SUNRPC: Fix ("SUNRPC: Add "@len" parameter to gss_unwrap()")

Braino when converting "buf->len -=" to "buf->len = len -".

The result is under-estimation of the ralign and rslack values. On
krb5p mounts, this has caused READDIR to fail with EIO, and KASAN
splats when decoding READLINK replies.

As a result of fixing this oversight, the gss_unwrap method now
returns a buf->len that can be shorter than priv_len for small
RPC messages. The additional adjustment done in unwrap_priv_data()
can underflow buf->len. This causes the nfsd_request_too_large
check to fail during some NFSv3 operations.

Reported-by: Marian Rainer-Harbach
Reported-by: Pierre Sauter <[e-mail]>
BugLink: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1886277
Fixes: 31c9590ae468 ("SUNRPC: Add "@len" parameter to gss_unwrap()")
Reviewed-by: J. Bruce Fields <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

net/sunrpc/auth_gss/gss_krb5_wrap.c

···
 							buf->head[0].iov_len);
 	memmove(ptr, ptr + GSS_KRB5_TOK_HDR_LEN + headskip, movelen);
 	buf->head[0].iov_len -= GSS_KRB5_TOK_HDR_LEN + headskip;
-	buf->len = len - GSS_KRB5_TOK_HDR_LEN + headskip;
+	buf->len = len - (GSS_KRB5_TOK_HDR_LEN + headskip);
 
 	/* Trim off the trailing "extra count" and checksum blob */
 	xdr_buf_trim(buf, ec + GSS_KRB5_TOK_HDR_LEN + tailskip);

net/sunrpc/auth_gss/svcauth_gss.c

···
 
 	maj_stat = gss_unwrap(ctx, 0, priv_len, buf);
 	pad = priv_len - buf->len;
-	buf->len -= pad;
 	/* The upper layers assume the buffer is aligned on 4-byte boundaries.
 	 * In the krb5p case, at least, the data ends up offset, so we need to
 	 * move it around. */