[LTS 8.8 RT] CVE-2023-4623, VULN-6710

[pvts-mat]

[LTS 8.8 RT]
CVE-2023-4623
VULN-6710

Problem

https://www.cve.org/CVERecord?id=CVE-2023-4623

A use-after-free vulnerability in the Linux kernel's net/sched: sch_hfsc (HFSC qdisc traffic control) component can be exploited to achieve local privilege escalation. If a class with a link-sharing curve (i.e. with the HFSC_FSC flag set) has a parent without a link-sharing curve, then init_vf() will call vttree_insert() on the parent, but vttree_remove() will be skipped in update_vf(). This leaves a dangling pointer that can cause a use-after-free.

Analysis and solution

A single commit was identified as a fix for this issue: b3d26c5702c7d6c45456326e56d2ccf3f103e60fnet/sched: sch_hfsc: Ensure inner classes have fsc curve.

The solution consisted of rejecting the addition of a class with a link-sharing curve to the class without it (see Specific tests for details):

[root@ciqlts8_8-rt pvts]# (
tc qdisc add dev lo root handle 1: hfsc default 10
tc class add dev lo parent 1: classid 1:1 hfsc rt m2 100kbps
tc class add dev lo parent 1:1 classid 1:10 hfsc ls m2 50kbps
echo $?
)
Error: Invalid parent - parent class must have FSC.
2

The fix introduced a problem with existing network setup scripts for some users https://lore.kernel.org/all/297D84E3-736E-4AB4-B825-264279E2043C@flyingcircus.io/:

I upgraded from 6.1.38 to 6.1.55 this morning and it broke my traffic shaping script, leaving me with a non-functional uplink on a remote router.

It was decided to fix the problem without breaking backwards compatibility https://lore.kernel.org/all/20231013151057.2611860-1-pctammela@mojatatu.com/:

Setting 'rt' as a parent is incorrect and the man page is explicit about
it as it doesn't make sense 'qdisc wise'. Being able to set it has
always been wrong unfortunately…

Sure but unfortunately "we don't break backward compat" means
we can't really argue. It will take us more time to debate this
than to fix it (assuming we understand the initial problem).

Frankly one can even argue whether "exploitable by root / userns"
is more important than single user's init scripts breaking.
The "security" issues for root are dime a dozen.

The solution was to change the erroneous qdisc hierarchy to a correct one when the possible UAF condition was detected https://lore.kernel.org/all/20231013151057.2611860-1-pctammela@mojatatu.com/:

As reported [1] disallowing 'rt' inner curves breaks already existing
scripts. Even though it doesn't make sense 'qdisc wise' users have been
relying on this behaviour since the qdisc inception.

We need users to update the scripts/applications to use 'sc' or 'ls'
as a inner curve instead of 'rt', but also avoid the UAF found by
Budimir, which was present since the qdisc inception.

The fix of the fix is given in the commit a13b67c9a015c4e21601ef9aa4ec9c5d972df1b4net/sched: sch_hfsc: upgrade 'rt' to 'sc' when it becomes a inner curve

While the changes could be squashed into a single commit it was decided to retain the sequence of two commits for more straightforward LTS 8.8 RT - mainline patches correspondence.

The same solution was used already in other "version 8" branches centos8, fips-8-complaint/4.18.0-553.16.1, fips-8/4.18.0-553.16.1, rocky8_10, sig-cloud-8/4.18.0-553.22.1.el8_10, sig-cloud-8/4.18.0-553.33.1.el8_10, sig-cloud-8/4.18.0-553.36.1.el8_10:

git --no-pager log --decorate --format="%h %cd %d %s" --date=short -n 2 ae71642ce
ae71642ce 2024-09-11 net/sched: sch_hfsc: upgrade 'rt' to 'sc' when it becomes a inner curve
1a61f0a5d 2024-09-11 net/sched: sch_hfsc: Ensure inner classes have fsc curve

kABI check: omitted (unstable ABI of RT kernels)

Boot test: passed

Refer to Specific tests for implicit boot test passing.

Kselftests: passed relative

Methodology

A mix of kernel-selftests-internal and source-compiled tests were used:

• kernel-selftests-internal:bpf tests, except:
  • bpf:test_kmod.sh: takes very long time to finish and always fails anyway,
  • bpf:test_progs: unstable, can crash the machine,
  • bpf:test_progs-no_alu32: unstable, can crash the machine.
• source-compiled: all the rest.

