Re: [PATCH 1/8] lkdtm: change snprintf to scnprintf for possible overflow

[Greg KH]

On Tue, Jan 15, 2019 at 12:47:34PM -0800, Kees Cook wrote:
> On Sat, Jan 12, 2019 at 7:28 AM Willy Tarreau <w@xxxxxx> wrote:
> >
> > From: Silvio Cesare <silvio.cesare@xxxxxxxxx>
> >
> > Change snprintf to scnprintf. There are generally two cases where using
> > snprintf causes problems.
> >
> > 1) Uses of size += snprintf(buf, SIZE - size, fmt, ...)
> > In this case, if snprintf would have written more characters than what the
> > buffer size (SIZE) is, then size will end up larger than SIZE. In later
> > uses of snprintf, SIZE - size will result in a negative number, leading
> > to problems. Note that size might already be too large by using
> > size = snprintf before the code reaches a case of size += snprintf.
> >
> > 2) If size is ultimately used as a length parameter for a copy back to user
> > space, then it will potentially allow for a buffer overflow and information
> > disclosure when size is greater than SIZE. When the size is used to index
> > the buffer directly, we can have memory corruption. This also means when
> > size = snprintf... is used, it may also cause problems since size may become
> > large.  Copying to userspace is mitigated by the HARDENED_USERCOPY kernel
> > configuration.
> >
> > The solution to these issues is to use scnprintf which returns the number of
> > characters actually written to the buffer, so the size variable will never
> > exceed SIZE.
> >
> > Signed-off-by: Silvio Cesare <silvio.cesare@xxxxxxxxx>
> > Cc: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Will Deacon <will.deacon@xxxxxxx>
> > Cc: Greg KH <greg@xxxxxxxxx>
> > Signed-off-by: Willy Tarreau <w@xxxxxx>
> 
> It looks like these are going via individual trees. Greg, can you
> please take this into your drivers-misc tree for lkdtm?
> 
> Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

Will do, thanks.

greg k-h