[dpdk-dev] [PATCH 2/8] vhost: avoid enum fields in VhostUserMsg

[Stefan Hajnoczi <stefanha@redhat.com>]

The VhostUserMsg struct binary representation must match the vhost-user
protocol specification since this struct is read from and written to the
socket.

The VhostUserMsg.request union contains enum fields.  Enum binary
representation is implementation-defined according to the C standard and
it is unportable to make assumptions about the representation:

  6.7.2.2 Enumeration specifiers
  ...
  Each enumerated type shall be compatible with char, a signed integer
  type, or an unsigned integer type. The choice of type is
  implementation-defined, but shall be capable of representing the
  values of all the members of the enumeration.

Additionally, librte_vhost relies on the enum type being unsigned when
validating untrusted inputs:

  if (ret <= 0 || msg.request.master >= VHOST_USER_MAX) {

If msg.request.master is signed then negative values pass this check!

Even if we assume gcc on x86_64 (SysV amd64 ABI) and don't care about
portability, the actual enum constants still affect the final type.  For
example, if we add a negative constant then its type changes to signed
int:

  typedef enum VhostUserRequest {
      ...
      VHOST_USER_INVALID = -1,
  };

This is very fragile and it's unlikely that anyone changing the code
would remember this.  A security hole can be introduced accidentally.

This patch switches VhostUserMsg.request fields to uint32_t to avoid the
portability and potential security issues.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 lib/librte_vhost/vhost_user.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/librte_vhost/vhost_user.h b/lib/librte_vhost/vhost_user.h
index d4bd604b9..0fafbe6e0 100644
--- a/lib/librte_vhost/vhost_user.h
+++ b/lib/librte_vhost/vhost_user.h
@@ -81,8 +81,8 @@ typedef struct VhostUserLog {
 
 typedef struct VhostUserMsg {
 	union {
-		VhostUserRequest master;
-		VhostUserSlaveRequest slave;
+		uint32_t master; /* a VhostUserRequest value */
+		uint32_t slave;  /* a VhostUserSlaveRequest value*/
 	} request;
 
 #define VHOST_USER_VERSION_MASK     0x3
-- 
2.14.3