Re: [PATCH] sctp: prefer struct_size over open coded arithmetic

[Kees Cook <keescook@chromium.org>]

On Sat, Apr 27, 2024 at 07:23:36PM +0200, Erick Archer wrote:
> This is an effort to get rid of all multiplications from allocation
> functions in order to prevent integer overflows [1][2].
> 
> As the "ids" variable is a pointer to "struct sctp_assoc_ids" and this
> structure ends in a flexible array:
> 
> struct sctp_assoc_ids {
        __u32           gaids_number_of_ids;
> 	sctp_assoc_t	gaids_assoc_id[];
> };

This could gain __counted_by:

diff --git a/include/uapi/linux/sctp.h b/include/uapi/linux/sctp.h
index b7d91d4cf0db..836173e73401 100644
--- a/include/uapi/linux/sctp.h
+++ b/include/uapi/linux/sctp.h
@@ -1007,7 +1007,7 @@ enum sctp_sstat_state {
  */
 struct sctp_assoc_ids {
 	__u32		gaids_number_of_ids;
-	sctp_assoc_t	gaids_assoc_id[];
+	sctp_assoc_t	gaids_assoc_id[] __counted_by(gaids_number_of_ids);
 };
 
 /*

> 
> the preferred way in the kernel is to use the struct_size() helper to
> do the arithmetic instead of the calculation "size + size * count" in
> the kmalloc() function.
> 
> Also, refactor the code adding the "ids_size" variable to avoid sizing
> twice.
> 
> This way, the code is more readable and safer.
> 
> This code was detected with the help of Coccinelle, and audited and
> modified manually.
> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
> Link: https://github.com/KSPP/linux/issues/160 [2]
> Signed-off-by: Erick Archer <erick.archer@outlook.com>
> ---
> Hi,
> 
> The Coccinelle script used to detect this code pattern is the following:
> 
> virtual report
> 
> @rule1@
> type t1;
> type t2;
> identifier i0;
> identifier i1;
> identifier i2;
> identifier ALLOC =~ "kmalloc|kzalloc|kmalloc_node|kzalloc_node|vmalloc|vzalloc|kvmalloc|kvzalloc";
> position p1;
> @@
> 
> i0 = sizeof(t1) + sizeof(t2) * i1;
> ...
> i2 = ALLOC@p1(..., i0, ...);
> 
> @script:python depends on report@
> p1 << rule1.p1;
> @@
> 
> msg = "WARNING: verify allocation on line %s" % (p1[0].line)
> coccilib.report.print_report(p1[0],msg)
> 
> Regards,
> Erick
> ---
>  net/sctp/socket.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index e416b6d3d270..64196b1dce1d 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -7119,6 +7119,7 @@ static int sctp_getsockopt_assoc_ids(struct sock *sk, int len,
>  	struct sctp_sock *sp = sctp_sk(sk);
>  	struct sctp_association *asoc;
>  	struct sctp_assoc_ids *ids;
> +	size_t ids_size;
>  	u32 num = 0;
>  
>  	if (sctp_style(sk, TCP))
> @@ -7131,11 +7132,11 @@ static int sctp_getsockopt_assoc_ids(struct sock *sk, int len,
>  		num++;
>  	}
>  
> -	if (len < sizeof(struct sctp_assoc_ids) + sizeof(sctp_assoc_t) * num)
> +	ids_size = struct_size(ids, gaids_assoc_id, num);
> +	if (len < ids_size)
>  		return -EINVAL;
>  
> -	len = sizeof(struct sctp_assoc_ids) + sizeof(sctp_assoc_t) * num;
> -
> +	len = ids_size;
>  	ids = kmalloc(len, GFP_USER | __GFP_NOWARN);
>  	if (unlikely(!ids))
>  		return -ENOMEM;

But yes, this looks fine to me.

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook