[RFC PATCH 6/7] execmem: add support for cache of large ROX pages

[Peter Zijlstra]

On Thu, Apr 11, 2024 at 07:05:25PM +0300, Mike Rapoport wrote:

> To populate the cache, a writable large page is allocated from vmalloc with
> VM_ALLOW_HUGE_VMAP, filled with invalid instructions and then remapped as
> ROX.

> +static void execmem_invalidate(void *ptr, size_t size, bool writable)
> +{
> +	if (execmem_info->invalidate)
> +		execmem_info->invalidate(ptr, size, writable);
> +	else
> +		memset(ptr, 0, size);
> +}

+static void execmem_invalidate(void *ptr, size_t size, bool writeable)
+{
+       /* fill memory with INT3 instructions */
+       if (writeable)
+               memset(ptr, 0xcc, size);
+       else
+               text_poke_set(ptr, 0xcc, size);
+}

Thing is, 0xcc (aka INT3_INSN_OPCODE) is not an invalid instruction.
It raises #BP not #UD.