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List:       linux-hardening
Subject:    Re: [PATCH] media: ngene: Fix out-of-bounds bug in ngene_command_config_free_buf()
From:       Kees Cook <keescook () chromium ! org>
Date:       2021-04-21 17:40:24
Message-ID: 202104211039.31E9785 () keescook
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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@vger.kernel.org
> Reported-by: kernel test robot <lkp@intel.com>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.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@chromium.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


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