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

Roberto Sassu
2022-12-20T10:37:42Z

On 12/20/2022 8:24 AM, Herbert Xu wrote:
> On Mon, Dec 19, 2022 at 09:49:29AM +0100, Roberto Sassu wrote:
>>
>> do you have any news on this bug?
>
> Thanks for the reminder. Could you please try this patch?

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).

Roberto

> ---8<---
> The helper mpi_read_raw_from_sgl ignores the second parameter
> nbytes when reading the SG list and may overrun its own buffer
> because it only allocates enough memory according to 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..6bffc68c1a5a 100644
> --- a/lib/mpi/mpicoder.c
> +++ b/lib/mpi/mpicoder.c
> @@ -494,17 +494,15 @@ MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int nbytes)
> val->sign = 0;
> val->nlimbs = nlimbs;
>
> - if (nbytes == 0)
> - return val;
> -
> j = nlimbs - 1;
> a = 0;
> z = BYTES_PER_MPI_LIMB - nbytes % BYTES_PER_MPI_LIMB;
> z %= BYTES_PER_MPI_LIMB;
>
> - while (sg_miter_next(&miter)) {
> + while (nbytes && 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;