linux-kernel - Re: [PATCH] efi: fix a race and a buffer overflow while reading efivars via sysfs

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Date:   Wed, 4 Mar 2020 14:51:22 +0800
From:   joeyli <jlee@...e.com>
To:     Vladis Dronov <vdronov@...hat.com>
CC:     Ard Biesheuvel <ardb@...nel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] efi: fix a race and a buffer overflow while reading
 efivars via sysfs

Hi all,

On Tue, Mar 03, 2020 at 05:24:58AM -0500, Vladis Dronov wrote:
> Hello, Ard, all,
> 
> > > Wouldn't it be easier to pass a var_data_size stack variable into
> > > efivar_entry_get(), and only update the value in 'var' if it is <=
> > > 1024?
> > > 
> > 
> > I was thinking about this approach, but this way we still do not protect
> > var from a concurrent access. For example, efivar_data_read() can race
> > with itself:
> 
> Oh, indeed, this race is not possible the way you sugget with a var_data_size
> stack variable. Unfortunately, AFAIU, the read/write race stays:
>  
> > ... efivar read functions still can race with the write function
> > efivar_store_raw(). Surely, the race window is much smaller but it is there.
> > I strongly believe we need to protect all data accesses here with a lock.
>

Looks that kernel uses EFI protocol to query variable everytime, then
why should kernel keeps a copy of variable data size, data and attributes in
memory? It makes sense to keep VariableName and VendorGuid, but why data?

The efi_variable can be used to interactive with userland. But we do not
need to keep a data copy in efi_variable with efivar_entry. e.g. The
efivarfs_file_read() allocates a buffer for reading variable instead
of using efi_variable->Data. 

Regards
Joey Lee