From: Ondrej Mosnacek <email@example.com> To: Paul Moore <firstname.lastname@example.org> Cc: Linux Security Module list <email@example.com>, James Morris <firstname.lastname@example.org>, Steven Rostedt <email@example.com>, Ingo Molnar <firstname.lastname@example.org>, Steffen Klassert <email@example.com>, Herbert Xu <firstname.lastname@example.org>, "David S . Miller" <email@example.com>, Stephen Smalley <firstname.lastname@example.org>, SElinux list <email@example.com>, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, Linux FS Devel <email@example.com>, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, bpf <email@example.com>, network dev <firstname.lastname@example.org>, email@example.com, Linux kernel mailing list <firstname.lastname@example.org>, Casey Schaufler <email@example.com>, Michael Ellerman <firstname.lastname@example.org> Subject: Re: [PATCH v3] lockdown,selinux: fix wrong subject in some SELinux lockdown checks Date: Tue, 31 Aug 2021 11:08:08 +0200 [thread overview] Message-ID: <CAFqZXNvJtMOfLk-SLt2S2qt=+-x8fm9jS3NKxFoT0_5d2=8Ckg@mail.gmail.com> (raw) In-Reply-To: <CAHC9VhSr2KpeBXuyoHR3_hs+qczFUaBx0oCSMfBBA5UNYU+0KA@mail.gmail.com> On Fri, Jun 18, 2021 at 5:40 AM Paul Moore <email@example.com> wrote: > On Wed, Jun 16, 2021 at 4:51 AM Ondrej Mosnacek <firstname.lastname@example.org> wrote: > > > > Commit 59438b46471a ("security,lockdown,selinux: implement SELinux > > lockdown") added an implementation of the locked_down LSM hook to > > SELinux, with the aim to restrict which domains are allowed to perform > > operations that would breach lockdown. > > > > However, in several places the security_locked_down() hook is called in > > situations where the current task isn't doing any action that would > > directly breach lockdown, leading to SELinux checks that are basically > > bogus. > > > > To fix this, add an explicit struct cred pointer argument to > > security_lockdown() and define NULL as a special value to pass instead > > of current_cred() in such situations. LSMs that take the subject > > credentials into account can then fall back to some default or ignore > > such calls altogether. In the SELinux lockdown hook implementation, use > > SECINITSID_KERNEL in case the cred argument is NULL. > > > > Most of the callers are updated to pass current_cred() as the cred > > pointer, thus maintaining the same behavior. The following callers are > > modified to pass NULL as the cred pointer instead: > > 1. arch/powerpc/xmon/xmon.c > > Seems to be some interactive debugging facility. It appears that > > the lockdown hook is called from interrupt context here, so it > > should be more appropriate to request a global lockdown decision. > > 2. fs/tracefs/inode.c:tracefs_create_file() > > Here the call is used to prevent creating new tracefs entries when > > the kernel is locked down. Assumes that locking down is one-way - > > i.e. if the hook returns non-zero once, it will never return zero > > again, thus no point in creating these files. Also, the hook is > > often called by a module's init function when it is loaded by > > userspace, where it doesn't make much sense to do a check against > > the current task's creds, since the task itself doesn't actually > > use the tracing functionality (i.e. doesn't breach lockdown), just > > indirectly makes some new tracepoints available to whoever is > > authorized to use them. > > 3. net/xfrm/xfrm_user.c:copy_to_user_*() > > Here a cryptographic secret is redacted based on the value returned > > from the hook. There are two possible actions that may lead here: > > a) A netlink message XFRM_MSG_GETSA with NLM_F_DUMP set - here the > > task context is relevant, since the dumped data is sent back to > > the current task. > > b) When adding/deleting/updating an SA via XFRM_MSG_xxxSA, the > > dumped SA is broadcasted to tasks subscribed to XFRM events - > > here the current task context is not relevant as it doesn't > > represent the tasks that could potentially see the secret. > > It doesn't seem worth it to try to keep using the current task's > > context in the a) case, since the eventual data leak can be > > circumvented anyway via b), plus there is no way for the task to > > indicate that it doesn't care about the actual key value, so the > > check could generate a lot of "false alert" denials with SELinux. > > Thus, let's pass NULL instead of current_cred() here faute de > > mieux. > > > > Improvements-suggested-by: Casey Schaufler <email@example.com> > > Improvements-suggested-by: Paul Moore <firstname.lastname@example.org> > > Fixes: 59438b46471a ("security,lockdown,selinux: