Parsing of /etc/gshadow can return bad pointers causing segfaults in applications

[Malte Schmidt]

This bug is already solved upstream (https://sourceware.org/bugzilla/show_bug.cgi?id=20338) in 2.32 and has to be backported.

It indirectly causes systemd-sysusers on 20.04/focal to fail (https://github.com/systemd/systemd/issues/6512).

[dm0 (fedora-dm0)]

Specifically structured /etc/gshadow entries can cause fgetgsent() to return invalid pointers that cause applications to segfault on dereference.

One line must fit into the character buffer (1024 bytes, unless a previous line was longer) but have enough group members such that

     line length + alignment + sizeof(char *) * (#adm + 1 + #mem + 1) > 1024.

The parser would return early to avoid overflow, leaving the static result struct pointing to pointers from the previous line which are now invalid, causing segfaults when those pointers are dereferenced.

See the following for a test program and a patch:

https://sourceware.org/ml/libc-alpha/2016-06/msg01015.html

[dm0 (fedora-dm0)]

Created attachment 9705
gshadow: Sync fgetsgent_r.c with grp/fgetgrent_r.c

[dm0 (fedora-dm0)]

Can this be applied to make it into the next release?

[Jason Perrin (jvperrin)]

This is affecting us too (specifically this bug, leading to https://github.com/systemd/systemd/issues/6512 in systemd, which then leads to https://bugs.launchpad.net/ubuntu/+source/tomcat9/+bug/1848614 when installing tomcat9 on Ubuntu bionic). Any updates on this, the patch attached, or anything we can do to help get the patch merged?

Thanks for your work on glibc!

[Florian Weimer (fweimer)]

Patches posted: https://sourceware.org/pipermail/libc-alpha/2020-July/116430.html

[Florian Weimer (fweimer)]

Fixed for glibc 2.32 via:

commit 2add4235ef674988948155f9a8f60a8c7b09bcff
Author: Florian Weimer <email address hidden>
Date: Thu Jul 16 17:31:20 2020 +0200

    gshadow: Implement fgetsgent_r using __nss_fgetent_r (bug 20338)

    Tested-by: Carlos O'Donell <email address hidden>
    Reviewed-by: Carlos O'Donell <email address hidden>

[Florian Weimer (fweimer)]

I'm flagging this as security- because the affected files contain trusted content.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Nicolas KAROLAK (nikaro)]

This bug affect us in Bionic, can the fix be backported for this release?

[Michael Hudson-Doyle (mwhudson)]

Do you have a test case for this? I tried the one from the systemd bug report in a focal container and it seemed to work fine.