Re: [PATCH v15 11/16] arm64: kexec_file: allow for loading Image-format kernel

[Mark Rutland]

On Wed, Oct 10, 2018 at 03:52:37PM +0900, AKASHI Takahiro wrote:
> Mark,
> 
> On Tue, Oct 09, 2018 at 04:28:00PM +0100, Mark Rutland wrote:
> > On Fri, Sep 28, 2018 at 03:48:36PM +0900, AKASHI Takahiro wrote:
> > > This patch provides kexec_file_ops for "Image"-format kernel. In this
> > > implementation, a binary is always loaded with a fixed offset identified
> > > in text_offset field of its header.
> > > 
> > > Regarding signature verification for trusted boot, this patch doesn't
> > > contains CONFIG_KEXEC_VERIFY_SIG support, which is to be added later
> > > in this series, but file-attribute-based verification is still a viable
> > > option by enabling IMA security subsystem.
> > > 
> > > You can sign(label) a to-be-kexec'ed kernel image on target file system
> > > with:
> > >     $ evmctl ima_sign --key /path/to/private_key.pem Image
> > > 
> > > On live system, you must have IMA enforced with, at least, the following
> > > security policy:
> > >     "appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig"
> > > 
> > > See more details about IMA here:
> > >     https://sourceforge.net/p/linux-ima/wiki/Home/
> > > 
> > > Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
> > > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > > Cc: Will Deacon <will.deacon@xxxxxxx>
> > > Reviewed-by: James Morse <james.morse@xxxxxxx>
> > > ---
> > >  arch/arm64/include/asm/kexec.h         |  28 +++++++
> > >  arch/arm64/kernel/Makefile             |   2 +-
> > >  arch/arm64/kernel/kexec_image.c        | 108 +++++++++++++++++++++++++
> > >  arch/arm64/kernel/machine_kexec_file.c |   1 +
> > >  4 files changed, 138 insertions(+), 1 deletion(-)
> > >  create mode 100644 arch/arm64/kernel/kexec_image.c
> > > 
> > > diff --git a/arch/arm64/include/asm/kexec.h b/arch/arm64/include/asm/kexec.h
> > > index 157b2897d911..5e673481b3a3 100644
> > > --- a/arch/arm64/include/asm/kexec.h
> > > +++ b/arch/arm64/include/asm/kexec.h
> > > @@ -101,6 +101,34 @@ struct kimage_arch {
> > >  	unsigned long dtb_mem;
> > >  };
> > >  
> > > +/**
> > > + * struct arm64_image_header - arm64 kernel image header
> > > + * See Documentation/arm64/booting.txt for details
> > > + *
> > > + * @mz_magic: DOS header magic number ('MZ', optional)
> > 
> > Please just call this code0. If CONFIG_EFI is disabled, it is not 'MZ'.
> 
> How about this?
> (This definition will go into a new header, asm/image.h.)
> 
> ---8<---
> /*
>  * struct arm64_image_header - arm64 kernel image header
>  * See Documentation/arm64/booting.txt for details
>  *
>  * @code0:		Executable code, or
>  *   @mz_header		  alternatively used for part of MZ header
>  * @code1:		Executable code
>  * @text_offset:	Image load offset
>  * @image_size:		Effective Image size
>  * @flags:		kernel flags
>  * @reserved:		reserved
>  * @magic:		Magic number
>  * @reserved5:		reserved, or
>  *   @pe_header:	  alternatively used for PE COFF offset
>  */
> 
> struct arm64_image_header {
> 	union {
> 		__le32 code0;
> 		struct {
> 			__le16 magic;
> 			__le16 pad;
> 		} mz_header;
> 	};
> 	__le32 code1;
> 	__le64 text_offset;
> 	__le64 image_size;
> 	__le64 flags;
> 	__le64 reserved[3];
> 	__le32 magic;
> 	union {
> 		__le32 reserved5;
> 		__le32 pe_header;
> 	};
> };

Do we care about the MZ header?

The definition of the Image header in Documentation/arm64/booting.txt is:

  u32 code0;                    /* Executable code */
  u32 code1;                    /* Executable code */
  u64 text_offset;              /* Image load offset, little endian */
  u64 image_size;               /* Effective Image size, little endian */
  u64 flags;                    /* kernel flags, little endian */
  u64 res2      = 0;            /* reserved */
  u64 res3      = 0;            /* reserved */
  u64 res4      = 0;            /* reserved */
  u32 magic     = 0x644d5241;   /* Magic number, little endian, "ARM\x64" */
  u32 res5;                     /* reserved (used for PE COFF offset) */

I'd prefer that we aligned our header definition with that, rather than
diverging from it.

If we need to look at the MZ magic to determine if the Image is a valid PE
binary, can't we just cast to the existing struct mz_hdr from <linux/pe.h> to
do that?

[...]

> > > +	/* Check cpu features */
> > > +	flags = le64_to_cpu(h->flags);
> > > +	value = head_flag_field(flags, HEAD_FLAG_BE);
> > > +	if (((value == HEAD_FLAG_BE) && !IS_ENABLED(CONFIG_CPU_BIG_ENDIAN)) ||
> > > +	    ((value != HEAD_FLAG_BE) && IS_ENABLED(CONFIG_CPU_BIG_ENDIAN)))
> > > +		if (!system_supports_mixed_endian())
> > > +			return ERR_PTR(-EINVAL);
> > 
> > I think this can be simplified:
> > 
> > 	bool be_image = head_flag_field(flags, HEAD_FLAG_BE);
> > 	bool be_kernel = IS_ENABLED(CONFIG_CPU_BIG_ENDIAN);
> > 
> > 	if ((be_image != be_kernel) && !system_supports_mixed_endian)
> > 	    		return ERR_PTR(-EINVAL);
> 
> Okay.
> 
> > ... though do we need to police this at all? It's arguably policy given
> > the new image has to be signed anyway), and there are other fields that
> > could fall into that category in future.
> 
> The aim here is to prevent any images from being loaded
> when there is no question that a new image will never be successfully
> kexec'ed since the core on a given SoC obviously doesn't support cpu
> features that are assumed and required by a image.
> 
> I believe that this check is a good and easy practice to avoid possible
> failures before execution.

My only concern is that this is arguably placing some policy in the
kernel, and I don't want to set the expectation that we'll do this for
other things in future, as that becomes a maintenance nightmare.

I'm not necessarily opposed to these specific checks, given they're
simple. Just wanted to make sure that we've thought about it.

Thanks,
Mark.