Re: [PATCH bpf-next v2 2/3] bpf/verifier: Use kmalloc_size_roundup() to match ksize() usage

[Daniel Borkmann <daniel@iogearbox.net>]

On 10/29/22 4:54 AM, Kees Cook wrote:
> Round up allocations with kmalloc_size_roundup() so that the verifier's
> use of ksize() is always accurate and no special handling of the memory
> is needed by KASAN, UBSAN_BOUNDS, nor FORTIFY_SOURCE. Pass the new size
> information back up to callers so they can use the space immediately,
> so array resizing to happen less frequently as well.
> 
[...]

The commit message is a bit cryptic here without further context. Is this
a bug fix or improvement? I read the latter, but it would be good to have
more context here for reviewers (maybe Link tag pointing to some discussion
or the like). Also, why is the kmalloc_size_roundup() not hidden for kmalloc
callers, isn't this a tree-wide issue?

Thanks,
Daniel

>   kernel/bpf/verifier.c | 12 ++++++++----
>   1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index eb8c34db74c7..1c040d27b8f6 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1008,9 +1008,9 @@ static void *copy_array(void *dst, const void *src, size_t n, size_t size, gfp_t
>   	if (unlikely(check_mul_overflow(n, size, &bytes)))
>   		return NULL;
>   
> -	if (ksize(dst) < bytes) {
> +	if (ksize(dst) < ksize(src)) {
>   		kfree(dst);
> -		dst = kmalloc_track_caller(bytes, flags);
> +		dst = kmalloc_track_caller(kmalloc_size_roundup(bytes), flags);
>   		if (!dst)
>   			return NULL;
>   	}
> @@ -1027,12 +1027,14 @@ static void *copy_array(void *dst, const void *src, size_t n, size_t size, gfp_t
>    */
>   static void *realloc_array(void *arr, size_t old_n, size_t new_n, size_t size)
>   {
> +	size_t alloc_size;
>   	void *new_arr;
>   
>   	if (!new_n || old_n == new_n)
>   		goto out;
>   
> -	new_arr = krealloc_array(arr, new_n, size, GFP_KERNEL);
> +	alloc_size = kmalloc_size_roundup(size_mul(new_n, size));
> +	new_arr = krealloc(arr, alloc_size, GFP_KERNEL);
>   	if (!new_arr) {
>   		kfree(arr);
>   		return NULL;
> @@ -2504,9 +2506,11 @@ static int push_jmp_history(struct bpf_verifier_env *env,
>   {
>   	u32 cnt = cur->jmp_history_cnt;
>   	struct bpf_idx_pair *p;
> +	size_t alloc_size;
>   
>   	cnt++;
> -	p = krealloc(cur->jmp_history, cnt * sizeof(*p), GFP_USER);
> +	alloc_size = kmalloc_size_roundup(size_mul(cnt, sizeof(*p)));
> +	p = krealloc(cur->jmp_history, alloc_size, GFP_USER);
>   	if (!p)
>   		return -ENOMEM;
>   	p[cnt - 1].idx = env->insn_idx;
>