console: avoid wrapping of console pointers

We particularly want/need to avoid accessing data outside (ahead of)
the ring buffer. Also latch both pointers into local variable to
avoid different steps of the calculation being done with different
values.

Reported-by: Quan Luo <a4651386@xxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/drivers/char/console.c
+++ b/xen/drivers/char/console.c
@@ -254,20 +254,23 @@ static void conring_puts(const char *str
 long read_console_ring(struct xen_sysctl_readconsole *op)
 {
     XEN_GUEST_HANDLE_PARAM(char) str;
-    uint32_t idx, len, max, sofar, c;
+    uint32_t idx, len, max, sofar, c, p;
 
     str   = guest_handle_cast(op->buffer, char),
     max   = op->count;
     sofar = 0;
 
-    c = conringc;
-    if ( op->incremental && ((int32_t)(op->index - c) > 0) )
+    c = read_atomic(&conringc);
+    p = read_atomic(&conringp);
+    if ( op->incremental &&
+         (c <= p ? c < op->index && op->index <= p
+                 : c < op->index || op->index <= p) )
         c = op->index;
 
-    while ( (c != conringp) && (sofar < max) )
+    while ( (c != p) && (sofar < max) )
     {
         idx = CONRING_IDX_MASK(c);
-        len = conringp - c;
+        len = p - c;
         if ( (idx + len) > conring_size )
             len = conring_size - idx;
         if ( (sofar + len) > max )
@@ -281,10 +284,7 @@ long read_console_ring(struct xen_sysctl
     if ( op->clear )
     {
         spin_lock_irq(&console_lock);
-        if ( (uint32_t)(conringp - c) > conring_size )
-            conringc = conringp - conring_size;
-        else
-            conringc = c;
+        conringc = p - c > conring_size ? p - conring_size : c;
         spin_unlock_irq(&console_lock);
     }