x86/paravirt: Detect over-sized patching bugs in paravirt_patch_insns()

So paravirt_patch_insns() contains this gem of logic:

unsigned paravirt_patch_insns(void *insnbuf, unsigned len,
                              const char *start, const char *end)
{
        unsigned insn_len = end - start;

        if (insn_len > len || start == NULL)
                insn_len = len;
        else
                memcpy(insnbuf, start, insn_len);

        return insn_len;
}

Note how 'len' (size of the original instruction) is checked against the new
instruction, and silently discarded with no warning printed whatsoever.

This crashes the kernel in funny ways if the patching template is buggy,
and usually in much later places.

Instead do a direct BUG_ON(), there's no way to continue successfully at that point.

I've tested this patch, with the vanilla kernel check never triggers, and
if I intentionally increase the size of one of the patch templates to a
too high value the assert triggers:

[    0.164385] kernel BUG at arch/x86/kernel/paravirt.c:167!

Without this patch a broken kernel randomly crashes in later places,
after the silent patching failure.

Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: Ingo Molnar

arch/x86/kernel/paravirt.c

@@ -163,10 +163,10 @@ unsigned paravirt_patch_insns(void *insnbuf, unsigned len,
 {
 	unsigned insn_len = end - start;
 
-	if (insn_len > len || start == NULL)
-		insn_len = len;
-	else
-		memcpy(insnbuf, start, insn_len);
+	/* Alternative instruction is too large for the patch site and we cannot continue: */
+	BUG_ON(insn_len > len || start == NULL);
+
+	memcpy(insnbuf, start, insn_len);
 
 	return insn_len;
 }