linux-kernel - Re: [PATCH] media: ngene: Fix out-of-bounds bug in ngene_command_config_free

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Date:   Wed, 21 Apr 2021 10:40:24 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Ralph Metzler <rjkm@...zlerbros.de>,
        Matthias Benesch <twoof7@...enet.de>,
        Oliver Endriss <o.endriss@....de>, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] media: ngene: Fix out-of-bounds bug in
 ngene_command_config_free_buf()

On Mon, Apr 19, 2021 at 07:16:31PM -0500, Gustavo A. R. Silva wrote:
> Fix an 11-year old bug in ngene_command_config_free_buf() while
> addressing the following warnings caught with -Warray-bounds:
> 
> arch/alpha/include/asm/string.h:22:16: warning: '__builtin_memcpy' offset [12, 16] from the object at 'com' is out of the bounds of referenced subobject 'config' with type 'unsigned char' at offset 10 [-Warray-bounds]
> arch/x86/include/asm/string_32.h:182:25: warning: '__builtin_memcpy' offset [12, 16] from the object at 'com' is out of the bounds of referenced subobject 'config' with type 'unsigned char' at offset 10 [-Warray-bounds]
> 
> The problem is that the original code is trying to copy 6 bytes of
> data into a one-byte size member _config_ of the wrong structue
> FW_CONFIGURE_BUFFERS, in a single call to memcpy(). This causes a
> legitimate compiler warning because memcpy() overruns the length
> of &com.cmd.ConfigureBuffers.config. It seems that the right
> structure is FW_CONFIGURE_FREE_BUFFERS, instead, because it contains
> 6 more members apart from the header _hdr_. Also, the name of
> the function ngene_command_config_free_buf() suggests that the actual
> intention is to ConfigureFreeBuffers, instead of ConfigureBuffers
> (which configuration takes place in the function ngene_command_config_buf(),
> above).
> 
> Fix this by enclosing those 6 members of struct FW_CONFIGURE_FREE_BUFFERS
> into new struct config, and use &com.cmd.ConfigureFreeBuffers.config as
> the destination address, instead of &com.cmd.ConfigureBuffers.config,
> when calling memcpy().
> 
> This also helps with the ongoing efforts to globally enable
> -Warray-bounds and get us closer to being able to tighten the
> FORTIFY_SOURCE routines on memcpy().
> 
> Link: https://github.com/KSPP/linux/issues/109
> Fixes: dae52d009fc9 ("V4L/DVB: ngene: Initial check-in")
> Cc: stable@...r.kernel.org
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Nice find! Yeah, this looks like a copy/paste bug but it went unnoticed
because it's occupying the same memory via the union. Heh.

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  drivers/media/pci/ngene/ngene-core.c |  2 +-
>  drivers/media/pci/ngene/ngene.h      | 14 ++++++++------
>  2 files changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/media/pci/ngene/ngene-core.c b/drivers/media/pci/ngene/ngene-core.c
> index 07f342db6701..7481f553f959 100644
> --- a/drivers/media/pci/ngene/ngene-core.c
> +++ b/drivers/media/pci/ngene/ngene-core.c
> @@ -385,7 +385,7 @@ static int ngene_command_config_free_buf(struct ngene *dev, u8 *config)
>  
>  	com.cmd.hdr.Opcode = CMD_CONFIGURE_FREE_BUFFER;
>  	com.cmd.hdr.Length = 6;
> -	memcpy(&com.cmd.ConfigureBuffers.config, config, 6);
> +	memcpy(&com.cmd.ConfigureFreeBuffers.config, config, 6);
>  	com.in_len = 6;
>  	com.out_len = 0;
>  
> diff --git a/drivers/media/pci/ngene/ngene.h b/drivers/media/pci/ngene/ngene.h
> index 84f04e0e0cb9..3d296f1998a1 100644
> --- a/drivers/media/pci/ngene/ngene.h
> +++ b/drivers/media/pci/ngene/ngene.h
> @@ -407,12 +407,14 @@ enum _BUFFER_CONFIGS {
>  
>  struct FW_CONFIGURE_FREE_BUFFERS {
>  	struct FW_HEADER hdr;
> -	u8   UVI1_BufferLength;
> -	u8   UVI2_BufferLength;
> -	u8   TVO_BufferLength;
> -	u8   AUD1_BufferLength;
> -	u8   AUD2_BufferLength;
> -	u8   TVA_BufferLength;
> +	struct {
> +		u8   UVI1_BufferLength;
> +		u8   UVI2_BufferLength;
> +		u8   TVO_BufferLength;
> +		u8   AUD1_BufferLength;
> +		u8   AUD2_BufferLength;
> +		u8   TVA_BufferLength;
> +	} __packed config;
>  } __attribute__ ((__packed__));
>  
>  struct FW_CONFIGURE_UART {
> -- 
> 2.27.0
> 

-- 
Kees Cook