Re: [PATCH 2/2] riscv: Add KASAN support

[Daniel Axtens]

Nick Hu <nickhu@xxxxxxxxxxxxx> writes:

> Hi Christoph,
>
> Thanks for your reply. I will answer one by one.
>
> Hi Alexander,
>
> Would you help me for the question about SOFTIRQENTRY_TEXT?
>
> On Mon, Aug 12, 2019 at 11:10:50PM +0800, Christoph Hellwig wrote:
>> > 2. KASAN can't debug the modules since the modules are allocated in VMALLOC
>> > area. We mapped the shadow memory, which corresponding to VMALLOC area,
>> > to the kasan_early_shadow_page because we don't have enough physical space
>> > for all the shadow memory corresponding to VMALLOC area.
>> 
>> How do other architectures solve this problem?
>> 
> Other archs like arm64 and x86 allocate modules in their module region.

I've run in to a similar difficulty in ppc64. My approach has been to
add a generic feature to allow kasan to handle vmalloc areas:

https://lore.kernel.org/linux-mm/20190903145536.3390-1-dja@xxxxxxxxxx/

I link this with ppc64 in this series:

https://lore.kernel.org/linuxppc-dev/20190806233827.16454-1-dja@xxxxxxxxxx/

However, see Christophe Leroy's comments: he thinks I should take a
different approach in a number of places, including just adding a
separate module area. I haven't had time to think through all of his
proposals yet; in particular I'd want to think through what the
implication of a separate module area is for KASLR.

Regards,
Daniel

>
>> > @@ -54,6 +54,8 @@ config RISCV
>> >  	select EDAC_SUPPORT
>> >  	select ARCH_HAS_GIGANTIC_PAGE
>> >  	select ARCH_WANT_HUGE_PMD_SHARE if 64BIT
>> > +	select GENERIC_STRNCPY_FROM_USER if KASAN
>> 
>> Is there any reason why we can't always enabled this?  Also just
>> enabling the generic efficient strncpy_from_user should probably be
>> a separate patch.
>> 
> You're right, always enable it would be better.
>
>> > +	select HAVE_ARCH_KASAN if MMU
>> 
>> Based on your cover letter this should be if MMU && 64BIT
>> 
>> >  #define __HAVE_ARCH_MEMCPY
>> >  extern asmlinkage void *memcpy(void *, const void *, size_t);
>> > +extern asmlinkage void *__memcpy(void *, const void *, size_t);
>> >  
>> >  #define __HAVE_ARCH_MEMMOVE
>> >  extern asmlinkage void *memmove(void *, const void *, size_t);
>> > +extern asmlinkage void *__memmove(void *, const void *, size_t);
>> > +
>> > +#define memcpy(dst, src, len) __memcpy(dst, src, len)
>> > +#define memmove(dst, src, len) __memmove(dst, src, len)
>> > +#define memset(s, c, n) __memset(s, c, n)
>> 
>> This looks weird and at least needs a very good comment.  Also
>> with this we effectively don't need the non-prefixed prototypes
>> anymore.  Also you probably want to split the renaming of the mem*
>> routines into a separate patch with a proper changelog.
>> 
> I made some mistakes on this porting, this would be better:
>
> #define __HAVE_ARCH_MEMSET
> extern asmlinkage void *memset(void *, int, size_t);
> extern asmlinkage void *__memset(void *, int, size_t);
>
> #define __HAVE_ARCH_MEMCPY
> extern asmlinkage void *memcpy(void *, const void *, size_t);
> extern asmlinkage void *__memcpy(void *, const void *, size_t);
>
> #define __HAVE_ARCH_MEMMOVE
> extern asmlinkage void *memmove(void *, const void *, size_t);
> extern asmlinkage void *__memmove(void *, const void *, size_t);
>
> #if defined(CONFIG_KASAN) && !defined(__SANITIZE_ADDRESS__)
>
> #define memcpy(dst, src, len) __memcpy(dst, src, len)
> #define memmove(dst, src, len) __memmove(dst, src, len)
> #define memset(s, c, n) __memset(s, c, n)
>
> #endif
>
>> >  #include <asm/tlbflush.h>
>> >  #include <asm/thread_info.h>
>> >  
>> > +#ifdef CONFIG_KASAN
>> > +#include <asm/kasan.h>
>> > +#endif
>> 
>> Any good reason to not just always include the header?
>>
> Nope, I would remove the '#ifdef CONFIG_KASAN', and do the logic in the header
> instead.
>
>> > +
>> >  #ifdef CONFIG_DUMMY_CONSOLE
>> >  struct screen_info screen_info = {
>> >  	.orig_video_lines	= 30,
>> > @@ -64,12 +68,17 @@ void __init setup_arch(char **cmdline_p)
>> >  
>> >  	setup_bootmem();
>> >  	paging_init();
>> > +
>> >  	unflatten_device_tree();
>> 
>> spurious whitespace change.
>> 
>> > diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
>> > index 23cd1a9..9700980 100644
>> > --- a/arch/riscv/kernel/vmlinux.lds.S
>> > +++ b/arch/riscv/kernel/vmlinux.lds.S
>> > @@ -46,6 +46,7 @@ SECTIONS
>> >  		KPROBES_TEXT
>> >  		ENTRY_TEXT
>> >  		IRQENTRY_TEXT
>> > +		SOFTIRQENTRY_TEXT
>> 
>> Hmm.  What is the relation to kasan here?  Maybe we should add this
>> separately with a good changelog?
>> 
> There is a commit for it:
>
> Author: Alexander Potapenko <glider@xxxxxxxxxx>
> Date:   Fri Mar 25 14:22:05 2016 -0700
>
>     arch, ftrace: for KASAN put hard/soft IRQ entries into separate sections
>
>     KASAN needs to know whether the allocation happens in an IRQ handler.
>     This lets us strip everything below the IRQ entry point to reduce the
>     number of unique stack traces needed to be stored.
>
>     Move the definition of __irq_entry to <linux/interrupt.h> so that the
>     users don't need to pull in <linux/ftrace.h>.  Also introduce the
>     __softirq_entry macro which is similar to __irq_entry, but puts the
>     corresponding functions to the .softirqentry.text section.
>
> After reading the patch I understand that soft/hard IRQ entries should be
> separated for KASAN to work, but why?
>
> Alexender, do you have any comments on this?
>
>> > +++ b/arch/riscv/mm/kasan_init.c
>> > @@ -0,0 +1,102 @@
>> > +// SPDX-License-Identifier: GPL-2.0
>> 
>> This probably also wants a copyright statement.
>> 
>> > +	// init for swapper_pg_dir
>> 
>> Please use /* */ style comments.
>
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