linux-kernel - Re: [PATCH 2/2] efi: Distinguish between "remaining space" and actually used space

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Date:	Mon, 01 Apr 2013 16:50:46 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Matthew Garrett <matthew.garrett@...ula.com>
Cc:	matt.fleming@...el.com, jwboyer@...hat.com,
	linux-efi@...r.kernel.org, seth.forshee@...onical.com,
	linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 2/2] efi: Distinguish between "remaining space" and
 actually used space

On Mon, 2013-04-01 at 11:14 -0400, Matthew Garrett wrote:
> EFI implementations distinguish between space that is actively used by a
> variable and space that merely hasn't been garbage collected yet. Space
> that hasn't yet been garbage collected isn't available for use and so isn't
> counted in the remaining_space field returned by QueryVariableInfo().
> 
> Combined with commit 68d9298 this can cause problems. Some implementations
> don't garbage collect until the remaining space is smaller than the maximum
> variable size, and as a result check_var_size() will always fail once more
> than 50% of the variable store has been used even if most of that space is
> marked as available for garbage collection. The user is unable to create
> new variables, and deleting variables doesn't increase the remaining space.
> 
> The problem that 68d9298 was attempting to avoid was one where certain
> platforms fail if the actively used space is greater than 50% of the
> available storage space. We should be able to calculate that by simply
> summing the size of each available variable and subtracting that from
> the total storage space. With luck this will fix the problem described in
> https://bugzilla.kernel.org/show_bug.cgi?id=55471 without permitting
> damage to occur to the machines 68d9298 was attempting to fix.
> 
> Signed-off-by: Matthew Garrett <matthew.garrett@...ula.com>
> ---
>  drivers/firmware/efivars.c | 41 ++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 38 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/firmware/efivars.c b/drivers/firmware/efivars.c
> index 60e7d8f..a6d8a58 100644
> --- a/drivers/firmware/efivars.c
> +++ b/drivers/firmware/efivars.c
[...]
> @@ -452,8 +462,33 @@ check_var_size_locked(struct efivars *efivars, u32 attributes,
>  	if (status != EFI_SUCCESS)
>  		return status;
>  
> -	if (!storage_size || size > remaining_size || size > max_size ||
> -	    (remaining_size - size) < (storage_size / 2))
> +	list_for_each_entry(entry, &efivars->list, list) {
> +		var = &entry->var;
> +		status = get_var_data_locked(efivars, var);
> +
> +		if (status || !(var->Attributes & EFI_VARIABLE_NON_VOLATILE))
> +			continue;
> +
> +		active_size += var->DataSize;
> +		active_size += utf16_strsize(var->VariableName, 1024);
> +		/*
> +		 * There's some additional metadata associated with each
> +		 * variable. Intel's reference implementation is 60 bytes -
> +		 * bump that to 128 to account for vendor additions and
> +		 * potential alignment constraints
> +		 */
> +		active_size += 128;
> +	}
> +
> +	/*
> +	 * Add on the size of any boot services-only variables
> +	 */
> +	active_size += boot_var_size;
> +
> +	active_available = storage_size - active_size;
> +
> +	if (!storage_size || size > remaining_size ||
> +	    (active_available - size) < (storage_size / 2))
>  		return EFI_OUT_OF_RESOURCES;

Both substractions here could quite possibly underflow due to
over-estimation of active_size.

We can rearrange the condition to be:

	active_size + size > storage_size / 2

and delete the active_available variable.  But I think metadata for the
new variable should be accounted as well:

	active_size + size + 128 > storage_size / 2

Since we'll now be using that magic number twice, it needs a name.

Ben.

>  	return status;

-- 
Ben Hutchings
DNRC Motto:  I can please only one person per day.
Today is not your day.  Tomorrow isn't looking good either.

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