** Changed in: linux (Ubuntu Kinetic)
Status: In Progress => Fix Committed
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https://bugs.launchpad.net/bugs/1987998
Title:
LSM: Configuring Too Many LSMs Causes Kernel Panic on Boot
Status in linux package in Ubuntu:
Fix Committed
Status in linux source package in Jammy:
In Progress
Status in linux source package in Kinetic:
Fix Committed
Bug description:
BugLink: https://bugs.launchpad.net/bugs/1987998
[Impact]
The Ubuntu kernel carries an out of tree patchet, known as "LSM:
Module stacking for AppArmor" upstream, to enable stackable LSMs for
containers. The revision the Ubuntu kernel carries is an older one,
from 2020, and has some slight divergences from the latest revision in
development.
One such divergence, is support for Landlock as a stackable LSM. When
the stackable LSM patchset was applied, Landlock was still in
development and not mainlined yet, and wasn't present in the earlier
revision of the "LSM: Module stacking for AppArmor" patchset. Support
for this was added by us.
There was a minor omission made during enabling support for Landlock.
The LSM slot type was marked as LSMBLOB_NEEDED, when it should have
been LSMBLOB_NOT_NEEDED.
Landlock itself does not provide any of the hooks that use a struct
lsmblob, such as secid_to_secctx, secctx_to_secid, inode_getsecid,
cred_getsecid, kernel_act_as task_getsecid_subj task_getsecid_obj and
ipc_getsecid.
When we set .slot = LSMBLOB_NEEDED, this indicates that we need an
entry in struct lsmblob, and we need to increment LSMBLOB_ENTRIES by
one to fit the entry into the secid array:
#define LSMBLOB_ENTRIES ( \
(IS_ENABLED(CONFIG_SECURITY_SELINUX) ? 1 : 0) + \
(IS_ENABLED(CONFIG_SECURITY_SMACK) ? 1 : 0) + \
(IS_ENABLED(CONFIG_SECURITY_APPARMOR) ? 1 : 0) + \
(IS_ENABLED(CONFIG_BPF_LSM) ? 1 : 0))
struct lsmblob {
u32 secid[LSMBLOB_ENTRIES];
};
Currently, we don't increment LSMBLOB_ENTRIES by one to make an entry
for Landlock, so for the Ubuntu kernel, we can fit a maximum of two
entries, one for Apparmor and one for bpf.
If you try and configure three LSMs like so and reboot:
GRUB_CMDLINE_LINUX_DEFAULT="lsm=landlock,bpf,apparmor"
You will receive the following panic:
LSM: Security Framework initializing
landlock: Up and running.
LSM support for eBPF active
Kernel panic - not syncing: security_add_hooks Too many LSMs registered.
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.15.0-46-generic #49-Ubuntu
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1 04/01/2014
Call Trace:
<TASK>
show_stack+0x52/0x5c
dump_stack_lvl+0x4a/0x63
dump_stack+0x10/0x16
panic+0x149/0x321
security_add_hooks+0x45/0x13a
apparmor_init+0x189/0x1ef
initialize_lsm+0x54/0x74
ordered_lsm_init+0x379/0x392
security_init+0x40/0x49
start_kernel+0x466/0x4dc
x86_64_start_reservations+0x24/0x2a
x86_64_start_kernel+0xe4/0xef
secondary_startup_64_no_verify+0xc2/0xcb
</TASK>
---[ end Kernel panic - not syncing: security_add_hooks Too many LSMs
registered. ]---
There is a check added in security_add_hooks() that makes sure that
you cannot configure too many LSMs:
if (lsmid->slot == LSMBLOB_NEEDED) {
if (lsm_slot >= LSMBLOB_ENTRIES)
panic("%s Too many LSMs registered.\n", __func__);
lsmid->slot = lsm_slot++;
init_debug("%s assigned lsmblob slot %d\n", lsmid->lsm,
lsmid->slot);
}
A workaround is to enable no more than 2 LSMs until this is fixed.
[Fix]
If you read the following mailing list thread on linux-security-
modules from May 2021:
https://lore.kernel.org/selinux/202105141224.942DE93@keescook/T/
It is explained that Landlock does not provide any of the hooks that
use a struct lsmblob, such as secid_to_secctx, secctx_to_secid,
inode_getsecid, cred_getsecid, kernel_act_as task_getsecid_subj
task_getsecid_obj and ipc_getsecid.
