Re: [bug report] [regression?] bpf lsm breaks /proc/*/attr/current with security= on commandline

[Filip Hejsek <filip.hejsek@gmail.com>]

On Wed, 2025-09-24 at 17:24 -0400, Paul Moore wrote:
> On Sat, Sep 13, 2025 at 1:01 PM Filip Hejsek <filip.hejsek@gmail.com> wrote:
> > 
> > Hello,
> > 
> > TLDR: because of bpf lsm, putting security=selinux on commandline
> >       results in /proc/*/attr/current returning errors.
> > 
> > When the legacy security= commandline option is used, the specified lsm
> > is added to the end of the lsm list. For example, security=apparmor
> > results in the following order of security modules:
> > 
> >    capability,landlock,lockdown,yama,bpf,apparmor
> > 
> > In particular, the bpf lsm will be ordered before the chosen major lsm.
> > 
> > This causes reads and writes of /proc/*/attr/current to fail, because
> > the bpf hook overrides the apparmor/selinux hook.
> 
> What kernel are you using?

I'm using Arch Linux kernel, which is very close to mainline. I have
also tested my own build from git sources (I used a stripped down
config which I based on config from Arch). Example in QEMU:

$ qemu-system-x86_64 -nodefaults -accel kvm -cpu host -smp cpus=2 -m 1G -display none -kernel ~/git/linux/arch/x86/boot/bzImage -initrd ./initramfs.img -serial mon:stdio -append 'console=ttyS0 security=selinux'
:: mounting '' on real root
mount: /new_root: no valid filesystem type specified.
ERROR: Failed to mount '' on real root
You are now being dropped into an emergency shell.
sh: can't access tty; job control turned off
[rootfs ~]# uname -a
Linux archlinux 6.17.0-rc7-00020-gcec1e6e5d1ab #3 SMP PREEMPT_DYNAMIC Thu Sep 25 16:28:02 CEST 2025 x86_64 GNU/Linux
[rootfs ~]# mount -t securityfs securityfs /sys/kernel/security
[rootfs ~]# cat /proc/cmdline
console=ttyS0 security=selinux
[rootfs ~]# cat /sys/kernel/security/lsm; echo
capability,landlock,lockdown,yama,bpf,selinux
[rootfs ~]# cat /proc/self/attr/current
cat: read error: Invalid argument

(Note: In this example, uname reports archlinux, but that's only
because I based the config on Arch config, it's not actually an Arch
kernel.)

Maybe the different behavior is caused by a different config? You can
find the Arch config at [1]. Based on Fedora package sources, I think
their config has

   CONFIG_LSM="lockdown,yama,integrity,selinux,bpf,landlock,ipe"

while the Arch config has

   CONFIG_LSM="landlock,lockdown,yama,integrity,bpf"

.

[1]: https://gitlab.archlinux.org/archlinux/packaging/packages/linux/-/raw/main/config?ref_type=heads

>   Things appear to work correctly on my
> kernel that is tracking upstream (Fedora Rawhide + some unrelated
> bits):
> 
> % uname -a
> Linux dev-rawhide-1.lan 6.17.0-0.rc7.250923gd1ab3.57.1.secnext.fc44.x86_64 #1 SM
> P PREEMPT_DYNAMIC Tue Sep 23 10:07:14 EDT 2025 x86_64 GNU/Linux
> % cat /proc/cmdline
> BOOT_IMAGE=(hd0,gpt4)/boot/vmlinuz-6.17.0-0.rc7.250923gd1ab3.57.1.secnext.fc44.x
> 86_64 root=UUID=285029fa-4431-45e9-af1b-298ab0caf16a ro console=ttyS0 mitigation
> s=off security=selinux
> % cat /sys/kernel/security/lsm; echo ""
> lockdown,capability,yama,selinux,bpf,landlock,ipe,ima,evm
> % id -Z
> unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
> % cat /proc/self/attr/current; echo ""
> unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
> 
> I even ran it against the LSM initialization rework that has been
> proposed, but has not yet been accepted/merged, and that worked the
> same as above.
> 
> Is this a distro kernel with a lot of "special" patches which aren't
> present upstream?