Re: [PATCH 07/32] mm: Bring back vmalloc_exec

[Kent Overstreet <kent.overstreet@linux.dev>]

On Thu, May 11, 2023 at 03:28:40PM -0700, Kees Cook wrote:
> On Wed, May 10, 2023 at 03:05:48PM +0000, Johannes Thumshirn wrote:
> > On 09.05.23 18:56, Kent Overstreet wrote:
> > > +/**
> > > + * vmalloc_exec - allocate virtually contiguous, executable memory
> > > + * @size:	  allocation size
> > > + *
> > > + * Kernel-internal function to allocate enough pages to cover @size
> > > + * the page level allocator and map them into contiguous and
> > > + * executable kernel virtual space.
> > > + *
> > > + * For tight control over page level allocator and protection flags
> > > + * use __vmalloc() instead.
> > > + *
> > > + * Return: pointer to the allocated memory or %NULL on error
> > > + */
> > > +void *vmalloc_exec(unsigned long size, gfp_t gfp_mask)
> > > +{
> > > +	return __vmalloc_node_range(size, 1, VMALLOC_START, VMALLOC_END,
> > > +			gfp_mask, PAGE_KERNEL_EXEC, VM_FLUSH_RESET_PERMS,
> > > +			NUMA_NO_NODE, __builtin_return_address(0));
> > > +}
> > > +EXPORT_SYMBOL_GPL(vmalloc_exec);
> > 
> > Uh W+X memory reagions.
> > The 90s called, they want their shellcode back.
> 
> Just to clarify: the kernel must never create W+X memory regions. So,
> no, do not reintroduce vmalloc_exec().
> 
> Dynamic code areas need to be constructed in a non-executable memory,
> then switched to read-only and verified to still be what was expected,
> and only then made executable.

So if we're opening this up to the topic if what an acceptible API would
look like - how hard is this requirement?

The reason is that the functions we're constructing are only ~50 bytes,
so we don't want to be burning a full page per function (particularly
for the 64kb page architectures...)

If we were to build an allocator for sub-page dynamically constructed
code, we'd have to flip the whole page to W+X while copying in the new
function. But, we could construct it in non executable memory and then
hand it off to this new allocator to do the copy, which would also do
the page permission flipping.

It seem like this is something BPF might want eventually too, depending
on average BPF program size...