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

[Kees Cook <keescook@chromium.org>]

On Wed, Jul 15, 2020 at 10:49:33PM +0300, Mike Rapoport wrote:
> On Wed, Jul 15, 2020 at 12:36:02PM -0700, Kees Cook wrote:
> > 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
> > }
> 
> This inverts the dependcy chain, IMHO, module_alloc() is a special case
> of text_alloc() and not vice versa.

Okay, sure. That's fine too. Whatever the case is, I want to make sure
we keep the KASLR offset though. I don't think that should be unique to
the modules logic.

-- 
Kees Cook