,6 +998,14 @@ static int ecryptfs_getattr_link(struct mnt_idmap *idmap,
 	return rc;
 }
 
+static int ecryptfs_do_getattr(const struct path *path, struct kstat *stat,
+			       u32 request_mask, unsigned int flags)
+{
+	if (flags & AT_GETATTR_NOSEC)
+		return vfs_getattr_nosec(path, stat, request_mask, flags);
+	return vfs_getattr(path, stat, request_mask, flags);
+}
+
 static int ecryptfs_getattr(struct mnt_idmap *idmap,
 			    const struct path *path, struct kstat *stat,
 			    u32 request_mask, unsigned int flags)
@@ -1006,8 +1014,8 @@ static int ecryptfs_getattr(struct mnt_idmap *idmap,
 	struct kstat lower_stat;
 	int rc;
 
-	rc = vfs_getattr(ecryptfs_dentry_to_lower_path(dentry), &lower_stat,
+	rc = ecryptfs_do_getattr(ecryptfs_dentry_to_lower_path(dentry),
-			 request_mask, flags);
+				 &lower_stat, request_mask, flags);
 	if (!rc) {
 		fsstack_copy_attr_all(d_inode(dentry),
 				      ecryptfs_inode_to_lower(d_inode(dentry)));

fs/overlayfs/inode.c

@@ -171,7 +171,7 @@ int ovl_getattr(struct mnt_idmap *idmap, const struct path *path,
 
 	type = ovl_path_real(dentry, &realpath);
 	old_cred = ovl_override_creds(dentry->d_sb);
-	err = vfs_getattr(&realpath, stat, request_mask, flags);
+	err = ovl_do_getattr(&realpath, stat, request_mask, flags);
 	if (err)
 		goto out;
 
@@ -196,8 +196,8 @@ int ovl_getattr(struct mnt_idmap *idmap, const struct path *path,
 					(!is_dir ? STATX_NLINK : 0);
 
 			ovl_path_lower(dentry, &realpath);
-			err = vfs_getattr(&realpath, &lowerstat,
+			err = ovl_do_getattr(&realpath, &lowerstat, lowermask,
-					  lowermask, flags);
+					     flags);
 			if (err)
 				goto out;
 
@@ -249,8 +249,8 @@ int ovl_getattr(struct mnt_idmap *idmap, const struct path *path,
 
 			ovl_path_lowerdata(dentry, &realpath);
 			if (realpath.dentry) {
-				err = vfs_getattr(&realpath, &lowerdatastat,
+				err = ovl_do_getattr(&realpath, &lowerdatastat,
-						  lowermask, flags);
+						     lowermask, flags);
 				if (err)
 					goto out;
 			} else {

fs/overlayfs/overlayfs.h

@@ -408,6 +408,14 @@ static inline bool ovl_open_flags_need_copy_up(int flags)
 	return ((OPEN_FMODE(flags) & FMODE_WRITE) || (flags & O_TRUNC));
 }
 
+static inline int ovl_do_getattr(const struct path *path, struct kstat *stat,
+				 u32 request_mask, unsigned int flags)
+{
+	if (flags & AT_GETATTR_NOSEC)
+		return vfs_getattr_nosec(path, stat, request_mask, flags);
+	return vfs_getattr(path, stat, request_mask, flags);
+}
+
 /* util.c */
 int ovl_get_write_access(struct dentry *dentry);
 void ovl_put_write_access(struct dentry *dentry);

fs/stat.c

@@ -133,7 +133,8 @@ int vfs_getattr_nosec(const struct path *path, struct kstat *stat,
 	idmap = mnt_idmap(path->mnt);
 	if (inode->i_op->getattr)
 		return inode->i_op->getattr(idmap, path, stat,
-					    request_mask, query_flags);
+					    request_mask,
+					    query_flags | AT_GETATTR_NOSEC);
 
 	generic_fillattr(idmap, request_mask, inode, stat);
 	return 0;
@@ -166,6 +167,9 @@ int vfs_getattr(const struct path *path, struct kstat *stat,
 {
 	int retval;
 
+	if (WARN_ON_ONCE(query_flags & AT_GETATTR_NOSEC))
+		return -EPERM;
+
 	retval = security_inode_getattr(path);
 	if (retval)
 		return retval;

include/uapi/linux/fcntl.h

@@ -116,5 +116,8 @@
 #define AT_HANDLE_FID		AT_REMOVEDIR	/* file handle is needed to
 					compare object identity and may not
 					be usable to open_by_handle_at(2) */
+#if defined(__KERNEL__)
+#define AT_GETATTR_NOSEC	0x80000000
+#endif
 
 #endif /* _UAPI_LINUX_FCNTL_H */