Re: [PATCH] ptrace: reintroduce usage of subjective credentials in ptrace_has_cap()

[Andrei Vagin]

On Wed, Jan 15, 2020 at 9:18 AM Christian Brauner
<christian.brauner@xxxxxxxxxx> wrote:
>
> Commit 69f594a38967 ("ptrace: do not audit capability check when outputing /proc/pid/stat")
> introduced the ability to opt out of audit messages for accesses to
> various proc files since they are not violations of policy.
> While doing so it somehow switched the check from ns_capable() to
> has_ns_capability{_noaudit}(). That means it switched from checking the
> subjective credentials of the task to using the objective credentials. I
> couldn't find the original lkml thread and so I don't know why this switch
> was done. But it seems wrong since ptrace_has_cap() is currently only used
> in ptrace_may_access(). And it's used to check whether the calling task
> (subject) has the CAP_SYS_PTRACE capability in the provided user namespace
> to operate on the target task (object). According to the cred.h comments
> this would mean the subjective credentials of the calling task need to be
> used.
> This switches it to use security_capable() because we only call
> ptrace_has_cap() in ptrace_may_access() and in there we already have a
> stable reference to the calling tasks creds under cred_guard_mutex so
> there's no need to go through another series of dereferences and rcu
> locking done in ns_capable{_noaudit}().

This patch breaks CRIU tests:

All CRIU tests fail because ptrace returns EPERM:

$ python test/zdtm.py run -t zdtm/static/env00 --sat
=== Run 1/1 ================ zdtm/static/env00
========================== Run zdtm/static/env00 in h ==========================
Start test
./env00 --pidfile=env00.pid --outfile=env00.out --envname=ENV_00_TEST
Run criu dump
=[strace]=> dump/zdtm/static/env00/44/1/dump.strace
=[log]=> dump/zdtm/static/env00/44/1/dump.log
------------------------ grep Error ------------------------
b'(00.014558) cg:     `- [net_cls,net_prio] -> [/] [0]'
b'(00.014634) cg:     `- [perf_event] -> [/] [0]'
b'(00.014713) cg:     `- [pids] ->
[/user.slice/user-0.slice/session-1.scope] [0]'
b'(00.014818) cg: Set 1 is criu one'
b'(00.015123) Warn  (compel/src/lib/infect.c:127): Unable to interrupt
task: 44 (Operation not permitted)'
b'(00.015302) Unlock network'
b'(00.015423) Unfreezing tasks into 1'
b'(00.015524) \tUnseizing 44 into 1'
b'(00.015701) Error (compel/src/lib/infect.c:346): Unable to detach
from 44: No such process'
b'(00.015864) Error (criu/cr-dump.c:1775): Dumping FAILED.'
------------------------ ERROR OVER ------------------------
################### Test zdtm/static/env00 FAIL at CRIU dump ###################
Send the 9 signal to  44
Wait for zdtm/static/env00(44) to die for 0.100000
##################################### FAIL #####################################

Here is a strace output for the criu dump process:

write(4, "(00.014482) cg:     `- [name=zdt"..., 61) = 61 <0.000028>
write(4, "(00.014558) cg:     `- [net_cls,"..., 53) = 53 <0.000028>
write(4, "(00.014634) cg:     `- [perf_eve"..., 47) = 47 <0.000031>
write(4, "(00.014713) cg:     `- [pids] ->"..., 80) = 80 <0.000034>
write(4, "(00.014818) cg: Set 1 is criu on"..., 34) = 34 <0.000028>
rt_sigaction(SIGALRM, {sa_handler=0x43de00, sa_mask=[ALRM],
sa_flags=SA_RESTORER, sa_restorer=0x7f962247c6b0}, NULL, 8) = 0
<0.000018>
alarm(10)                               = 0 <0.000025>
ptrace(PTRACE_SEIZE, 44, NULL, 0)       = -1 EPERM (Operation not
permitted) <0.000022>
write(4, "(00.015123) Warn  (compel/src/li"..., 104) = 104 <0.000029>
alarm(0)                                = 10 <0.000032>
write(4, "(00.015302) Unlock network\n", 27) = 27 <0.000043>
write(4, "(00.015423) Unfreezing tasks int"..., 36) = 36 <0.000036>

The criu process is started with all capabilities in the root user namespace.

I don't have time to investigate this issue right now, will provide
more details next Tuesday.

The issue has been detected by our travis-c job:
https://travis-ci.org/avagin/linux/jobs/638547093

Thanks,
Andrei