Re: [syzbot] general protection fault in legacy_parse_param
From: Christian Brauner <hidden>
Date: 2021-08-30 16:57:42
Also in:
bpf, linux-fsdevel, lkml, netdev, selinux
Possibly related (same subject, not in this thread)
- 2021-08-27 · Re: [syzbot] general protection fault in legacy_parse_param · Christian Brauner <hidden>
- 2021-08-27 · Re: [syzbot] general protection fault in legacy_parse_param · Casey Schaufler <casey@schaufler-ca.com>
- 2021-08-27 · Re: [syzbot] general protection fault in legacy_parse_param · Christian Brauner <hidden>
- 2021-08-27 · Re: [syzbot] general protection fault in legacy_parse_param · syzbot <hidden>
- 2021-07-06 · Re: [syzbot] general protection fault in legacy_parse_param · Paul Moore <paul@paul-moore.com>
On Mon, Aug 30, 2021 at 09:40:57AM -0700, Casey Schaufler wrote:
On 8/30/2021 7:25 AM, Casey Schaufler wrote:quoted
On 8/30/2021 5:23 AM, Christian Brauner wrote:quoted
On Fri, Aug 27, 2021 at 07:11:18PM -0700, syzbot wrote:quoted
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#bisectionSo 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().).The security_fs_context_parse_param() function is incorrectly implemented using the call_int_hook() macro. It should return zero if any of the modules return zero. It does not follow the usual failure model of LSM hooks. It could be argued that the code was fine before the addition of the BPF hook, but it was going to fail as soon as any two security modules provided mount options. Regardless, I will have a patch later today. Thank you for tracking this down.Here's my proposed patch. I'll tidy it up with a proper commit message if it looks alright to y'all. I've tested with Smack and with and without BPF.
Looks good to me. On question, in contrast to smack, selinux returns 1 instead of 0 on success. So selinux would cause an early return preventing other hooks from running. Just making sure that this is intentional. Iirc, this would mean that selinux causes fsconfig() to return a positive value to userspace which I think is a bug; likely in selinux. So I think selinux should either return 0 or the security hook itself needs to overwrite a positive value with a sensible errno that can be seen by userspace.
quoted hunk ↗ jump to hunk
security/security.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-)diff --git a/security/security.c b/security/security.c index 09533cbb7221..3cf0faaf1c5b 100644 --- a/security/security.c +++ b/security/security.c@@ -885,7 +885,19 @@ int security_fs_context_dup(struct fs_context *fc, struct fs_context *src_fc) int security_fs_context_parse_param(struct fs_context *fc, struct fs_parameter *param) { - return call_int_hook(fs_context_parse_param, -ENOPARAM, fc, param); + struct security_hook_list *hp; + int trc; + int rc = -ENOPARAM; + + hlist_for_each_entry(hp, &security_hook_heads.fs_context_parse_param, + list) { + trc = hp->hook.fs_context_parse_param(fc, param); + if (trc == 0) + rc = 0; + else if (trc != -ENOPARAM) + return trc; + } + return rc; } int security_sb_alloc(struct super_block *sb)
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