Re: [PATCH v3 1/3] kprobes: Add text_alloc() and text_free()

[Kees Cook <keescook@chromium.org>]

On Wed, Jul 15, 2020 at 01:32:27AM +0300, Jarkko Sakkinen wrote:
> Introduce new API for allocating space for code generaed at run-time
> leveraging from module_alloc() and module_memfree() code. Use this to
> perform memory allocations in the kprobes code in order to loose the
> bound between kprobes and modules subsystem.
> 
> Initially, use this API only with arch/x86 and define a new config
> flag CONFIG_ARCH_HAS_TEXT_ALLOC to promote the availability of the
> new API.
> [...]
> diff --git a/include/linux/text.h b/include/linux/text.h
> new file mode 100644
> index 000000000000..a27d4a42ecda
> --- /dev/null
> +++ b/include/linux/text.h
> @@ -0,0 +1,17 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +
> +#ifndef _LINUX_TEXT_H
> +#define _LINUX_TEXT_H
> +
> +/*
> + * An allocator used for allocating modules, core sections and init sections.
> + * Returns NULL on failure.
> + */
> +void *text_alloc(unsigned long size);
> +
> +/*
> + * Free memory returned from text_alloc().
> + */
> +void text_free(void *region);
> +
> +#endif /* _LINUX_TEXT_H */
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 2e97febeef77..fa7687eb0c0e 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -35,6 +35,7 @@
>  #include <linux/ftrace.h>
>  #include <linux/cpu.h>
>  #include <linux/jump_label.h>
> +#include <linux/text.h>
>  
>  #include <asm/sections.h>
>  #include <asm/cacheflush.h>
> @@ -111,12 +112,20 @@ enum kprobe_slot_state {
>  
>  void __weak *alloc_insn_page(void)
>  {
> +#ifdef CONFIG_ARCH_HAS_TEXT_ALLOC
> +	return text_alloc(PAGE_SIZE);
> +#else
>  	return module_alloc(PAGE_SIZE);
> +#endif

This seems like it shouldn't be needed. I think text_alloc() should
always be visible. In the ARCH_HAS... case it will call the arch
implementation, and without it should just call module_alloc():

For example:
void *text_alloc(unsigned long size)
{
#ifdef CONFIG_ARCH_HAS_TEXT_ALLOC
	return arch_text_alloc(size);
#else
	return module_alloc(size);
#endif
}

-- 
Kees Cook