Re: [PATCH v2] bpf: btf: Fix a missing-check bug

[Martin Lau <kafai@fb.com>]

On Wed, Oct 24, 2018 at 08:00:19AM -0500, Wenwen Wang wrote:
> In btf_parse(), the header of the user-space btf data 'btf_data' is firstly
> parsed and verified through btf_parse_hdr(). In btf_parse_hdr(), the header
> is copied from user-space 'btf_data' to kernel-space 'btf->hdr' and then
> verified. If no error happens during the verification process, the whole
> data of 'btf_data', including the header, is then copied to 'data' in
> btf_parse(). It is obvious that the header is copied twice here. More
> importantly, no check is enforced after the second copy to make sure the
> headers obtained in these two copies are same. Given that 'btf_data'
> resides in the user space, a malicious user can race to modify the header
> between these two copies. By doing so, the user can inject inconsistent
> data, which can cause undefined behavior of the kernel and introduce
> potential security risk.
> 
> To avoid the above issue, this patch copies the parsed header from
> 'btf->hdr' to 'data'. The remaining part in 'data' is still copied from the
> user-space 'btf_data'.
LGTM.

Acked-by: Martin KaFai Lau <kafai@fb.com>

> 
> Signed-off-by: Wenwen Wang <wang6495@umn.edu>
> ---
>  kernel/bpf/btf.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 378cef7..b52a834a 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -2152,6 +2152,7 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size,
>  	struct btf_verifier_env *env = NULL;
>  	struct bpf_verifier_log *log;
>  	struct btf *btf = NULL;
> +	u32 hdr_len;
>  	u8 *data;
>  	int err;
>  
> @@ -2200,7 +2201,15 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size,
>  	btf->data_size = btf_data_size;
>  	btf->nohdr_data = btf->data + btf->hdr.hdr_len;
>  
> -	if (copy_from_user(data, btf_data, btf_data_size)) {
> +	/*
> +	 * The header at btf_data could be modified by a malicious user
> +	 * after it is parsed. So we copy the parsed header here. The
> +	 * remaining part is still copied from btf_data.
> +	 */
> +	hdr_len = min_t(u32, btf->hdr.hdr_len, sizeof(btf->hdr));
> +	memcpy(data, &btf->hdr, hdr_len);
> +	if (copy_from_user(data + hdr_len, (u8 __user *)btf_data + hdr_len,
> +				btf_data_size - hdr_len)) {
>  		err = -EFAULT;
>  		goto errout;
>  	}
> -- 
> 2.7.4
>