Re: [v2 PATCH] lib/mpi: Fix buffer overrun when SG is too long

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Dec 21, 2022 at 02:53:58PM +0800, Herbert Xu wrote:
> On Tue, Dec 20, 2022 at 08:30:16PM +0000, Eric Biggers wrote:
> >
> > > Tried, could not boot the UML kernel.
> > > 
> > > After looking, it seems we have to call sg_miter_stop(). Or alternatively,
> > > we could let sg_miter_next() be called but not writing anything inside the
> > > loop.
> > > 
> > > With either of those fixes, the tests pass (using one scatterlist).
> 
> Thanks for the quick feedback Roberto!
> 
> > I think it should look like:
> > 
> > 	while (nbytes) {
> > 		sg_miter_next(&miter);
> > 		...
> > 	}
> > 	sg_miter_stop(&miter);
> 
> You're right Eric.  However, we could also do it by simply not
> checking nbytes since we already set nents according to nbytes
> at the top of the function.
> 
> ---8<---
> The helper mpi_read_raw_from_sgl sets the number of entries in
> the SG list according to nbytes.  However, if the last entry
> in the SG list contains more data than nbytes, then it may overrun
> the buffer because it only allocates enough memory for nbytes.
> 
> Fixes: 2d4d1eea540b ("lib/mpi: Add mpi sgl helpers")
> Reported-by: Roberto Sassu <roberto.sassu@huaweicloud.com>
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
> 
> diff --git a/lib/mpi/mpicoder.c b/lib/mpi/mpicoder.c
> index 39c4c6731094..157ef532a6a2 100644
> --- a/lib/mpi/mpicoder.c
> +++ b/lib/mpi/mpicoder.c
> @@ -504,7 +501,8 @@ MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int nbytes)
>  
>  	while (sg_miter_next(&miter)) {
>  		buff = miter.addr;
> -		len = miter.length;
> +		len = min_t(unsigned, miter.length, nbytes);
> +		nbytes -= len;
>  
>  		for (x = 0; x < len; x++) {
>  			a <<= 8;

That's fine, I guess.  One quirk of the above approach is that if the last
needed element of the scatterlist has a lot of extra pages, this will iterate
through all those extra pages, processing 0 bytes from each.  It could just stop
when done.  I suppose it's not worth worrying about that case, though.

- Eric