implement SELinux lockdown") > > Signed-off-by: Ondrej Mosnacek <email@example.com> > > This seems reasonable to me, but before I merge it into the SELinux > tree I think it would be good to get some ACKs from the relevant > subsystem folks. I don't believe we ever saw a response to the last > question for the PPC folks, did we? Can we move this forward somehow, please? Quoting the yet-unanswered question from the v2 thread for convenience: > > > The callers migrated to the new hook, passing NULL as cred: > > > 1. arch/powerpc/xmon/xmon.c [...] > > > > This definitely sounds like kernel_t based on the description above. > > Here I'm a little concerned that the hook might be called from some > unusual interrupt, which is not masked by spin_lock_irqsave()... We > ran into this with PMI (Platform Management Interrupt) before, see > commit 5ae5fbd21079 ("powerpc/perf: Fix handling of privilege level > checks in perf interrupt context"). While I can't see anything that > would suggest something like this happening here, the whole thing is > so foreign to me that I'm wary of making assumptions :) > > @Michael/PPC devs, can you confirm to us that xmon_is_locked_down() is > only called from normal syscall/interrupt context (as opposed to > something tricky like PMI)? I strongly suspect the answer will be just "Of course it is, why would you even ask such a silly question?", but please let's have it on record so we can finally get this patch merged... > > --- > > > > v3: > > - add the cred argument to security_locked_down() and adapt all callers > > - keep using current_cred() in BPF, as the hook calls have been shifted > > to program load time (commit ff40e51043af ("bpf, lockdown, audit: Fix > > buggy SELinux lockdown permission checks")) > > - in SELinux, don't ignore hook calls where cred == NULL, but use > > SECINITSID_KERNEL as the subject instead > > - update explanations in the commit message > > > > v2: https://firstname.lastname@example.org/ > > - change to a single hook based on suggestions by Casey Schaufler > > > > v1: https://email@example.com/ > > > > arch/powerpc/xmon/xmon.c | 4 ++-- > > arch/x86/kernel/ioport.c | 4 ++-- > > arch/x86/kernel/msr.c | 4 ++-- > > arch/x86/mm/testmmiotrace.c | 2 +- > > drivers/acpi/acpi_configfs.c | 2 +- > > drivers/acpi/custom_method.c | 2 +- > > drivers/acpi/osl.c | 3 ++- > > drivers/acpi/tables.c | 2 +- > > drivers/char/mem.c | 2 +- > > drivers/cxl/mem.c | 2 +- > > drivers/firmware/efi/efi.c | 2 +- > > drivers/firmware/efi/test/efi_test.c | 2 +- > > drivers/pci/pci-sysfs.c | 6 +++--- > > drivers/pci/proc.c | 6 +++--- > > drivers/pci/syscall.c | 2 +- > > drivers/pcmcia/cistpl.c | 2 +- > > drivers/tty/serial/serial_core.c | 2 +- > > fs/debugfs/file.c | 2 +- > > fs/debugfs/inode.c | 2 +- > > fs/proc/kcore.c | 2 +- > > fs/tracefs/inode.c | 2 +- > > include/linux/lsm_hook_defs.h | 2 +- > > include/linux/lsm_hooks.h | 1 + > > include/linux/security.h | 4 ++-- > > kernel/bpf/helpers.c | 10 ++++++---- > > kernel/events/core.c | 2 +- > > kernel/kexec.c | 2 +- > > kernel/kexec_file.c | 2 +- > > kernel/module.c | 2 +- > > kernel/params.c | 2 +- > > kernel/power/hibernate.c | 3 ++- > > kernel/trace/bpf_trace.c | 20 ++++++++++++-------- > > kernel/trace/ftrace.c | 4 ++-- > > kernel/trace/ring_buffer.c | 2 +- > > kernel/trace/trace.c | 10 +++++----- > > kernel/trace/trace_events.c | 2 +- > > kernel/trace/trace_events_hist.c | 4 ++-- > > kernel/trace/trace_events_synth.c | 2 +- > > kernel/trace/trace_events_trigger.c | 2 +- > > kernel/trace/trace_kprobe.c | 6 +++--- > > kernel/trace/trace_printk.c | 2 +- > > kernel/trace/trace_stack.c | 2 +- > > kernel/trace/trace_stat.c | 2 +- > > kernel/trace/trace_uprobe.c | 4 ++-- > > net/xfrm/xfrm_user.c | 11 +++++++++-- > > security/lockdown/lockdown.c | 3 ++- > > security/security.c | 4 ++-- > > security/selinux/hooks.c | 7 +++++-- > > 48 files changed, 97 insertions(+), 77 deletions(-) > > -- > paul moore > www.paul-moore.com > -- Ondrej Mosnacek Software Engineer, Linux Security - SELinux kernel Red Hat, Inc
next prev parent reply other threads:[~2021-08-31 9:08 UTC|newest] Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-06-16 8:51 Ondrej Mosnacek 2021-06-18 3:40 ` Paul Moore 2021-08-31 9:08 ` Ondrej Mosnacek [this message] 2021-08-31 13:49 ` Paul Moore 2021-06-18 22:18 ` Dan Williams 2021-08-31 9:09 ` Ondrej Mosnacek 2021-08-31 13:53 ` Paul Moore 2021-08-31 18:58 ` Dan Williams 2021-08-31 18:59 ` Paul Moore 2021-06-19 17:00 ` Thomas Gleixner 2021-07-13 2:34 ` Paul Moore 2021-06-21 8:35 ` Steffen Klassert
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