linux-kernel - Re: [PATCH 3/4] EFI: Add support for variables longer than 1024 bytes

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Message-ID: <20111214223950.GA29264@srcf.ucam.org>
Date:	Wed, 14 Dec 2011 22:39:50 +0000
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Mike Waychison <mikew@...gle.com>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org, hpa@...or.com
Subject: Re: [PATCH 3/4] EFI: Add support for variables longer than 1024
 bytes

On Wed, Dec 14, 2011 at 02:14:27PM -0800, Mike Waychison wrote:

> >  static efi_status_t
> > -get_var_data_locked(struct efivars *efivars, struct efi_variable *var)
> > +get_var_data_locked(struct efivars *efivars, struct extended_efi_variable **var)
> >  {
> >        efi_status_t status;
> > +       unsigned long length;
> > +
> > +       if (!*var)
> > +               *var = kmalloc(sizeof(struct extended_efi_variable), GFP_KERNEL);
> 
> Aren't we holding a spinlock here?

Good point.

> > +
> > +       (*var)->header.DataSize = 0;
> > +       status = efivars->ops->get_variable((*var)->header.VariableName,
> > +                                           &(*var)->header.VendorGuid,
> > +                                           &(*var)->header.Attributes,
> > +                                           &(*var)->header.DataSize,
> > +                                           (*var)->Data);
> 
> This doesn't look right.  ->Data here is after the Data[1024] buffer
> embedded in (*var)->header, and a read into this buffer will corrupt
> the heap.

DataSize is 0, so we'll never actually read anything back here.

> > +
> > +       if (status == EFI_BUFFER_TOO_SMALL) {
> > +               *var = krealloc(*var, sizeof(struct extended_efi_variable) +
> > +                               (*var)->header.DataSize, GFP_KERNEL);
> > +               status = efivars->ops->get_variable((*var)->header.VariableName,
> > +                                                   &(*var)->header.VendorGuid,
> > +                                                   &(*var)->header.Attributes,
> > +                                                   &(*var)->header.DataSize,
> > +                                                   (*var)->Data);
> > +       }
> > +
> > +       length = ((*var)->header.DataSize < 1024) ? (*var)->header.DataSize :
> > +               1024;
> > +
> > +       memcpy(&(*var)->header.Data, &(*var)->Data, length);
> 
> This memcpy clobbers the header.Data with the corrupted data when we
> didn't use the second path.

We'll always follow the second path providing there's actually data to 
read back. If there isn't then length will be 0.

> > +       if (count == sizeof(struct efi_variable)) {
> > +               tmp_var = (struct efi_variable *)buf;
> > +               new_var = kmalloc(sizeof(struct efi_variable) +
> > +                                 tmp_var->DataSize, GFP_KERNEL);
> > +               memcpy(&new_var->header, tmp_var, sizeof(struct efi_variable));
> > +               memcpy(&new_var->Data, tmp_var->Data, tmp_var->DataSize);
> > +       } else if (count > sizeof(struct efi_variable)) {
> > +               new_var = (struct extended_efi_variable *)buf;
> > +       } else {
> >                return -EINVAL;
> > +       }
> 
> Ugh.  This is difficult to follow, and complicates the memory freeing path :(

Entirely agreed.

> We need to be careful here.  The store_raw ABI is broken, in the sense
> that the ABI from compat mode differs from that in 32bit mode (there
> is a long in the efi_variable structure which changes the offsets).  I
> don't know how to fix it properly and still maintain proper ABI
> compatibility.

True.

> What are your thoughts on _not_ wrapping efi_variable with
> extended_efi_variable, and instead just using a
> "internal_efi_variable" structure that we copy stuff into/outof.  I
> think that would make the memory management for dealing with the
> different sizes a lot easier to follow.

Hm. I think that'd only work if we expose a new interface. Writes would 
be easy enough to handle, but reads still need to work for old apps.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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