On Fri, Aug 27, 2021 at 07:11:18PM -0700, syzbot wrote: > syzbot has bisected this issue to: > > commit 54261af473be4c5481f6196064445d2945f2bdab > Author: KP Singh <kpsingh@xxxxxxxxxx> > 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@xxxxxxxxxxxxxxxxxxxxxxxxx > 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
