[bug #62318] Tftp module should not expect OACK packet

[Pierre]

URL:
  <https://savannah.gnu.org/bugs/?62318>

                 Summary: Tftp module should not expect OACK packet
                 Project: GNU GRUB
            Submitted by: pierrearm
            Submitted on: Fri 15 Apr 2022 10:15:05 AM UTC
                Category: Network
                Severity: Major
                Priority: 5 - Normal
              Item Group: None
                  Status: None
                 Privacy: Public
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
                 Release: 
                 Release: Git master
         Discussion Lock: Any
         Reproducibility: None
         Planned Release: None

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Details:

Hello,
I think there is a bug in the tftp module. Here is my setup:
Tftp server:
xinetd daemon used
It contains a file named 'file' containing the string 'salut'.

grub.efi built with the following commands (built from latest master:
37ddd9457f13f92fd176bbb2951cfc1fbc93e6f7 kern/efi/init: Log a console error
during a stack check failure):
./bootstrap
./configure --target=aarch64-linux-gnu --with-platform=efi 
--prefix={path_to_grub_dir}out
make
make install
grub-mkstandalone -d out/lib/grub/arm64-efi/ -O arm64-efi -o grub.efi

Commands issued after launching grub (the ip of the tftp server is
192.168.1.11):
insmod tftp
insmod efinet
net_dhcp 
linux (tftp,192.168.1.11)file
> error: time out opening `file'.

I was trying to fetch the file 'file' from the tftp server, which resulted in
an error.
In grub-core/net/tftp.c, the tftp module expects to receive an OACK packet and
retries GRUB_NET_TRIES (=40) times if no OACK packet is received.

However, from RFC 2347 (https://datatracker.ietf.org/doc/html/rfc2347):
"""
When the client appends options to the end of a Read Request packet,
three possible responses may be returned by the server:
OACK  - acknowledge of Read Request and the options;
DATA  - acknowledge of Read Request, but not the options;
ERROR - the request has been denied.
"""
it is possible for the server to answer to a Read Request packet with a DATA
packet. The xinetd daemon answers with a DATA packet (so no OACK). This
results in the grub tftp module repeatedly trying to initiate a transfer and
finally aborting.
When using the tftpd-hpa dameon, an OACK packet is sent, which results in a
working tftp transfer.

I join to wireshark traces. The tftp server is at 192.168.1.11 and the client
at 192.168.1.16:
- one when using the xinetd daemon. You can see that the client sends a Read
Request packet, the server directly starts sending the content of file, but
the client re-sends a Read Request packet (40 times)
- one when using the tftpd-hpa daemon. An OACK packet is sent from the server,
and everything works as expected.

Regards,
Pierre







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File Attachments:


-------------------------------------------------------
Date: Fri 15 Apr 2022 10:15:05 AM UTC  Name: tftp_hpa.pcapng  Size: 3KiB   By:
pierrearm

<http://savannah.gnu.org/bugs/download.php?file_id=53090>
-------------------------------------------------------
Date: Fri 15 Apr 2022 10:15:05 AM UTC  Name: xinetd.pcapng  Size: 68KiB   By:
pierrearm

<http://savannah.gnu.org/bugs/download.php?file_id=53091>

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