Bug 3431

Summary:

scp truncates file when copying file over itself

Product:

Portable OpenSSH

Reporter:

Christof Schmitt <christof.schmitt>

Component:

scp

Assignee:

Assigned to nobody <unassigned-bugs>

Status:

CLOSED FIXED

Severity:

major

CC:

djm, dtucker

Priority:

Version:

9.0p1

Hardware:

All

OS:

Linux

Bug Depends on:

Bug Blocks:

3418

Attachments:

Description	Flags
don't overwrite files	dtucker: ok+

Description Christof Schmitt 2022-05-11 08:19:00 AEST

With the new sftp based scp, using scp to copy a file over
itself, truncates that file:

Steps to Reproduce:

1. echo foo > /tmp/bar
2. scp /tmp/bar $(hostname):/tmp/bar
3. cat /tmp/bar

All data in that file is then lost.

This is a problem for scripts, e.g. cluster automation tools
that rely on the fact that a file /tmp/bar can be copied with
scp to all cluster nodes as /tmp/bar, including the node itself.

Changing this behavior is unexpected, and silently dropping the
data seems to be the worst possibly outcome.

In addition to scripts, admins might also be surprised that
copying files (maybe accidentally) to the same host, leaves those
files corrupted.

Would it be possible to change the behavior in this case?
Either explicitly returning an error, or finding a way
to preserve the data would be a better outcome.

https://lists.mindrot.org/pipermail/openssh-unix-dev/2022-February/040059.html
suggests that possibly the sftp server could use a temp file.

Comment 1 Dmitry Belyavskiy 2022-05-11 17:08:13 AEST

Not sure that assignment to an external contributor is a good idea :)

Comment 2 Damien Miller 2022-05-13 13:10:49 AEST

Created attachment 3594 [details]
don't overwrite files

This fixes it for 'scp foo localhost:foo' and 'scp localhost:foo foo', but not 'scp localhost:foo localhost:foo'

Comment 3 Damien Miller 2022-05-13 17:01:49 AEST

This fix has been committed and will be in OpenSSH 9.1, due in a few months.

commit 56a0697fe079ff3e1ba30a2d5c26b5e45f7b71f8 (HEAD -> master, origin/master, origin/HEAD)
Author: djm@openbsd.org <djm@openbsd.org>
Date:   Fri May 13 06:31:50 2022 +0000

    upstream: arrange for scp, when in sftp mode, to not ftruncate(3) files
    
    early
    
    previous behavious of unconditionally truncating the destination file
    would cause "scp ~/foo localhost:" and "scp localhost:foo ~/" to
    delete all the contents of their destination.
    
    spotted by solene@ sthen@, also bz3431; ok dtucker@
    
    OpenBSD-Commit-ID: ca39fdd39e0ec1466b9666f15cbcfddea6aaa179

Comment 4 Damien Miller 2022-10-04 21:59:23 AEDT

Closing bugs from openssh-9.1 release cycle

Comment 5 Damien Miller 2023-03-17 13:38:32 AEDT

OpenSSH 9.3 has been released. Close resolved bugs