Re: [PATCH v3 1/3] ptrace: Provide ___ptrace_may_access() that can be applied on arbitrary tasks

Peter Zijlstra
2018-09-05T19:03:18Z

On Wed, Sep 05, 2018 at 02:40:18PM -0400, Andrea Arcangeli wrote:
> [ 1838.769917] <<EOE>> [<ffffffff816391e5>] avc_compute_av+0x126/0x1b5

That does read_lock(), which is not allowed from scheduler context.

> [ 1838.777125] [<ffffffff810b842e>] ? walk_tg_tree_from+0xbe/0x110
> [ 1838.783828] [<ffffffff8128b9c4>] avc_has_perm_noaudit+0xc4/0x110

In current code this can end up in avc_update_node() which uses
spin_lock(), which is a bug from scheduler context.o

> [ 1838.790628] [<ffffffff8128f1fb>] cred_has_capability+0x6b/0x120
> [ 1838.797331] [<ffffffff810db71c>] ? ktime_get+0x4c/0xd0
> [ 1838.803160] [<ffffffff810e167b>] ? clockevents_program_event+0x6b/0xf0
> [ 1838.810532] [<ffffffff8128f2de>] selinux_capable+0x2e/0x40
> [ 1838.816748] [<ffffffff81288f65>] security_capable_noaudit+0x15/0x20
> [ 1838.823829] [<ffffffff8108b975>] has_ns_capability_noaudit+0x15/0x20
> [ 1838.831014] [<ffffffff8108bc55>] ptrace_has_cap+0x35/0x40
> [ 1838.837126] [<ffffffff8108c717>] ___ptrace_may_access+0xa7/0x1e0
> [ 1838.843925] [<ffffffff8163f0ae>] __schedule+0x26e/0xa00
> [ 1838.849855] [<ffffffff81640949>] schedule_preempt_disabled+0x29/0x70
> [ 1838.857041] [<ffffffff810d9324>] cpu_startup_entry+0x184/0x290
> [ 1838.863637] [<ffffffff8104891a>] start_secondary+0x1da/0x250

So yes, looks like all that security LSM nonsense isn't going to work
here.