[PATCH 2/4] SUNRPC: harden gss_unwrap_resp_priv length checks

[Chuck Lever <cel@kernel.org>]

From: Chris Mason <clm@meta.com>

gss_unwrap_resp_priv() validates the RPCSEC_GSS opaque length with

    offset = (u8 *)(p) - (u8 *)head->iov_base;
    if (offset + opaque_len > rcv_buf->len)
            goto unwrap_failed;
    maj_stat = gss_unwrap(ctx->gc_gss_ctx, offset,
                          offset + opaque_len, rcv_buf);

Both operands are u32 and the sum is computed in u32. A reply with
opaque_len near 0xffffffff makes offset + opaque_len wrap to a small
value that is below rcv_buf->len, so the bound check passes and
gss_unwrap() is called with end < begin. The check also lacks a
lower bound, so any opaque_len in [0, GSS_KRB5_TOK_HDR_LEN) is
accepted and forwarded to gss_krb5_unwrap_v2(), whose pre-decrypt
header reads at ptr+4 and ptr+6 then run past the token.

A krb5p NFS server returning a crafted RPCSEC_GSS reply can drive
the client into out-of-bounds reads in gss_krb5_unwrap_v2() and the
rotate_left() loop that follows.

Fix by replacing the single combined check with three guards that
are safe in u32 arithmetic and that enforce the RFC 4121 minimum
outer token length:

    if (offset > rcv_buf->len)
            goto unwrap_failed;
    if (opaque_len > rcv_buf->len - offset)
            goto unwrap_failed;
    if (opaque_len < GSS_KRB5_TOK_HDR_LEN)
            goto unwrap_failed;

The first guard makes the subtraction in the second guard
unconditionally safe; offset is derived from a successful
xdr_inline_decode() in the head kvec, so in practice it already
satisfies the bound. The floor mirrors the server-side check added
in commit 7507012eeb98 ("SUNRPC: svcauth_gss: enforce krb5 token
minimum length").

Fixes: 2d2da60c63b6 ("RPCSEC_GSS: client-side privacy support")
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <clm@meta.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 net/sunrpc/auth_gss/auth_gss.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/auth_gss/auth_gss.c b/net/sunrpc/auth_gss/auth_gss.c
index 9d3fb6848f40..8ddc65e894da 100644
--- a/net/sunrpc/auth_gss/auth_gss.c
+++ b/net/sunrpc/auth_gss/auth_gss.c
@@ -2072,7 +2072,11 @@ gss_unwrap_resp_priv(struct rpc_task *task, struct rpc_cred *cred,
 		goto unwrap_failed;
 	opaque_len = be32_to_cpup(p++);
 	offset = (u8 *)(p) - (u8 *)head->iov_base;
-	if (offset + opaque_len > rcv_buf->len)
+	if (offset > rcv_buf->len)
+		goto unwrap_failed;
+	if (opaque_len > rcv_buf->len - offset)
+		goto unwrap_failed;
+	if (opaque_len <= GSS_KRB5_TOK_HDR_LEN)
 		goto unwrap_failed;
 
 	maj_stat = gss_unwrap(ctx->gc_gss_ctx, offset,
-- 
2.54.0