This bug was fixed in the package glib2.0 - 2.64.6-1~ubuntu20.04.4
---------------
glib2.0 (2.64.6-1~ubuntu20.04.4) focal; urgency=medium
* Initialise memory used for file builder buffers to zero, since memory
artifacts found themseleves into gschema.compiled files, leading to glib
being unable to parse the gschema.compiled files, causing gdm, gnome-shell
and various gnome applications to fail to
start. (LP: #1930359)
- d/p/gvdb-builder-Initialise-some-memory-to-zero-in-the-bloom-.patch
-- Matthew Ruffell <matthew.ruff...@canonical.com> Mon, 12 Jul 2021
15:56:16 +0100
** Changed in: glib2.0 (Ubuntu Focal)
Status: Fix Committed => Fix Released
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https://bugs.launchpad.net/bugs/1930359
Title:
glib2.0: Uninitialised memory is written to gschema.compiled, failure
to parse this file leads to gdm, gnome-shell failing to start
Status in GLib:
Unknown
Status in glib2.0 package in Ubuntu:
Fix Released
Status in glib2.0 source package in Focal:
Fix Released
Bug description:
[Impact]
A recent SRU of mutter 3.36.9-0ubuntu0.20.04.1 caused an outage for a
user with 300 VDIs running Focal, where GNOME applications would fail
to start, and if you reboot, gdm and gnome-shell both fail to start,
and you are left with a black screen and a blinking cursor.
After much investigation, mutter was not at fault. Instead, mutter-
common calls the libglib2.0-0 hook on upgrade:
Processing triggers for libglib2.0-0:amd64 (2.64.6-1~ubuntu20.04.3)
...
This in turn calls glib-compile-schemas to recompile the gsettings
gschema cache, from the files in /usr/share/glib-2.0/schemas/. The
result is a binary gschemas.compiled file, which is loaded by
libglib2.0 on every invocation of a GNOME application, or gdm or
gnome-shell to fetch application default settings.
Now, glib2.0 2.64.6-1~ubuntu20.04.3 in Focal has some non-
deterministic behaviour when calling glib-compile-schemas, causing
generated gschemas.compiled files to have differing contents on each
run:
# glib-compile-schemas /usr/share/glib-2.0/schemas
# cmp -l /home/ubuntu/schemas/gschemas.compiled
/usr/share/glib-2.0/schemas/gschemas.compiled | gawk '{printf "%08X %02X
%02X\n", $1, strtonum(0$2), strtonum(0$3)}'
0000376F E3 D0
00003771 A4 DB
# glib-compile-schemas /usr/share/glib-2.0/schemas
# cmp -l /home/ubuntu/schemas/gschemas.compiled
/usr/share/glib-2.0/schemas/gschemas.compiled | gawk '{printf "%08X %02X
%02X\n", $1, strtonum(0$2), strtonum(0$3)}'
0000376F E3 C3
00003771 A4 98
# glib-compile-schemas /usr/share/glib-2.0/schemas
# cmp -l /home/ubuntu/schemas/gschemas.compiled
/usr/share/glib-2.0/schemas/gschemas.compiled | gawk '{printf "%08X %02X
%02X\n", $1, strtonum(0$2), strtonum(0$3)}'
0000376F E3 68
00003771 A4 30
00003772 55 56
The bytes on the left are from a corrupted gschemas.compiled provided
by an affected user. The changing bytes on the right are non-
deterministic.
I ran valgrind over glib-compile-schemas, and found that we are
writing to uninitialised memory.
https://paste.ubuntu.com/p/hvZccwdzxz/
What is happening is that a submodule of glib, gvdb, contains the
logic for serialising the gschema data structures, and when it
allocates a buffer to store the eventual gschemas.compiled file, it
does not initialise it.
When we populate the fields in the buffer, some bytes are never
overwritten, and these junk bytes find themselves written to
gschemas.compiled.
On boot, when gdm and gnome-shell attempt to parse and load this
corrupted gschemas.compiled file, it can't parse the junk bytes, and
raises and error, which propagates up to a breakpoint in glib logging,
but no debugger is present, so the kernel traps the breakpoint, and
terminates the library, and the calling application, e.g. gdm.
The result is that the user is left starting at a black screen with a
blinking pointer.
[Testcase]
On a Focal system, simply run valgrind over glib-compile-schemas:
# valgrind glib-compile-schemas /usr/share/glib-2.0/schemas
You will get output like this, with the warning "Syscall param
write(buf) points to uninitialised byte(s)":
https://paste.ubuntu.com/p/hvZccwdzxz/
If you happen to have a large amount of gschema overrides present on
your system, like my affected user does, you can save a copy of a
generated gschema.compiled to your home directory and bindiff it
against recompiles:
# glib-compile-schemas /usr/share/glib-2.0/schemas
# cp /usr/share/glib-2.0/schemas/gschema.compiled
/home/ubuntu/schemas/gschemas.compiled
# glib-compile-schemas /usr/share/glib-2.0/schemas
# cmp -l /home/ubuntu/schemas/gschemas.compiled
/usr/share/glib-2.0/schemas/gschemas.compiled | gawk '{printf "%08X %02X
%02X\n", $1, strtonum(0$2), strtonum(0$3)}'
0000376F E3 C3
00003771 A4 98
If you install the test package from the following ppa:
https://launchpad.net/~mruffell/+archive/ubuntu/sf311791-test
When you run valgrind, it will report a clean run with no writing to
uninitialised buffers, and all invocations of glib-compile-schemas
will be deterministic, and generate the file same with the same sha256
hash every time. The unwritten bytes if you do a bindiff from before
and after will be all set to zero.
[Where problems can occur]
I am doubtful that any programs are relying on buggy non-deterministic
behaviour from random bytes found in uninitialised memory, so this
should be a relatively safe change.
Since we are updating glib, which all GNOME applications, gdm and
gnome-shell link to, if we introduce an error, it could cause these
applications to stop working, and at a worse case, see the symptoms
this bug is trying to fix, which is a blinking cursor on a blank
screen.
Installing any updates to glib also causes the gsettings gschema cache
to be re-generated, and from this bug, we know that libglib seems to
trust the gschema.compiled file and doesn't perform much validation,
if the user has bad data in their gschema files, it could lead to
their systems having issues on next boot.
If a regression occurs, users should first attempt to re-generate
their schemas like so:
glib-compile-schemas /usr/share/glib-2.0/schemas
and if that fails, then they should downgrade their libglib2.0-0
libglib2.0-bin libglib2.0-data packages.
[Other info]
This was fixed by the commit:
commit ea64c739239faea463f3cb9154a12cc4532ba525
Author: Philip Withnall <withn...@endlessm.com>
Date: Wed Mar 18 09:15:59 2020 +0000
Subject: gvdb-builder: Initialise some memory to zero in the bloom filter
Link:
https://github.com/GNOME/glib/commit/ea64c739239faea463f3cb9154a12cc4532ba525
Only Focal needs this patch, Groovy and up are unaffected.
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