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

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Date:   Tue, 20 Dec 2022 11:36:50 +0100
From:   Roberto Sassu <roberto.sassu@...weicloud.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Eric Biggers <ebiggers@...nel.org>, dhowells@...hat.com,
        davem@...emloft.net, zohar@...ux.ibm.com,
        dmitry.kasatkin@...il.com, paul@...l-moore.com, jmorris@...ei.org,
        serge@...lyn.com, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, keyrings@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        Roberto Sassu <roberto.sassu@...wei.com>,
        Tadeusz Struk <tadeusz.struk@...el.com>
Subject: Re: [PATCH] lib/mpi: Fix buffer overrun when SG is too long

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@...weicloud.com>
> Signed-off-by: Herbert Xu <herbert@...dor.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;