[PATCH net] tls: Preserve sk_err across recvmsg() when data has been copied

[Chuck Lever <cel@kernel.org>]

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

The sk_err check in tls_rx_rec_wait() consumes the error via
sock_error(), which clears sk_err atomically. When the caller
(tls_sw_recvmsg, tls_sw_splice_read, or tls_sw_read_sock) already
has bytes copied to userspace, it returns those bytes and discards
the error from this call. sk_err is now zero on the socket, so the
next read syscall observes only RCV_SHUTDOWN and reports a clean
EOF instead of the actual error (typically -ECONNRESET).

The race is reachable when tls_read_flush_backlog()'s periodic
sk_flush_backlog() triggers tcp_reset() in the middle of a
multi-record read.

Pass a has_copied flag to tls_rx_rec_wait(). When has_copied is
false, consume sk_err via sock_error() as before. When has_copied
is true, report the error from READ_ONCE() but leave sk_err set:
the caller returns the byte count and discards the err from this
call, and the next read syscall surfaces the preserved sk_err. This
mirrors the tcp_recvmsg() preserve-and-surface pattern.

The decrypt-abort path is unaffected: tls_err_abort() raises
sk_err to EBADMSG after tls_rx_rec_wait() returns, and nothing
on the caller's return path consumes it, so the EBADMSG surfaces
on the next read.

tls_sw_splice_read() passes has_copied=false: it processes
one record per call, so no bytes have been copied within the
function when tls_rx_rec_wait() runs. A reset that arrives
between iterations of splice_direct_to_actor() (the sendfile()
path) is still consumed by sock_error() in the later call, and the
outer loop returns the prior iterations' byte count and drops the
error. tcp_splice_read() exhibits the same pattern at the iteration
boundary; addressing it belongs at the splice_direct_to_actor()
layer and is out of scope here.

Fixes: c46b01839f7a ("tls: rx: periodically flush socket backlog")
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 net/tls/tls_sw.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
index 2590e855f6a5..c4cc4e357848 100644
--- a/net/tls/tls_sw.c
+++ b/net/tls/tls_sw.c
@@ -1356,9 +1356,14 @@ void tls_sw_splice_eof(struct socket *sock)
 	mutex_unlock(&tls_ctx->tx_lock);
 }
 
+/* When has_copied is true the caller has already moved bytes to
+ * userspace. Report sk_err but leave it set so the next read
+ * surfaces it instead of a spurious EOF, otherwise sk_err is
+ * consumed via sock_error().
+ */
 static int
 tls_rx_rec_wait(struct sock *sk, struct sk_psock *psock, bool nonblock,
-		bool released)
+		bool released, bool has_copied)
 {
 	struct tls_context *tls_ctx = tls_get_ctx(sk);
 	struct tls_sw_context_rx *ctx = tls_sw_ctx_rx(tls_ctx);
@@ -1376,8 +1381,11 @@ tls_rx_rec_wait(struct sock *sk, struct sk_psock *psock, bool nonblock,
 		if (!sk_psock_queue_empty(psock))
 			return 0;
 
-		if (sk->sk_err)
+		if (sk->sk_err) {
+			if (has_copied)
+				return -READ_ONCE(sk->sk_err);
 			return sock_error(sk);
+		}
 
 		if (ret < 0)
 			return ret;
@@ -1413,7 +1421,7 @@ tls_rx_rec_wait(struct sock *sk, struct sk_psock *psock, bool nonblock,
 	}
 
 	if (unlikely(!tls_strp_msg_load(&ctx->strp, released)))
-		return tls_rx_rec_wait(sk, psock, nonblock, false);
+		return tls_rx_rec_wait(sk, psock, nonblock, false, has_copied);
 
 	return 1;
 }
@@ -2100,7 +2108,7 @@ int tls_sw_recvmsg(struct sock *sk,
 		int to_decrypt, chunk;
 
 		err = tls_rx_rec_wait(sk, psock, flags & MSG_DONTWAIT,
-				      released);
+				      released, !!(decrypted + copied));
 		if (err <= 0) {
 			if (psock) {
 				chunk = sk_msg_recvmsg(sk, psock, msg, len,
@@ -2287,7 +2295,7 @@ ssize_t tls_sw_splice_read(struct socket *sock,  loff_t *ppos,
 		struct tls_decrypt_arg darg;
 
 		err = tls_rx_rec_wait(sk, NULL, flags & SPLICE_F_NONBLOCK,
-				      true);
+				      true, false);
 		if (err <= 0)
 			goto splice_read_end;
 
@@ -2373,7 +2381,7 @@ int tls_sw_read_sock(struct sock *sk, read_descriptor_t *desc,
 		} else {
 			struct tls_decrypt_arg darg;
 
-			err = tls_rx_rec_wait(sk, NULL, true, released);
+			err = tls_rx_rec_wait(sk, NULL, true, released, !!copied);
 			if (err <= 0)
 				goto read_sock_end;
 
-- 
2.54.0