I verified this with:
ubuntu-jammy$ grep -Rin "secid_to_secctx" security/landlock/
ubuntu-jammy$ grep -Rin "secctx_to_secid" security/landlock/
ubuntu-jammy$ grep -Rin "inode_getsecid" security/landlock/
ubuntu-jammy$ grep -Rin "cred_getsecid" security/landlock/
ubuntu-jammy$ grep -Rin "kernel_act_as" security/landlock/
ubuntu-jammy$ grep -Rin "task_getsecid_subj" security/landlock/
ubuntu-jammy$ grep -Rin "task_getsecid_obj" security/landlock/
ubuntu-jammy$ grep -Rin "ipc_getsecid" security/landlock/
The fix is to change Landlock from LSMBLOB_NEEDED to
LSMBLOB_NOT_NEEDED.
Due to the "LSM: Module stacking for AppArmor" patchset being 25
patches long, it was impractical to revert just the below patch and
reapply with the fix, due to a large amount of conflicts:
commit f17b27a2790e72198d2aaf45242453e5a9043049
Author: Casey Schaufler <ca...@schaufler-ca.com>
Date: Mon Aug 17 16:02:56 2020 -0700
Subject: UBUNTU: SAUCE: LSM: Create and manage the lsmblob data structure.
Link:
https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/jammy/commit/?id=f17b27a2790e72198d2aaf45242453e5a9043049
So instead, I wrote up a fix that just changes the Landlock LSM slots
to follow the latest upstream development, from V37 of the patchset:
https://lore.kernel.org/selinux/20220628005611.13106-4-casey@schaufler-
ca.com/
I refactored the landlock_lsmid struct to only be in one place, and to
be marked as extern from security/landlock/setup.h.
[Testcase]
Launch a Jammy or Kinetic VM.
1. Edit /etc/default/grub and append the following to
GRUB_CMDLINE_LINUX_DEFAULT:
"lsm=landlock,bpf,apparmor"
2. sudo update-grub
3. reboot
The system will panic on boot.
If you install the test kernel from the following ppa:
https://launchpad.net/~mruffell/+archive/ubuntu/sf343286-test
Instead of a panic occurring, the kernel should display all LSMs
initialising, and continue to boot.
[ 0.288224] LSM: Security Framework initializing
[ 0.289457] landlock: Up and running.
[ 0.290290] LSM support for eBPF active
[ 0.291189] AppArmor: AppArmor initialized
[Where problems could occur]
The risk of regression in changing Landlock from LSMBLOB_NEEDED to
LSMBLOB_NOT_NEEDED is low, due to Landlock not needing a slot in the
secid array in struct lsmblob in the first place.
The refactor is minor and unlikely to introduce any issues with
Landlock or its security promises.
I feel that simply fixing this small bug is less regression risk than
reverting the entire 25 patch patchset and applying the latest V37
upstream patchset, which has undergone significant changes from mid
2020. I think its best we consume the newer patchset once it makes its
way into mainline in a future kernel instead.
If a regression were to occur, users could configure 2 LSMs instead of
all 3, or not enable Landlock.
[Other Info]
This patchset was originally NACKed by Stefan Bader on request of John
Johansen due to a minor misunderstanding on how LSMBLOB_NEEDED worked
with blob slot registration.
ACK:
https://lists.ubuntu.com/archives/kernel-team/2022-August/132898.html
Applied:
https://lists.ubuntu.com/archives/kernel-team/2022-August/132887.html
NACK:
https://lists.ubuntu.com/archives/kernel-team/2022-September/132965.html
Revert:
https://lists.ubuntu.com/archives/kernel-team/2022-September/132966.html
This led to a off list discussion between myself, Stefan bader, Andrea
Righi, John Johansen, Jay Vosburgh and Tim Gardner:
Subject:
Please Re-review LSM Stacking Patchset fix - Change Landlock from
LSMBLOB_NEEDED to LSMBLOB_NOT_NEEDED
The discussion uses largely information found in this SRU template,
plus some additional information from mailing list threads
highlighting why it is safe to move to LSMBLOB_NOT_NEEDED.
From Matthew Ruffell
https://pastebin.canonical.com/p/hxMgk2tDG9/
From John Johansen
https://pastebin.canonical.com/p/ZM4qCMrx2M/
https://pastebin.canonical.com/p/q6485wDwdt/
From Matthew Ruffell
https://pastebin.canonical.com/p/y2HxnDMS6T/
From Stefan Bader
https://pastebin.canonical.com/p/CCvVh8SvhB/
Now that the misunderstanding has been cleared up, and the patchset
has been double checked and approved by the Security Team,
resubmitting the initial patch as V2.
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