Re: [PATCH nf 1/1] netfilter: ebtables: fix OOB read in compat_mtw_from_user

[Florian Westphal <fw@strlen.de>]

Ren Wei <n05ec@lzu.edu.cn> wrote:
> From: Luxiao Xu <rakukuip@gmail.com>
> 
> The function compat_mtw_from_user() converts ebtables extensions from
> 32-bit user structures to kernel native structures. However, it lacks
> proper validation of the user-supplied match_size/target_size.
> 
> When certain extensions are processed, the kernel-side translation
> logic may perform memory accesses based on the extension's expected
> size. If the user provides a size smaller than what the extension
> requires, it results in an out-of-bounds read as reported by KASAN.
> 
> This fix introduces a check to ensure match_size is at least as large
> as the extension's required compatsize. This covers matches, watchers,
> and targets, while maintaining compatibility with standard targets.
> 
> Fixes: 81e675c227ec ("netfilter: ebtables: add CONFIG_COMPAT support")
> Cc: stable@kernel.org
> Reported-by: Yuan Tan <yuantan098@gmail.com>
> Reported-by: Yifan Wu <yifanwucs@gmail.com>
> Reported-by: Juefei Pu <tomapufckgml@gmail.com>
> Reported-by: Xin Liu <bird@lzu.edu.cn>
> Signed-off-by: Luxiao Xu <rakukuip@gmail.com>
> Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
> ---
>  net/bridge/netfilter/ebtables.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/net/bridge/netfilter/ebtables.c b/net/bridge/netfilter/ebtables.c
> index aea3e19875c6..80cd0233c088 100644
> --- a/net/bridge/netfilter/ebtables.c
> +++ b/net/bridge/netfilter/ebtables.c
> @@ -1977,6 +1977,11 @@ static int compat_mtw_from_user(const struct compat_ebt_entry_mwt *mwt,
>  		if (IS_ERR(match))
>  			return PTR_ERR(match);
>  
> +		if (match_size < match->compatsize) {
> +			module_put(match->me);
> +			return -EINVAL;
> +		}
> +

Are you sure this catches all bad requests? AFAIR compatsize is 0
in most cases, which bypasses this test.

should this be:

u16 csize = match->compatsize ? : match->matchsize;
...
if (match_size < csize) {
...

?

@Pablo: I think the 32bit compat layer should be removed in -next, or
at least strongly discouraged and slated for removal soon.