On Fri, Aug 27, 2021 at 07:11:18PM -0700, syzbot wrote:

syzbot has bisected this issue to:

commit 54261af473be4c5481f6196064445d2945f2bdab
Author: KP Singh [off-list ref]
Date:   Thu Apr 30 15:52:40 2020 +0000

    security: Fix the default value of fs_context_parse_param hook

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=160c5d75300000
start commit:   77dd11439b86 Merge tag 'drm-fixes-2021-08-27' of git://ano..
git tree:       upstream
final oops:     https://syzkaller.appspot.com/x/report.txt?x=150c5d75300000
console output: https://syzkaller.appspot.com/x/log.txt?x=110c5d75300000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2fd902af77ff1e56
dashboard link: https://syzkaller.appspot.com/bug?extid=d1e3b1d92d25abf97943
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=126d084d300000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16216eb1300000

Reported-by: syzbot+d1e3b1d92d25abf97943@syzkaller.appspotmail.com
Fixes: 54261af473be ("security: Fix the default value of fs_context_parse_param hook")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection

So ok, this seems somewhat clear now. When smack and 
CONFIG_BPF_LSM=y
is selected the bpf LSM will register NOP handlers including

bpf_lsm_fs_context_fs_param()

for the

fs_context_fs_param

LSM hook. The bpf LSM runs last, i.e. after smack according to:

CONFIG_LSM="landlock,lockdown,yama,safesetid,integrity,tomoyo,smack,bpf"

in the appended config. The smack hook runs and sets

param->string = NULL

then the bpf NOP handler runs returning -ENOPARM indicating to the vfs
parameter parser that this is not a security module option so it should
proceed processing the parameter subsequently causing the crash because
param->string is not allowed to be NULL (Which the vfs parameter parser
verifies early in fsconfig().).

If you take the appended syzkaller config and additionally select
kprobes you can observe this by registering bpf kretprobes for:
security_fs_context_parse_param()
smack_fs_context_parse_param()
bpf_lsm_fs_context_parse_param()
in different terminal windows and then running the syzkaller provided
reproducer:

root@f2-vm:~# bpftrace -e 'kretprobe:smack_fs_context_parse_param { printf("returned: %d\n", retval); }'
Attaching 1 probe...
returned: 0

root@f2-vm:~# bpftrace -e 'kretprobe:bpf_lsm_fs_context_parse_param { printf("returned: %d\n", retval); }'
Attaching 1 probe...
returned: -519

root@f2-vm:~# bpftrace -e 'kretprobe:security_fs_context_parse_param { printf("returned: %d\n", retval); }'
Attaching 1 probe...
returned: -519

^^^^^
This will ultimately tell the vfs to move on causing the crash because
param->string is null at that point.

Unless I missed something why that can't happen.

Christian