Coverage (including tests skipped during execution)

android, bpf, breakpoints, capabilities, cgroup, core, cpu-hotplug, cpufreq, drivers/net/bonding, drivers/net/team, efivarfs, exec, filesystems, firmware, fpu, ftrace, futex, gpio, intel_pstate, ipc, kcmp, kvm, lib, livepatch, membarrier, memfd, memory-hotplug, mount, mqueue, net, net/forwarding, net/mptcp, netfilter, nsfs, proc, pstore, ptrace, rseq, rtc, sgx, sigaltstack, size, splice, static_keys, sync, sysctl, tc-testing, tdx, timens, timers, tpm2, user, vm, x86, zram

Reference ciqlts8_8-rt (65d2b53323ce7b26a298afe3f8f7222f33d69f03)

Two test runs were conducted on the reference kernel.
kselftests–mixed–ciqlts8_8-rt–run1.log
kselftests–mixed–ciqlts8_8-rt–run2.log

Patch ciqlts8_8-rt-CVE-2023-4623 (6e54bd4657815ea115cd4e58ab1afcb8648f9277)

Two test runs were conducted on the patched kernel.
kselftests–mixed–ciqlts8_8-rt-CVE-2023-4623–run1.log
kselftests–mixed–ciqlts8_8-rt-CVE-2023-4623–run2.log

Comparison

ktests.xsh table --where "Summary = 'diff'" kselftests*.log
Column File
-------- -------------------------------------------------------
Status0 kselftests--mixed--ciqlts8_8-rt--run1.log
Status1 kselftests--mixed--ciqlts8_8-rt--run2.log
Status2 kselftests--mixed--ciqlts8_8-rt-CVE-2023-4623--run1.log
Status3 kselftests--mixed--ciqlts8_8-rt-CVE-2023-4623--run2.log
TestCase Status0 Status1 Status2 Status3 Summary
bpf:test_tcpnotify_user fail fail pass fail diff
kvm:hardware_disable_test pass fail pass pass diff
net/mptcp:simult_flows.sh fail fail fail pass diff
net:gro.sh pass pass fail fail diff
net:reuseport_addr_any.sh pass fail fail fail diff
net:tls pass pass pass fail diff

Tests bpf:test_tcpnotify_user, kvm:hardware_disable_test, net:gro.sh have already shown inconsistent behavior before https://docs.google.com/spreadsheets/d/1tUwJ2rV57cYZXh7momPtraSjZcHDjMYHLeHA3DYWrUU/edit?pli=1&gid=0#gid=0&range=H:I

The net:reuseport_addr_any.sh test showed inconsistent behavior in the reference tests set itself. Added to the list of "flappy" tests for the ciqlts8_8-rt platform.

Specific tests: passed

The potential UAF condition was found to be reproducible with the following tc commands sequence:

tc qdisc add dev lo root handle 1: hfsc default 10
# Inner hfsc class constructed with 'rt' - realtime service
tc class add dev lo parent 1: classid 1:1 hfsc rt m2 100kbps
# Leaf hfsc class constructed with 'ls' - linksharing service
tc class add dev lo parent 1:1 classid 1:10 hfsc ls m2 50kbps

The "100kbps", "50kbps" parts are arbitrary. What's important is the use of rt for the inner class and ls for the leaf class. While the exact UAF was not obtained the commands helped confirm the efficacy of the patch.

Reference

The incorrect qdisc hierarchy can be created without any guardrails.

[root@ciqlts8_8-rt pvts]# (
tc qdisc add dev lo root handle 1: hfsc default 10
tc class add dev lo parent 1: classid 1:1 hfsc rt m2 100kbps
tc class add dev lo parent 1:1 classid 1:10 hfsc ls m2 50kbps
echo $?
)
0
[root@ciqlts8_8-rt pvts]# tc -g class show dev lo
tc -g class show dev lo
+---(1:) hfsc +---(1:1) hfsc rt m1 0bit d 0us m2 800Kbit +---(1:10) hfsc ls m1 0bit d 0us m2 400Kbit 

Full logs:
fix-replicate–ciqlts8_8-rt.log

Patch

Creating the incorrect qdisc hierarchy raises a warning, but succeeds. Notice the type of inner class being sc instead of rt as shown by tc -g class show dev lo command.

[root@ciqlts8_8-rt pvts]# (
tc qdisc add dev lo root handle 1: hfsc default 10
tc class add dev lo parent 1: classid 1:1 hfsc rt m2 100kbps
tc class add dev lo parent 1:1 classid 1:10 hfsc ls m2 50kbps
echo $?
)
Warning: Forced curve change on parent 'rt' to 'sc'.
0
[root@ciqlts8_8-rt pvts]# tc -g class show dev lo
tc -g class show dev lo
+---(1:) hfsc +---(1:1) hfsc sc m1 0bit d 0us m2 800Kbit +---(1:10) hfsc ls m1 0bit d 0us m2 400Kbit 

Full logs:
fix-replicate–ciqlts8_8-rt-CVE-2023-4623.log

[gvrose8192]

LGTM - Putting into review so that it can be merged later after reviews. Checks have passed.

[bmastbergen]

🥌

[gvrose8192]

Checks passed and approved - Rebasing and merging.