linux-kernel - Re: [PATCH] x86/efi: initialize status to ensure garbage is not returned on small size

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Message-ID: <ada10a97-f3d3-91af-51b8-6d2f973c95a1@canonical.com>
Date:	Wed, 27 Jul 2016 15:50:16 +0100
From:	Colin Ian King <colin.king@...onical.com>
To:	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	"H . Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/efi: initialize status to ensure garbage is not
 returned on small size

On 27/07/16 15:38, Matt Fleming wrote:
> On Wed, 20 Jul, at 11:11:06AM, Colin Ian King wrote:
>> From: Colin Ian King <colin.king@...onical.com>
>>
>> Although very unlikey, if size is too small or zero, then we end up with
>> status not being set and returning garbage. Instead, initializing status to
>> EFI_INVALID_PARAMETER to indicate that size is invalid in the calls to
>> setup_uga32 and setup_uga64.
>>
>> Signed-off-by: Colin Ian King <colin.king@...onical.com>
>> ---
>>  arch/x86/boot/compressed/eboot.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/boot/compressed/eboot.c b/arch/x86/boot/compressed/eboot.c
>> index ff574da..ec6d2ef 100644
>> --- a/arch/x86/boot/compressed/eboot.c
>> +++ b/arch/x86/boot/compressed/eboot.c
>> @@ -578,7 +578,7 @@ setup_uga32(void **uga_handle, unsigned long size, u32 *width, u32 *height)
>>  	efi_guid_t uga_proto = EFI_UGA_PROTOCOL_GUID;
>>  	unsigned long nr_ugas;
>>  	u32 *handles = (u32 *)uga_handle;;
>> -	efi_status_t status;
>> +	efi_status_t status = EFI_INVALID_PARAMETER;
>>  	int i;
>>  
>>  	first_uga = NULL;
>> @@ -623,7 +623,7 @@ setup_uga64(void **uga_handle, unsigned long size, u32 *width, u32 *height)
>>  	efi_guid_t uga_proto = EFI_UGA_PROTOCOL_GUID;
>>  	unsigned long nr_ugas;
>>  	u64 *handles = (u64 *)uga_handle;;
>> -	efi_status_t status;
>> +	efi_status_t status = EFI_INVALID_PARAMETER;
>>  	int i;
>>  
>>  	first_uga = NULL;
> 
> Can this ever happen in practice? This would imply that
> locate_protocol() found EFI_UGA_PROTOCOL_GUID but that the size
> returned is utterly bogus?

I just wanted to guard against the call
efi_call_early(locate_handle, EFI_LOCATE_BY_PROTOCOL, &uga_proto, NULL,
&size, uga_handle) returning a bogus size of less than a u32/u64
(depending on the setup_uga32/setup_uga64), which I was not 100% sure if
this could happen or not, so I always assume efi_call_early calls can go
wrong when you least expect them.

> 
> If so, I have no problem applying the patch but want to make sure
> we're not tricking ourselves into thinking we're being protected from
> something when we're not.
> 

I'd rather put extra guarding in rather than getting some potential
garbage return from the stack, but I am playing it rather conservatively
here.

Colin