Re: [PATCH net] net/atm: fix slab-out-of-bounds read in vcc_setsockopt()

On Tue, Aug 4, 2026 at 4:58 PM Eric Dumazet <[email protected]> wrote:
>
> vcc_setsockopt() never checked optlen for ATM-specific options (SO_ATMQOS
> and SO_SETCLP) and called copy_from_sockptr() assuming optval contained
> sufficient space for struct atm_qos or unsigned long.
>
> While copy_from_user() previously copied from a user buffer without checking
> optlen, the introduction of sockptr_t and cgroup BPF setsockopt filters allows
> optval to point to a kernel memory buffer allocated with a reduced optlen.
> Because copy_from_sockptr() on kernel pointers uses memcpy(), this leads to a
> KASAN slab-out-of-bounds read when optlen is smaller than the expected structure
> size.
>
> Fix this by using copy_safe_from_sockptr(), which validates that optlen is at
> least the expected size before copying.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: [email protected]
> Closes: https://lore.kernel.org/netdev/[email protected]/T/#u
> Signed-off-by: Eric Dumazet <[email protected]>
> ---
>  net/atm/common.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/net/atm/common.c b/net/atm/common.c
> index c7f92405daf050ecd2a507e41efdd622ab8dc6a8..6a0c5184c0d6380deaa95f68cb97c7745b4f7b56 100644
> --- a/net/atm/common.c
> +++ b/net/atm/common.c
> @@ -722,8 +722,9 @@ int vcc_setsockopt(struct socket *sock, int level, int optname,
>         {
>                 struct atm_qos qos;
>
> -               if (copy_from_sockptr(&qos, optval, sizeof(qos)))
> -                       return -EFAULT;
> +               error = copy_safe_from_sockptr(&qos, sizeof(qos), optval, optlen);
> +               if (error)
> +                       return error;
>                 error = check_qos(&qos);
>                 if (error)
>                         return error;
> @@ -737,8 +738,9 @@ int vcc_setsockopt(struct socket *sock, int level, int optname,
>                 return 0;
>         }
>         case SO_SETCLP:
> -               if (copy_from_sockptr(&value, optval, sizeof(value)))
> -                       return -EFAULT;
> +               error = copy_safe_from_sockptr(&value, sizeof(value), optval, optlen);

I will send a V2, because @value is an "unsigned long", while a prior
check uses an "int"

#define SO_SETCLP   __SO_ENCODE(SOL_ATM,0,int)

I will change @value to an "int" in V2.

pw-bot: cr

> +               if (error)
> +                       return error;
>                 if (value)
>                         vcc->atm_options |= ATM_ATMOPT_CLP;
>                 else
> --
> 2.55.0.571.g244d577d93-goog
>