From: "Ludovic Courtès" <ludo@gnu.org>
To: ulfvonbelow <striness@tilde.club>
Cc: 65221@debbugs.gnu.org
Subject: [bug#65221] [PATCH 0/2] Fix EXTRA-PORTS edge cases
Date: Tue, 15 Aug 2023 12:55:03 +0200 [thread overview]
Message-ID: <87y1ic8tiw.fsf_-_@gnu.org> (raw)
In-Reply-To: <20230811090615.3707-1-striness@tilde.club> (ulfvonbelow's message of "Fri, 11 Aug 2023 04:06:14 -0500")
Hi,
ulfvonbelow <striness@tilde.club> skribis:
> EXEC-COMMAND (and, by extension, FORK+EXEC-COMMAND) has several issues:
> 1. Despite it being documented that "all other file descriptors are closed
> prior to yielding control to COMMAND", this is not currently the case -
> only other file descriptors that are already marked as FD_CLOEXEC are
> closed. For example, if user code happens to have a file descriptor open,
> for example with call-with-input-file, while EXEC-COMMAND is run, the new
> process image will inherit that file descriptor. This may cause some
> resource waste, but more importantly may cause security issues in certain
> situations.
Yes. This has been the case since 0.9.2, as noted in ‘NEWS’:
Previously, services started indirectly with ‘exec-command’ (which is usually
the case) would not inherit any file descriptor from shepherd because
‘exec-command’ would explicitly close all of them. However, services started
with ‘make-system-constructor’ and processes created by some other means, such
as calling ‘system*’, would inherit some of those descriptors, giving them
more authority than intended.
The change here consists in marking all internally-used file descriptors as
“close-on-exec” (O_CLOEXEC), a feature that’s been available on GNU/Linux and
GNU/Hurd for years but that so far wasn’t used consistently in shepherd. This
is now fixed. As a side-effect, the file-descriptor-closing loop in
‘exec-command’ is now gone.
The FD-closing loop was removed on purpose, in
2c0354258047133db8b885bcc11afdf0def5d885.
Now, as you write, it means that service writers must be careful now and
not leave any non-CLOEXEC file descriptor behind them.
At the time I audited Guix System to check that this was a reasonable
thing to expect and that we could indeed ensure no file descriptors were
leaked. There’s also ‘tests/close-on-exec.sh’.
If you found cases where it would be necessary, what we could do is have
‘shepherd’ replace ‘call-with-input-file’ & co. with a variant that
opens files as O_CLOEXEC by default. WDYT?
> 2. EXTRA-PORTS is only honored when either LOG-PORT or LOG-FILE is passed. I
> have no idea why this is the case, it isn't documented anywhere, and it
> isn't intuitive.
#:extra-ports wasn’t really made to be exposed I guess; it was added for
use by systemd-style services in 965f6b61a473ee57a1fc6ec3ea1ad6e35d596031.
> 3. Even when LOG-PORT or LOG-FILE is passed, EXTRA-PORTS may not work as
> described, because it copies file descriptor contents in an arbitrary
> order. For example, suppose that (map fileno EXTRA-PORTS) is (7 6 5 4 3).
> If the underlying file originally stored in fd N is represented by F(N), it
> will assign
> 3 <-- F(7)
> 4 <-- F(6)
> 5 <-- F(5)
> 6 <-- F(6)
> 7 <-- F(7)
>
> In other words, the copying of earlier FDs in EXTRA-PORTS may overwrite
> later FDs in EXTRA-PORTS.
Good catch!
Could you make a more minimal patch fixing this specific issue, also
adding a test reproducing the problem being fixed?
Thanks for your work!
Ludo’.
next prev parent reply other threads:[~2023-08-15 10:56 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-11 9:03 [bug#65221] [PATCH 0/2] Fix EXTRA-PORTS edge cases ulfvonbelow
2023-08-11 9:06 ` [bug#65221] [PATCH 1/2] service: make EXTRA-PORTS work as advertised ulfvonbelow
2023-08-11 9:06 ` [bug#65221] [PATCH 2/2] service: use PRESERVE-PORTS for redirecting FDs 0-2 ulfvonbelow
2023-08-15 10:55 ` Ludovic Courtès [this message]
2023-08-18 20:21 ` [bug#65221] [PATCH 0/2] Fix EXTRA-PORTS edge cases Ulf Herrman
2023-08-18 20:22 ` [bug#65221] [PATCH 1/6] tests: add extra-ports.sh test ulfvonbelow
2023-08-18 20:22 ` [bug#65221] [PATCH 2/6] service: don't let earlier ports clobber later ones in EXTRA-PORTS ulfvonbelow
2023-08-18 20:22 ` [bug#65221] [PATCH 3/6] Makefile.am: enable extra-ports.sh test ulfvonbelow
2023-08-18 20:22 ` [bug#65221] [PATCH 4/6] service: honor EXTRA-PORTS regardless of log-port and log-file ulfvonbelow
2023-08-18 20:22 ` [bug#65221] [PATCH 5/6] service: use RECONFIGURE-FDS for redirecting FDs 0-2 ulfvonbelow
2023-08-18 20:22 ` [bug#65221] [PATCH 6/6] service: exec-command: close other file descriptors by default ulfvonbelow
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