Summary

xfrmnl_sa_parse() computes allocation and copy sizes from length fields
embedded in the netlink attribute payload (alg_key_len, ctx_len, bmp_len)
without comparing them against the actual attribute length returned by nla_len().

The nla_policy for these attributes sets only .minlen, never .maxlen, so
nlmsg_parse() cannot enforce an upper bound on attribute payload size. This
means a caller supplying an attribute whose payload is shorter than the claimed
key/context/bitmap length causes:

• OOB heap read (all algo/sec_ctx/ESN paths): the calloc destination is
  sized to the attacker-controlled value, and memcpy() reads that many bytes
  past the end of the (shorter) source attribute, dragging adjacent heap contents
  into sa->aead/auth/crypt/comp/sec_ctx/replay_state_esn.

• OOB heap write (XFRMA_ALG_AUTH, line 905): the old
  strcpy(sa->auth->alg_name, auth->alg_name) had no NUL-termination
  guarantee on the 64-byte source field, allowing a write past the alg_name
  destination in the freshly allocated sa->auth.

• Integer overflow (XFRMA_REPLAY_ESN_VAL): bmp_len is uint32_t, and
  sizeof(uint32_t) * bmp_len can wrap to a small value, producing an
  undersized allocation followed by a large memcpy.

Fix

For every variable-length attribute block, validate the payload-embedded length
field against nla_len() before computing the allocation size:

int attr_len = nla_len(tb[XFRMA_ALG_AEAD]);
int key_bytes;

if (attr_len < (int)sizeof(struct xfrm_algo_aead))
    return -NLE_INVAL;
key_bytes = (aead->alg_key_len + 7) / 8;
if (key_bytes > attr_len - (int)sizeof(struct xfrm_algo_aead))
    return -NLE_INVAL;
len = sizeof(struct xfrmnl_algo_aead) + key_bytes;

This pattern is applied to all seven affected blocks:
XFRMA_ALG_AEAD, XFRMA_ALG_AUTH_TRUNC, XFRMA_ALG_AUTH,
XFRMA_ALG_CRYPT, XFRMA_ALG_COMP, XFRMA_SEC_CTX, XFRMA_REPLAY_ESN_VAL.

For XFRMA_ALG_AUTH, strcpy is replaced with a bounded memcpy into the
calloc'd (zero-initialised) alg_name field:

memcpy(sa->auth->alg_name, auth->alg_name, sizeof(sa->auth->alg_name) - 1);

For XFRMA_REPLAY_ESN_VAL, an explicit overflow guard is added before the
bmp_len multiply:

if (esn->bmp_len > (UINT32_MAX / sizeof(uint32_t)))
    return -NLE_INVAL;

Normal well-formed SA messages from the kernel are unaffected: the validated
key_bytes/bmp_bytes values are identical to what the old code computed
whenever the attribute is correctly sized.

Context

This was noted in issue #453, which identified the strcpy on line 905. This
patch addresses the full family of unbounded copies in the same function.
The setter paths were hardened in an earlier PR; this hardens the corresponding
parser/receive path.