linux-kernel - Re: [PATCH 09/14] arm64: kexec_file: add sha256 digest check in purgatory

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Message-ID: <20170825104133.GB3127@leverpostej>
Date:   Fri, 25 Aug 2017 11:41:33 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     AKASHI Takahiro <takahiro.akashi@...aro.org>,
        catalin.marinas@....com, will.deacon@....com,
        bauerman@...ux.vnet.ibm.com, dhowells@...hat.com,
        vgoyal@...hat.com, herbert@...dor.apana.org.au,
        davem@...emloft.net, akpm@...ux-foundation.org, mpe@...erman.id.au,
        dyoung@...hat.com, bhe@...hat.com, arnd@...db.de,
        ard.biesheuvel@...aro.org, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 09/14] arm64: kexec_file: add sha256 digest check in
 purgatory

On Fri, Aug 25, 2017 at 10:21:06AM +0900, AKASHI Takahiro wrote:
> On Thu, Aug 24, 2017 at 06:04:40PM +0100, Mark Rutland wrote:
> > On Thu, Aug 24, 2017 at 05:18:06PM +0900, AKASHI Takahiro wrote:
> > > Most of sha256 code is based on crypto/sha256-glue.c, particularly using
> > > non-neon version.
> > > 
> > > Please note that we won't be able to re-use lib/mem*.S for purgatory
> > > because unaligned memory access is not allowed in purgatory where mmu
> > > is turned off.
> > > 
> > > Since purgatory is not linked with the other part of kernel, care must be
> > > taken of selecting an appropriate set of compiler options in order to
> > > prevent undefined symbol references from being generated.
> > 
> > What is the point in performing this check in the purgatory code, when
> > this will presumably have been checked when the image is loaded?
> 
> Well, this is what x86 does :)
> On powerpc, meanwhile, they don't have this check.
> 
> Maybe to avoid booting corrupted kernel after loading?
> (loaded data are now protected by making them unmapped, though.)

I'd really prefer to avoid this, since it seems to be what necessitates
all the complexity for executing C code (linking and all), and it's
going to be very slow to execute with the MMU off.

If you can deliberately corrupt the next kernel, you could also have
corrupted the purgatory to skip the check.

Unless we have a strong reason to want the hash check, I think it should
be dropped.

> > > diff --git a/arch/arm64/purgatory/entry.S b/arch/arm64/purgatory/entry.S
> > > index bc4e6b3bf8a1..74d028b838bd 100644
> > > --- a/arch/arm64/purgatory/entry.S
> > > +++ b/arch/arm64/purgatory/entry.S
> > > @@ -6,6 +6,11 @@
> > >  .text
> > >  
> > >  ENTRY(purgatory_start)
> > > +	adr	x19, .Lstack
> > > +	mov	sp, x19
> > > +
> > > +	bl	purgatory
> > > +
> > >  	/* Start new image. */
> > >  	ldr	x17, arm64_kernel_entry
> > >  	ldr	x0, arm64_dtb_addr
> > > @@ -15,6 +20,14 @@ ENTRY(purgatory_start)
> > >  	br	x17
> > >  END(purgatory_start)
> > >  
> > > +.ltorg
> > > +
> > > +.align 4
> > > +	.rept	256
> > > +	.quad	0
> > > +	.endr
> > > +.Lstack:
> > > +
> > >  .data
> > 
> > Why is the stack in .text?
> 
> to call verify_sha256_digest() from asm

Won't that also work if the stack is in .data? or .bss?

... or is there a particular need for it to be in .text?

> > Does this need to be zeroed?
> 
> No :)

Ok, so we can probably do:

	.data
	.align 4
	. += PURGATORY_STACK_SIZE
.Lstack_ptr:

... assuming we need to run C code.

[...]

> > > diff --git a/arch/arm64/purgatory/sha256.c b/arch/arm64/purgatory/sha256.c
> > > new file mode 100644
> > > index 000000000000..5d20d81767e3
> > > --- /dev/null
> > > +++ b/arch/arm64/purgatory/sha256.c
> > > @@ -0,0 +1,79 @@
> > > +#include <linux/kexec.h>
> > > +#include <linux/purgatory.h>
> > > +#include <linux/types.h>
> > > +
> > > +/*
> > > + * Under KASAN, those are defined as un-instrumented version, __memxxx()
> > > + */
> > > +#undef memcmp
> > > +#undef memcpy
> > > +#undef memset
> > 
> > This doesn't look like the right place for this undeffery; it looks
> > rather fragile.
> 
> Yeah, I agree, but if not there, __memxxx() are used.

Ok, but we'll have to add this to every C file used in the purgatory
code, or at the start of any header that uses a memxxx() function, or it
might still be overridden to use __memxxx(), before the undef takes
effect.

Can we define __memxxx() instead?

[...]

> > > +void *memcpy(void *dst, const void *src, size_t len)
> > > +{
> > > +	int i;
> > > +
> > > +	for (i = 0; i < len; i++)
> > > +		((u8 *)dst)[i] = ((u8 *)src)[i];
> > > +
> > > +	return NULL;
> > > +}
> > > +
> > > +void *memset(void *dst, int c, size_t len)
> > > +{
> > > +	int i;
> > > +
> > > +	for (i = 0; i < len; i++)
> > > +		((u8 *)dst)[i] = (u8)c;
> > > +
> > > +	return NULL;
> > > +}
> > > +
> > > +int memcmp(const void *src, const void *dst, size_t len)
> > > +{
> > > +	int i;
> > > +
> > > +	for (i = 0; i < len; i++)
> > > +		if (*(char *)src != *(char *)dst)
> > > +			return 1;
> > > +
> > > +	return 0;
> > > +}
> > 
> > How is the compiler prevented from "optimising" these into calls to
> > themselves?
> 
> I don't get what you mean by "calls to themselves."

There are compiler optimizations that recognise sequences like:

	for (i = 0; i < len; i++)
		dst[i] = src[i];

... and turn those into:

	memcpy(dst, src, len);

... these have been known to "optimize" memcpy implementations into
calls to themselves. Likewise for other string operations.

One way we avoid that today is by writing our memcpy in assembly.

Do we have a guarnatee that this will not happen here? e.g. do we pass
some compiler flag that prevents this?

Thanks,
Mark.