[PATCH] chardev/char-socket: Fix TLS io channels sending too much data to the backend

[Thomas Huth <thuth@redhat.com>]

Commit ffda5db65a ("io/channel-tls: fix handling of bigger read buffers")
changed the behavior of the TLS io channels to schedule a second reading
attempt if there is still incoming data pending. This caused a regression
with backends like the sclpconsole that check in their read function that
the sender does not try to write more bytes to it than the device can
currently handle.

The problem can be reproduced like this:

 1) In one terminal, do this:

  mkdir qemu-pki
  cd qemu-pki
  openssl genrsa 2048 > ca-key.pem
  openssl req -new -x509 -nodes -days 365000 -key ca-key.pem -out ca-cert.pem
  # enter some dummy value for the cert
  openssl genrsa 2048 > server-key.pem
  openssl req -new -x509 -nodes -days 365000 -key server-key.pem \
    -out server-cert.pem
  # enter some other dummy values for the cert

  gnutls-serv --echo --x509cafile ca-cert.pem --x509keyfile server-key.pem \
              --x509certfile server-cert.pem -p 8338

 2) In another terminal, do this:

  wget https://download.fedoraproject.org/pub/fedora-secondary/releases/39/Cloud/s390x/images/Fedora-Cloud-Base-39-1.5.s390x.qcow2

  qemu-system-s390x -nographic -nodefaults \
    -hda Fedora-Cloud-Base-39-1.5.s390x.qcow2 \
    -object tls-creds-x509,id=tls0,endpoint=client,verify-peer=false,dir=$PWD/qemu-pki \
    -chardev socket,id=tls_chardev,host=localhost,port=8338,tls-creds=tls0 \
    -device sclpconsole,chardev=tls_chardev,id=tls_serial

QEMU then aborts after a second or two with:

  qemu-system-s390x: ../hw/char/sclpconsole.c:73: chr_read: Assertion
   `size <= SIZE_BUFFER_VT220 - scon->iov_data_len' failed.
 Aborted (core dumped)

It looks like the second read does not trigger the chr_can_read() function
to be called before the second read, which should normally always be done
before sending bytes to a character device to see how much it can handle,
so the s->max_size in tcp_chr_read() still contains the old value from the
previous read. Let's make sure that we use the up-to-date value by calling
tcp_chr_read_poll() again here.

Fixes: ffda5db65a ("io/channel-tls: fix handling of bigger read buffers")
Buglink: https://issues.redhat.com/browse/RHEL-24614
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 Sorry if you've got this mail twice - I forgot to CC: qemu-devel when
 I sent it out the first time ... *facepalm*

 chardev/char-socket.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/chardev/char-socket.c b/chardev/char-socket.c
index 67e3334423..8a0406cc1e 100644
--- a/chardev/char-socket.c
+++ b/chardev/char-socket.c
@@ -496,9 +496,9 @@ static gboolean tcp_chr_read(QIOChannel *chan, GIOCondition cond, void *opaque)
         s->max_size <= 0) {
         return TRUE;
     }
-    len = sizeof(buf);
-    if (len > s->max_size) {
-        len = s->max_size;
+    len = tcp_chr_read_poll(opaque);
+    if (len > sizeof(buf)) {
+        len = sizeof(buf);
     }
     size = tcp_chr_recv(chr, (void *)buf, len);
     if (size == 0 || (size == -1 && errno != EAGAIN)) {
-- 
2.44.0