Merge tag 'keys-fixes-20200329' of git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs

Pull keyrings fixes from David Howells:
"Here's a couple of patches that fix a circular dependency between
holding key->sem and mm->mmap_sem when reading data from a key.

One potential issue is that a filesystem looking to use a key inside,
say, ->readpages() could deadlock if the key being read is the key
that's required and the buffer the key is being read into is on a page
that needs to be fetched.

The case actually detected is a bit more involved - with a filesystem
calling request_key() and locking the target keyring for write - which
could be being read"

* tag 'keys-fixes-20200329' of git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs:
KEYS: Avoid false positive ENOMEM error on key read
KEYS: Don't write out to userspace while holding key semaphore

include/keys/big_key-type.h

···
 extern void big_key_revoke(struct key *key);
 extern void big_key_destroy(struct key *key);
 extern void big_key_describe(const struct key *big_key, struct seq_file *m);
-extern long big_key_read(const struct key *key, char __user *buffer, size_t buflen);
+extern long big_key_read(const struct key *key, char *buffer, size_t buflen);
 
 #endif /* _KEYS_BIG_KEY_TYPE_H */

include/keys/user-type.h

···
 extern void user_revoke(struct key *key);
 extern void user_destroy(struct key *key);
 extern void user_describe(const struct key *user, struct seq_file *m);
-extern long user_read(const struct key *key,
-		      char __user *buffer, size_t buflen);
+extern long user_read(const struct key *key, char *buffer, size_t buflen);
 
 static inline const struct user_key_payload *user_key_payload_rcu(const struct key *key)
 {

include/linux/key-type.h

···
 	 *   much is copied into the buffer
 	 * - shouldn't do the copy if the buffer is NULL
 	 */
-	long (*read)(const struct key *key, char __user *buffer, size_t buflen);
+	long (*read)(const struct key *key, char *buffer, size_t buflen);
 
 	/* handle request_key() for this type instead of invoking
 	 * /sbin/request-key (optional)

net/dns_resolver/dns_key.c

···
  * - the key's semaphore is read-locked
  */
 static long dns_resolver_read(const struct key *key,
-			      char __user *buffer, size_t buflen)
+			      char *buffer, size_t buflen)
 {
 	int err = PTR_ERR(key->payload.data[dns_key_error]);
 

net/rxrpc/key.c

···
 static void rxrpc_destroy(struct key *);
 static void rxrpc_destroy_s(struct key *);
 static void rxrpc_describe(const struct key *, struct seq_file *);
-static long rxrpc_read(const struct key *, char __user *, size_t);
+static long rxrpc_read(const struct key *, char *, size_t);
 
 /*
  * rxrpc defined keys take an arbitrary string as the description and an
···
  * - this returns the result in XDR form
  */
 static long rxrpc_read(const struct key *key,
-		       char __user *buffer, size_t buflen)
+		       char *buffer, size_t buflen)
 {
 	const struct rxrpc_key_token *token;
 	const struct krb5_principal *princ;
 	size_t size;
-	__be32 __user *xdr, *oldxdr;
+	__be32 *xdr, *oldxdr;
 	u32 cnlen, toksize, ntoks, tok, zero;
 	u16 toksizes[AFSTOKEN_MAX];
 	int loop;
···
 	if (!buffer || buflen < size)
 		return size;
 
