Fwd: TPM unaligned memory accesses

Fwd: TPM unaligned memory accesses

[Andrew James]

Hi everyone,

I noticed the following potential unaligned memory access in
tpm-interface.c, and I was wondering how it would be handled on
architectures that don't support unaligned accesses.  Is this TPM code
expected to work on all architectures?

ssize_t tpm_transmit_cmd(...)
{
    const struct tpm_header *header = (struct tpm_header *)buf->data;
    int err;
    ssize_t len;

     len = tpm_transmit(chip, buf->data, PAGE_SIZE);
     if (len <  0)
         return len;

     err = be32_to_cpu(header->return_code);
        ...
}

I'm referring to the line at the bottom, before the ellipsis where we
read 'return_code'.

struct tpm_header has a __be16 tag followed by a __be32 return code.
If we are reading 'return_code', is this an unaligned access?
Similarly this would apply to the 'length' member too?

Documentation/unaligned-memory-access.txt recommends going through the
kernel API get_unaligned() and put_unaligned() in <asm/unaligned.h> to
avoid unaligned accesses, but I don't see this anywhere in the TPM
code.  I'm just trying to wrap my head around this.

Thanks!.
Andrew

Re: Fwd: TPM unaligned memory accesses

[Jarkko Sakkinen]

On Wed, Mar 10, 2021 at 04:41:54PM +1100, Andrew James wrote:
> Hi everyone,
> 
> I noticed the following potential unaligned memory access in
> tpm-interface.c, and I was wondering how it would be handled on
> architectures that don't support unaligned accesses.  Is this TPM code
> expected to work on all architectures?
> 
> ssize_t tpm_transmit_cmd(...)
> {
>     const struct tpm_header *header = (struct tpm_header *)buf->data;
>     int err;
>     ssize_t len;
> 
>      len = tpm_transmit(chip, buf->data, PAGE_SIZE);
>      if (len <  0)
>          return len;
> 
>      err = be32_to_cpu(header->return_code);
>         ...
> }
> 
> I'm referring to the line at the bottom, before the ellipsis where we
> read 'return_code'.
> 
> struct tpm_header has a __be16 tag followed by a __be32 return code.
> If we are reading 'return_code', is this an unaligned access?
> Similarly this would apply to the 'length' member too?
> 
> Documentation/unaligned-memory-access.txt recommends going through the
> kernel API get_unaligned() and put_unaligned() in <asm/unaligned.h> to
> avoid unaligned accesses, but I don't see this anywhere in the TPM
> code.  I'm just trying to wrap my head around this.

There's a patch for this in existence:

https://lore.kernel.org/tpmdd-devel/1479899094-9486-1-git-send-email-tomas.winkler@intel.com/

It's years since this was sent but I recall the issues were fairly
easy to fix.

/Jarkko