Merge pull request #4180 from gilles-peskine-arm/net_poll-fd_setsize-2.16

Backport 2.16: Fix stack corruption in mbedtls_net_poll with large file descriptor

ChangeLog.d/net_poll-fd_setsize.txt

@@ -0,0 +1,4 @@
+Security
+   * Fix a stack buffer overflow with mbedtls_net_poll() and
+     mbedtls_net_recv_timeout() when given a file descriptor that is
+     beyond FD_SETSIZE. Reported by FigBug in #4169.

include/mbedtls/net_sockets.h

@@ -151,6 +151,7 @@ int mbedtls_net_connect( mbedtls_net_context *ctx, const char *host, const char
  *
  * \return         0 if successful, or one of:
  *                      MBEDTLS_ERR_NET_SOCKET_FAILED,
+ *                      MBEDTLS_ERR_NET_UNKNOWN_HOST,
  *                      MBEDTLS_ERR_NET_BIND_FAILED,
  *                      MBEDTLS_ERR_NET_LISTEN_FAILED
  *
@@ -170,6 +171,8 @@ int mbedtls_net_bind( mbedtls_net_context *ctx, const char *bind_ip, const char
  *                  can be NULL if client_ip is null
  *
  * \return          0 if successful, or
+ *                  MBEDTLS_ERR_NET_SOCKET_FAILED,
+ *                  MBEDTLS_ERR_NET_BIND_FAILED,
  *                  MBEDTLS_ERR_NET_ACCEPT_FAILED, or
  *                  MBEDTLS_ERR_NET_BUFFER_TOO_SMALL if buf_size is too small,
  *                  MBEDTLS_ERR_SSL_WANT_READ if bind_fd was set to
@@ -182,6 +185,10 @@ int mbedtls_net_accept( mbedtls_net_context *bind_ctx,
 /**
  * \brief          Check and wait for the context to be ready for read/write
  *
+ * \note           The current implementation of this function uses
+ *                 select() and returns an error if the file descriptor
+ *                 is \c FD_SETSIZE or greater.
+ *
  * \param ctx      Socket to check
  * \param rw       Bitflag composed of MBEDTLS_NET_POLL_READ and
  *                 MBEDTLS_NET_POLL_WRITE specifying the events
@@ -263,16 +270,21 @@ int mbedtls_net_send( void *ctx, const unsigned char *buf, size_t len );
  *                 'timeout' seconds. If no error occurs, the actual amount
  *                 read is returned.
  *
+ * \note           The current implementation of this function uses
+ *                 select() and returns an error if the file descriptor
+ *                 is \c FD_SETSIZE or greater.
+ *
  * \param ctx      Socket
  * \param buf      The buffer to write to
  * \param len      Maximum length of the buffer
  * \param timeout  Maximum number of milliseconds to wait for data
  *                 0 means no timeout (wait forever)
  *
- * \return         the number of bytes received,
+ * \return         The number of bytes received if successful.
- *                 or a non-zero error code:
+ *                 MBEDTLS_ERR_SSL_TIMEOUT if the operation timed out.
- *                 MBEDTLS_ERR_SSL_TIMEOUT if the operation timed out,
  *                 MBEDTLS_ERR_SSL_WANT_READ if interrupted by a signal.
+ *                 Another negative error code (MBEDTLS_ERR_NET_xxx)
+ *                 for other failures.
  *
  * \note           This function will block (until data becomes available or
  *                 timeout is reached) even if the socket is set to

library/net_sockets.c

@@ -496,6 +496,13 @@ int mbedtls_net_poll( mbedtls_net_context *ctx, uint32_t rw, uint32_t timeout )
     if( fd < 0 )
         return( MBEDTLS_ERR_NET_INVALID_CONTEXT );
 
+    /* A limitation of select() is that it only works with file descriptors
+     * that are strictly less than FD_SETSIZE. This is a limitation of the
+     * fd_set type. Error out early, because attempting to call FD_SET on a
+     * large file descriptor is a buffer overflow on typical platforms. */
+    if( fd >= FD_SETSIZE )
+        return( MBEDTLS_ERR_NET_POLL_FAILED );
+
 #if defined(__has_feature)
 #if __has_feature(memory_sanitizer)
     /* Ensure that memory sanitizers consider read_fds and write_fds as
@@ -615,6 +622,13 @@ int mbedtls_net_recv_timeout( void *ctx, unsigned char *buf,
     if( fd < 0 )
         return( MBEDTLS_ERR_NET_INVALID_CONTEXT );
 
+    /* A limitation of select() is that it only works with file descriptors
+     * that are strictly less than FD_SETSIZE. This is a limitation of the
+     * fd_set type. Error out early, because attempting to call FD_SET on a
+     * large file descriptor is a buffer overflow on typical platforms. */
+    if( fd >= FD_SETSIZE )
+        return( MBEDTLS_ERR_NET_POLL_FAILED );
+
     FD_ZERO( &read_fds );
     FD_SET( fd, &read_fds );