> Looks like both of us sponsored it for the queue. I'd appreciate rejecting either one of them.
I pushed to the VCS[0] at the same time as sponsoring, so I'll reject the other one which doesn't quite match up with it. [0] https://salsa.debian.org/gnome- team/glib/-/commits/ubuntu/2.64.6-1_ubuntu20.04.4/ -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to glib2.0 in Ubuntu. 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: In Progress 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. To manage notifications about this bug go to: https://bugs.launchpad.net/glib/+bug/1930359/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp
