[PATCH 0/4] sunrpc: close length validation gaps in krb5p unwrap

[Chuck Lever <cel@kernel.org>]

From: Chuck Lever <chuck.lever@oracle.com>

Neither svcauth_gss_unwrap_priv() nor gss_unwrap_resp_priv()
enforces a minimum on the wire-supplied opaque length before
forwarding it to the krb5 unwrap core.  An authenticated krb5p
peer -- NFS client or server -- can send a token shorter than the
16-byte RFC 4121 header and reach gss_krb5_unwrap_v2() with a
length that drives pre-decrypt out-of-bounds reads, a
divide-by-zero in _rotate_left(), and a post-decrypt unsigned
underflow in the movelen computation that feeds a multi-gigabyte
memmove.

The fixes are layered so that neither the caller boundary nor
the unwrap core depends on the other for safety.  The caller-side
floor checks (patches 1, 2) are sufficient to close the attack,
but gss_krb5_unwrap_v2() must also be self-defending because it
is reachable through gss_unwrap() from any future caller that
might not enforce the same floor.  The xdr_buf_trim() clamp
(patch 3) is independent of krb5 -- it corrects a generic
invariant violation that gss_krb5_unwrap_v2() happens to be the
only current trigger for, but that any caller could hit if
buf->len and the component iov_lens get out of sync.

The client side (patch 2) has an additional u32 integer-overflow
bypass that the server side does not: the original
`offset + opaque_len > rcv_buf->len` wraps when opaque_len is
near UINT_MAX, accepting a huge token length that re-enters the
same underflow chain.  The replacement splits the check into
separate offset and length guards that are safe in u32 arithmetic.

Chris Mason (4):
  SUNRPC: svcauth_gss: enforce krb5 token minimum length
  SUNRPC: harden gss_unwrap_resp_priv length checks
  SUNRPC: xdr_buf_trim: clamp buf->len to avoid underflow
  SUNRPC: harden gss_krb5_unwrap_v2 against short tokens

 net/sunrpc/auth_gss/auth_gss.c      |  6 +++++-
 net/sunrpc/auth_gss/gss_krb5_wrap.c | 11 +++++++++--
 net/sunrpc/auth_gss/svcauth_gss.c   |  2 ++
 net/sunrpc/xdr.c                    |  2 +-
 4 files changed, 17 insertions(+), 4 deletions(-)

-- 
2.54.0