linux-kernel - Re: [PATCH v3 1/3] kprobes: Add text_alloc() and text

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Message-ID: <202007151232.0DA220B2@keescook>
Date:   Wed, 15 Jul 2020 12:36:02 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, x86@...r.kernel.org,
        Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jessica Yu <jeyu@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Will Deacon <will@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Alexandre Ghiti <alex@...ti.fr>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Stephen Boyd <sboyd@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Babu Moger <Babu.Moger@....com>,
        Omar Sandoval <osandov@...com>,
        Nayna Jain <nayna@...ux.ibm.com>,
        Marco Elver <elver@...gle.com>,
        Brian Gerst <brgerst@...il.com>, Jiri Kosina <jkosina@...e.cz>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH v3 1/3] kprobes: Add text_alloc() and text_free()

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