-	xdr = (__be32 __user *) buffer;
+	xdr = (__be32 *)buffer;
 	zero = 0;
 #define ENCODE(x)				\
 	do {					\
-		__be32 y = htonl(x);		\
-		if (put_user(y, xdr++) < 0)	\
-			goto fault;		\
+		*xdr++ = htonl(x);		\
 	} while(0)
 #define ENCODE_DATA(l, s)						\
 	do {								\
 		u32 _l = (l);						\
 		ENCODE(l);						\
-		if (copy_to_user(xdr, (s), _l) != 0)			\
-			goto fault;					\
-		if (_l & 3 &&						\
-		    copy_to_user((u8 __user *)xdr + _l, &zero, 4 - (_l & 3)) != 0) \
-			goto fault;					\
+		memcpy(xdr, (s), _l);					\
+		if (_l & 3)						\
+			memcpy((u8 *)xdr + _l, &zero, 4 - (_l & 3));	\
 		xdr += (_l + 3) >> 2;					\
 	} while(0)
 #define ENCODE64(x)					\
 	do {						\
 		__be64 y = cpu_to_be64(x);		\
-		if (copy_to_user(xdr, &y, 8) != 0)	\
-			goto fault;			\
+		memcpy(xdr, &y, 8);			\
 		xdr += 8 >> 2;				\
 	} while(0)
 #define ENCODE_STR(s)				\
···
 	ASSERTCMP((char __user *) xdr - buffer, ==, size);
 	_leave(" = %zu", size);
 	return size;
-
-fault:
-	_leave(" = -EFAULT");
-	return -EFAULT;
 }

security/keys/big_key.c

···
  * read the key data
  * - the key's semaphore is read-locked
  */
-long big_key_read(const struct key *key, char __user *buffer, size_t buflen)
+long big_key_read(const struct key *key, char *buffer, size_t buflen)
 {
 	size_t datalen = (size_t)key->payload.data[big_key_len];
 	long ret;
···
 
 		ret = datalen;
 
-		/* copy decrypted data to user */
-		if (copy_to_user(buffer, buf->virt, datalen) != 0)
-			ret = -EFAULT;
+		/* copy out decrypted data */
+		memcpy(buffer, buf->virt, datalen);
 
 err_fput:
 		fput(file);
···
 		big_key_free_buffer(buf);
 	} else {
 		ret = datalen;
-		if (copy_to_user(buffer, key->payload.data[big_key_data],
-				 datalen) != 0)
-			ret = -EFAULT;
+		memcpy(buffer, key->payload.data[big_key_data], datalen);
 	}
 
 	return ret;

security/keys/encrypted-keys/encrypted.c

···
 }
 
 /*
- * encrypted_read - format and copy the encrypted data to userspace
+ * encrypted_read - format and copy out the encrypted data
  *
  * The resulting datablob format is:
  * <master-key name> <decrypted data length> <encrypted iv> <encrypted data>
  *
  * On success, return to userspace the encrypted key datablob size.
  */
-static long encrypted_read(const struct key *key, char __user *buffer,
+static long encrypted_read(const struct key *key, char *buffer,
 			   size_t buflen)
 {
 	struct encrypted_key_payload *epayload;
···
 	key_put(mkey);
 	memzero_explicit(derived_key, sizeof(derived_key));
 
-	if (copy_to_user(buffer, ascii_buf, asciiblob_len) != 0)
-		ret = -EFAULT;
+	memcpy(buffer, ascii_buf, asciiblob_len);
 	kzfree(ascii_buf);
 
 	return asciiblob_len;

security/keys/internal.h

···
 #include <linux/keyctl.h>
 #include <linux/refcount.h>
 #include <linux/compat.h>
+#include <linux/mm.h>
+#include <linux/vmalloc.h>
 
 struct iovec;
 
···
 
 #endif
 
+/*
+ * Helper function to clear and free a kvmalloc'ed memory object.
+ */
+static inline void __kvzfree(const void *addr, size_t len)
+{
+	if (addr) {
+		memset((void *)addr, 0, len);
+		kvfree(addr);
+	}
+}
 #endif /* _INTERNAL_H */

security/keys/keyctl.c

···
 	payload = NULL;
 	if (plen) {
 		ret = -ENOMEM;
-		payload = kmalloc(plen, GFP_KERNEL);
+		payload = kvmalloc(plen, GFP_KERNEL);
 		if (!payload)
 			goto error;
 
