[PATCH v2] SUNRPC: Fix ("SUNRPC: Add "@len" parameter to gss_unwrap()")

[Chuck Lever <chuck.lever@oracle.com>]

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 <pierre.sauter@stwm.de>
BugLink: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1886277
Fixes: 31c9590ae468 ("SUNRPC: Add "@len" parameter to gss_unwrap()")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 net/sunrpc/auth_gss/gss_krb5_wrap.c |    2 +-
 net/sunrpc/auth_gss/svcauth_gss.c   |    1 -
 2 files changed, 1 insertion(+), 2 deletions(-)

Hi Bruce-

I can take this in v5.9 if you agree it's a righteous fix.

Changes since RFC:
- series squashed into a single patch
- "pad" is still calculated in unwrap_priv_data for later use

diff --git a/net/sunrpc/auth_gss/gss_krb5_wrap.c b/net/sunrpc/auth_gss/gss_krb5_wrap.c
index cf0fd170ac18..90b8329fef82 100644
--- a/net/sunrpc/auth_gss/gss_krb5_wrap.c
+++ b/net/sunrpc/auth_gss/gss_krb5_wrap.c
@@ -584,7 +584,7 @@ gss_unwrap_kerberos_v2(struct krb5_ctx *kctx, int offset, int len,
 							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);
diff --git a/net/sunrpc/auth_gss/svcauth_gss.c b/net/sunrpc/auth_gss/svcauth_gss.c
index 7d83f54aaaa6..258b04372f85 100644
--- a/net/sunrpc/auth_gss/svcauth_gss.c
+++ b/net/sunrpc/auth_gss/svcauth_gss.c
@@ -990,7 +990,6 @@ unwrap_priv_data(struct svc_rqst *rqstp, struct xdr_buf *buf, u32 seq, struct gs
 
 	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. */