linux-kernel - Re: [PATCH] DAC960: Fix undefined behavior on empty string

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Date:	Thu, 23 Jul 2009 14:26:03 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michael Buesch <mb@...sch.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] DAC960: Fix undefined behavior on empty string

On Sun, 19 Jul 2009 15:05:47 +0200
Michael Buesch <mb@...sch.de> wrote:

> This patch fixes undefined behavior due to buffer underrun,
> if an empty string is written to the proc file.
> 
> Signed-off-by: Michael Buesch <mb@...sch.de>
> Cc: stable@...nel.org
> 
> ---
> 
> This patch is untested, because I do not have the hardware.
> 
> ---
>  drivers/block/DAC960.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- linux-2.6.orig/drivers/block/DAC960.c
> +++ linux-2.6/drivers/block/DAC960.c
> @@ -6555,21 +6555,21 @@ static int DAC960_ProcWriteUserCommand(s
>  				       const char __user *Buffer,
>  				       unsigned long Count, void *Data)
>  {
>    DAC960_Controller_T *Controller = (DAC960_Controller_T *) Data;
>    unsigned char CommandBuffer[80];
>    int Length;
>    if (Count > sizeof(CommandBuffer)-1) return -EINVAL;
>    if (copy_from_user(CommandBuffer, Buffer, Count)) return -EFAULT;
>    CommandBuffer[Count] = '\0';
>    Length = strlen(CommandBuffer);
> -  if (CommandBuffer[Length-1] == '\n')
> +  if (Length > 0 && CommandBuffer[Length-1] == '\n')
>      CommandBuffer[--Length] = '\0';
>    if (Controller->FirmwareType == DAC960_V1_Controller)
>      return (DAC960_V1_ExecuteUserCommand(Controller, CommandBuffer)
>  	    ? Count : -EBUSY);
>    else
>      return (DAC960_V2_ExecuteUserCommand(Controller, CommandBuffer)
>  	    ? Count : -EBUSY);
>  }

I suspect this is NotABug, as it requires that
DAC960_ProcWriteUserCommand() be called in response to a zero-length
write, and various bits of code will terminate early if they see such a
write go past.  But we shouldn't rely on that here.

Surely we have a library function somewhere which will remove any
terminating whitespace from a C string?  Sigh.

I note that you cc'ed stable@...nel.org on this patch.  Why was that? 
I assume that this pseudo-file is root-only, in which case the fix
isn't particularly urgent?

Thanks.
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