···
 
 	key_ref_put(key_ref);
 error2:
-	kzfree(payload);
+	__kvzfree(payload, plen);
 error:
 	return ret;
 }
···
 }
 
 /*
+ * Call the read method
+ */
+static long __keyctl_read_key(struct key *key, char *buffer, size_t buflen)
+{
+	long ret;
+
+	down_read(&key->sem);
+	ret = key_validate(key);
+	if (ret == 0)
+		ret = key->type->read(key, buffer, buflen);
+	up_read(&key->sem);
+	return ret;
+}
+
+/*
  * Read a key's payload.
  *
  * The key must either grant the caller Read permission, or it must grant the
···
 	struct key *key;
 	key_ref_t key_ref;
 	long ret;
+	char *key_data = NULL;
+	size_t key_data_len;
 
 	/* find the key first */
 	key_ref = lookup_user_key(keyid, 0, 0);
 	if (IS_ERR(key_ref)) {
 		ret = -ENOKEY;
-		goto error;
+		goto out;
 	}
 
 	key = key_ref_to_ptr(key_ref);
 
 	ret = key_read_state(key);
 	if (ret < 0)
-		goto error2; /* Negatively instantiated */
+		goto key_put_out; /* Negatively instantiated */
 
 	/* see if we can read it directly */
 	ret = key_permission(key_ref, KEY_NEED_READ);
 	if (ret == 0)
 		goto can_read_key;
 	if (ret != -EACCES)
-		goto error2;
+		goto key_put_out;
 
 	/* we can't; see if it's searchable from this process's keyrings
 	 * - we automatically take account of the fact that it may be
···
 	 */
 	if (!is_key_possessed(key_ref)) {
 		ret = -EACCES;
-		goto error2;
+		goto key_put_out;
 	}
 
 	/* the key is probably readable - now try to read it */
 can_read_key:
-	ret = -EOPNOTSUPP;
-	if (key->type->read) {
-		/* Read the data with the semaphore held (since we might sleep)
-		 * to protect against the key being updated or revoked.
-		 */
-		down_read(&key->sem);
-		ret = key_validate(key);
-		if (ret == 0)
-			ret = key->type->read(key, buffer, buflen);
-		up_read(&key->sem);
+	if (!key->type->read) {
+		ret = -EOPNOTSUPP;
+		goto key_put_out;
 	}
 
-error2:
+	if (!buffer || !buflen) {
+		/* Get the key length from the read method */
+		ret = __keyctl_read_key(key, NULL, 0);
+		goto key_put_out;
+	}
+
+	/*
+	 * Read the data with the semaphore held (since we might sleep)
+	 * to protect against the key being updated or revoked.
+	 *
+	 * Allocating a temporary buffer to hold the keys before
+	 * transferring them to user buffer to avoid potential
+	 * deadlock involving page fault and mmap_sem.
+	 *
+	 * key_data_len = (buflen <= PAGE_SIZE)
+	 *		? buflen : actual length of key data
+	 *
+	 * This prevents allocating arbitrary large buffer which can
+	 * be much larger than the actual key length. In the latter case,
+	 * at least 2 passes of this loop is required.
+	 */
+	key_data_len = (buflen <= PAGE_SIZE) ? buflen : 0;
+	for (;;) {
+		if (key_data_len) {
+			key_data = kvmalloc(key_data_len, GFP_KERNEL);
+			if (!key_data) {
+				ret = -ENOMEM;
+				goto key_put_out;
+			}
+		}
+
+		ret = __keyctl_read_key(key, key_data, key_data_len);
+
+		/*
+		 * Read methods will just return the required length without
+		 * any copying if the provided length isn't large enough.
+		 */
+		if (ret <= 0 || ret > buflen)
+			break;
+
+		/*
+		 * The key may change (unlikely) in between 2 consecutive
+		 * __keyctl_read_key() calls. In this case, we reallocate
+		 * a larger buffer and redo the key read when
+		 * key_data_len < ret <= buflen.
+		 */
+		if (ret > key_data_len) {
+			if (unlikely(key_data))
+				__kvzfree(key_data, key_data_len);
+			key_data_len = ret;
+			continue;	/* Allocate buffer */
+		}
+
+		if (copy_to_user(buffer, key_data, ret))
+			ret = -EFAULT;
+		break;
+	}
+	__kvzfree(key_data, key_data_len);
+
+key_put_out:
 	key_put(key);
-error:
+out:
 	return ret;
 }
 

