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

Kees Cook
2023-05-16T21:02:17Z

On Fri, May 12, 2023 at 02:41:50PM -0400, Kent Overstreet wrote:
> 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...)

For something that small, why not use the text_poke API?

--
Kees Cook