s390/uv: Panic for set and remove shared access UVC errors

The return value uv_set_shared() and uv_remove_shared() (which are
wrappers around the share() function) is not always checked. The system
integrity of a protected guest depends on the Share and Unshare UVCs
being successful. This means that any caller that fails to check the
return value will compromise the security of the protected guest.

No code path that would lead to such violation of the security
guarantees is currently exercised, since all the areas that are shared
never get unshared during the lifetime of the system. This might
change and become an issue in the future.

The Share and Unshare UVCs can only fail in case of hypervisor
misbehaviour (either a bug or malicious behaviour). In such cases there
is no reasonable way forward, and the system needs to panic.

This patch replaces the return at the end of the share() function with
a panic, to guarantee system integrity.

Fixes: 5abb9351 ("s390/uv: introduce guest side ultravisor code")
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Link: https://lore.kernel.org/r/20240801112548.85303-1-imbrenda@linux.ibm.com
Message-ID: <20240801112548.85303-1-imbrenda@linux.ibm.com>
[frankja@linux.ibm.com: Fixed up patch subject]
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>

arch/s390/include/asm/uv.h

@@ -441,7 +441,10 @@ static inline int share(unsigned long addr, u16 cmd)
 
 	if (!uv_call(0, (u64)&uvcb))
 		return 0;
-	return -EINVAL;
+	pr_err("%s UVC failed (rc: 0x%x, rrc: 0x%x), possible hypervisor bug.\n",
+	       uvcb.header.cmd == UVC_CMD_SET_SHARED_ACCESS ? "Share" : "Unshare",
+	       uvcb.header.rc, uvcb.header.rrc);
+	panic("System security cannot be guaranteed unless the system panics now.\n");
 }
 
 /*