SUNRPC: fix gssx_dec_option_array error path bugs

Four coupled defects in the gssx XDR option-array decoder make the
error paths unsafe: a NULL deref in the caller, a refcount leak on
the decoded group_info, and a latent use-after-free that the leak
fix would otherwise expose.

gssx_dec_option_array() sets oa->count = 1 before allocating
oa->data.  If that allocation fails, -ENOMEM is returned with
oa->count == 1 and oa->data == NULL.  All other error paths jump
to free_oa: which frees oa->data and NULLs it but also leaves
oa->count == 1.  The caller trusts the count:

    gssp_accept_sec_context_upcall()
      gssx_dec_accept_sec_context()
        gssx_dec_option_array()        /* fails, count=1 data=NULL */
      data = res.options.data[0].value /* NULL deref */

Independently, free_creds: releases the partially decoded svc_cred
with a bare kfree(creds).  gssx_dec_linux_creds() installs a
groups_alloc() result into creds->cr_group_info; that object is
kvmalloc-backed and refcounted, and only put_group_info() reaches
kvfree().  A plain kfree(creds) drops the wrapper and leaks the
group_info allocation.

The natural fix for the leak is to call free_svc_cred(creds) before
kfree(creds), but free_svc_cred() invokes put_group_info() on
creds->cr_group_info unconditionally when non-NULL.  The existing
out_free_groups: path in gssx_dec_linux_creds() already called
groups_free() on that pointer without clearing it, so once
free_svc_cred() is wired in, the subsequent put_group_info() would
touch freed memory.

Fix all four together:

  - Move the oa->count = 1 assignment below the oa->data allocation
    so it is never set when oa->data is NULL.
  - Reset oa->count to 0 at free_oa: so count and data stay
    coherent and the caller sees an empty option array.
  - Call free_svc_cred(creds) before kfree(creds) at free_creds:
    so the refcounted cr_group_info is released.  free_svc_cred()
    either NULL-guards each field explicitly (cr_group_info has
    an if() check) or delegates to a helper that is NULL-safe
    itself (kfree for the string fields, gss_mech_put() which
    guards with if(gm) at gss_mech_switch.c:342), so it is safe
    to call on a partially decoded svc_cred where only
    cr_uid/cr_gid/cr_group_info have been written and everything
    else is zero from kzalloc.
  - In gssx_dec_linux_creds()'s out_free_groups: path, release
    cr_group_info with put_group_info() rather than groups_free()
    so the teardown matches free_svc_cred()'s refcount-aware path,
    and clear the pointer so a later free_svc_cred() on the same
    creds does not release it a second time.

Fixes: 3cfcfc102a ("SUNRPC: fix some memleaks in gssx_dec_option_array")
Cc: stable@vger.kernel.org
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <clm@meta.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260528-tier2-v1-2-d026a1415e0b@oracle.com
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/auth_gss/gss_rpc_xdr.c

@@ -222,7 +222,8 @@ static int gssx_dec_linux_creds(struct xdr_stream *xdr,
 
 	return 0;
 out_free_groups:
-	groups_free(creds->cr_group_info);
+	put_group_info(creds->cr_group_info);
+	creds->cr_group_info = NULL;
 	return err;
 }
 
@@ -242,12 +243,12 @@ static int gssx_dec_option_array(struct xdr_stream *xdr,
 		return 0;
 
 	/* we recognize only 1 currently: CREDS_VALUE */
-	oa->count = 1;
-
 	oa->data = kmalloc_obj(struct gssx_option);
 	if (!oa->data)
 		return -ENOMEM;
 
+	oa->count = 1;
+
 	creds = kzalloc_obj(struct svc_cred);
 	if (!creds) {
 		err = -ENOMEM;
@@ -294,8 +295,10 @@ static int gssx_dec_option_array(struct xdr_stream *xdr,
 	return 0;
 
 free_creds:
+	free_svc_cred(creds);
 	kfree(creds);
 free_oa:
+	oa->count = 0;
 	kfree(oa->data);
 	oa->data = NULL;
 	return err;