bpf: Fix out-of-bounds dynptr write in bpf_crypto_crypt

Stanislav reported that in bpf_crypto_crypt() the destination dynptr's
size is not validated to be at least as large as the source dynptr's
size before calling into the crypto backend with 'len = src_len'. This
can result in an OOB write when the destination is smaller than the
source.

Concretely, in mentioned function, psrc and pdst are both linear
buffers fetched from each dynptr:

  psrc = __bpf_dynptr_data(src, src_len);
  [...]
  pdst = __bpf_dynptr_data_rw(dst, dst_len);
  [...]
  err = decrypt ?
        ctx->type->decrypt(ctx->tfm, psrc, pdst, src_len, piv) :
        ctx->type->encrypt(ctx->tfm, psrc, pdst, src_len, piv);

The crypto backend expects pdst to be large enough with a src_len length
that can be written. Add an additional src_len > dst_len check and bail
out if it's the case. Note that these kfuncs are accessible under root
privileges only.

Fixes: 3e1c6f3 ("bpf: make common crypto API for TC/XDP programs")
Reported-by: Stanislav Fort <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Cc: Vadim Fedorenko <[email protected]>
Reviewed-by: Vadim Fedorenko <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: borkmann

kernel/bpf/crypto.c

@@ -278,7 +278,7 @@ static int bpf_crypto_crypt(const struct bpf_crypto_ctx *ctx,
 	siv_len = siv ? __bpf_dynptr_size(siv) : 0;
 	src_len = __bpf_dynptr_size(src);
 	dst_len = __bpf_dynptr_size(dst);
-	if (!src_len || !dst_len)
+	if (!src_len || !dst_len || src_len > dst_len)
 		return -EINVAL;
 
 	if (siv_len != ctx->siv_len)