Re: [Xen-devel] [PATCH] console: avoid buffer overflow in guest_console_write()

[Jürgen Groß <jgross@xxxxxxxx>]

On 29.11.19 14:26, Jan Beulich wrote:
> On 29.11.2019 13:37, Andrew Cooper wrote:
> > On 29/11/2019 12:19, Jan Beulich wrote:
> > > On 29.11.2019 13:15, Andrew Cooper wrote:
> > > > On 29/11/2019 12:13, Jan Beulich wrote:
> > > > > On 29.11.2019 13:01, Ian Jackson wrote:
> > > > > > Jan Beulich writes ("Re: [PATCH] console: avoid buffer overflow in 
> > > > > > guest_console_write()"):
> > > > > > > On 29.11.2019 11:22, Andrew Cooper wrote:
> > > > > > > > Is sizeof(array[0]) always 0, or is this just a GCC-ism ?  Godbolt
> > > > > > > > suggests is 0 on all compiler we support.
> > > > > > > >
> > > > > > > > Either way, isn't the more common idiom + 0ul ?  Personally, I feel that
> > > > > > > > is clearer to follow.
> > > > > > > I decided against + 0ul or alike because in principle size_t
> > > > > > > and unsigned long are different types. In particular 32-bit
> > > > > > > x86 gcc uses unsigned int for size_t, and hence min()'s
> > > > > > > type safety check would cause the build to fail there. The
> > > > > > > same risk obviously exists for any 32-bit arch (e.g. Arm32,
> > > > > > > but I haven't checked what type it actually uses).
> > > > > > I don't know what i wrong with
> > > > > >     (size_t)0
> > > > > > which is shorter, even !
> > > > > True. Yet it contains a cast, no matter how risk-free it may be
> > > > > in this case. With a cast, I could as well have written (yet
> > > > > shorter) (size_t)count.
> > > > Given that min() has a very strict typecheck, I think we should permit
> > > > any use of an explicit cast in a single operand, because it *is* safer
> > > > than switching to the min_t() route to make things compile.
> > > Well, I can switch to (size_t)count if this is liked better
> > > overall.
> > Personally, I'd prefer this option most of all.
> Okay, I've switched to this, but while doing so I started wondering
> why we'd then not use
>
>          kcount = min(count, (unsigned int)sizeof(kbuf) - 1);
>
> which is an (often slightly cheaper) 32-bit operation (and which
> is what I had actually started from).
While modifying guest_console_write(), would you mind writing a '\0'
to kbuf[kcount]? There is a "conring_puts(kbuf);" later in this
function which would like a 0 terminated string as input.


Juergen


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