security/keys/keyring.c

···
 {
 	struct keyring_read_iterator_context *ctx = data;
 	const struct key *key = keyring_ptr_to_key(object);
-	int ret;
 
 	kenter("{%s,%d},,{%zu/%zu}",
 	       key->type->name, key->serial, ctx->count, ctx->buflen);
···
 	if (ctx->count >= ctx->buflen)
 		return 1;
 
-	ret = put_user(key->serial, ctx->buffer);
-	if (ret < 0)
-		return ret;
-	ctx->buffer++;
+	*ctx->buffer++ = key->serial;
 	ctx->count += sizeof(key->serial);
 	return 0;
 }

security/keys/request_key_auth.c

···
 static void request_key_auth_describe(const struct key *, struct seq_file *);
 static void request_key_auth_revoke(struct key *);
 static void request_key_auth_destroy(struct key *);
-static long request_key_auth_read(const struct key *, char __user *, size_t);
+static long request_key_auth_read(const struct key *, char *, size_t);
 
 /*
  * The request-key authorisation key type definition.
···
  * - the key's semaphore is read-locked
  */
 static long request_key_auth_read(const struct key *key,
-				  char __user *buffer, size_t buflen)
+				  char *buffer, size_t buflen)
 {
 	struct request_key_auth *rka = dereference_key_locked(key);
 	size_t datalen;
···
 		if (buflen > datalen)
 			buflen = datalen;
 
-		if (copy_to_user(buffer, rka->callout_info, buflen) != 0)
-			ret = -EFAULT;
+		memcpy(buffer, rka->callout_info, buflen);
 	}
 
 	return ret;

security/keys/trusted-keys/trusted_tpm1.c

···
  * trusted_read - copy the sealed blob data to userspace in hex.
  * On success, return to userspace the trusted key datablob size.
  */
-static long trusted_read(const struct key *key, char __user *buffer,
+static long trusted_read(const struct key *key, char *buffer,
 			 size_t buflen)
 {
 	const struct trusted_key_payload *p;
-	char *ascii_buf;
 	char *bufp;
 	int i;
 
···
 		return -EINVAL;
 
 	if (buffer && buflen >= 2 * p->blob_len) {
-		ascii_buf = kmalloc_array(2, p->blob_len, GFP_KERNEL);
-		if (!ascii_buf)
-			return -ENOMEM;
-
-		bufp = ascii_buf;
+		bufp = buffer;
 		for (i = 0; i < p->blob_len; i++)
 			bufp = hex_byte_pack(bufp, p->blob[i]);
-		if (copy_to_user(buffer, ascii_buf, 2 * p->blob_len) != 0) {
-			kzfree(ascii_buf);
-			return -EFAULT;
-		}
-		kzfree(ascii_buf);
 	}
 	return 2 * p->blob_len;
 }

security/keys/user_defined.c

···
  * read the key data
  * - the key's semaphore is read-locked
  */
-long user_read(const struct key *key, char __user *buffer, size_t buflen)
+long user_read(const struct key *key, char *buffer, size_t buflen)
 {
 	const struct user_key_payload *upayload;
 	long ret;
···
 		if (buflen > upayload->datalen)
 			buflen = upayload->datalen;
 
-		if (copy_to_user(buffer, upayload->data, buflen) != 0)
-			ret = -EFAULT;
+		memcpy(buffer, upayload->data, buflen);
 	}
 
 	return ret;