On Sat, May 15, 2021 at 10:49 AM Andy Lutomirski [off-list ref] wrote:
On 5/10/21 10:21 PM, YiFei Zhu wrote:
quoted
On Mon, May 10, 2021 at 12:47 PM Andy Lutomirski [off-list ref] wrote:
quoted
On Mon, May 10, 2021 at 10:22 AM YiFei Zhu [off-list ref] wrote:
quoted
From: YiFei Zhu <redacted>
Based on: https://urldefense.com/v3/__https://lists.linux-foundation.org/pipermail/containers/2018-February/038571.html__;!!DZ3fjg!thbAoRgmCeWjlv0qPDndNZW1j6Y2Kl_huVyUffr4wVbISf-aUiULaWHwkKJrNJyo$
This patchset enables seccomp filters to be written in eBPF.
Supporting eBPF filters has been proposed a few times in the past.
The main concerns were (1) use cases and (2) security. We have
identified many use cases that can benefit from advanced eBPF
filters, such as:
I haven't reviewed this carefully, but I think we need to distinguish
a few things:
1. Using the eBPF *language*.
2. Allowing the use of stateful / non-pure eBPF features.
3. Allowing the eBPF programs to read the target process' memory.
I'm generally in favor of (1). I'm not at all sure about (2), and I'm
even less convinced by (3).
quoted
* exec-only-once filter / apply filter after exec
This is (2). I'm not sure it's a good idea.
The basic idea is that for a container runtime it may wait to execute
a program in a container without that program being able to execve
another program, stopping any attack that involves loading another
binary. The container runtime can block any syscall but execve in the
exec-ed process by using only cBPF.
The use case is suggested by Andrea Arcangeli and Giuseppe Scrivano.
@Andrea and @Giuseppe, could you clarify more in case I missed
something?
We've discussed having a notifier-using filter be able to replace its
filter. This would allow this and other use cases without any
additional eBPF or cBPF code.
A notifier is not always a solution (even ignoring its perf overhead).
One problem, pointed out by Andrea Arcangeli, is that notifiers need
userspace daemons. So, it can hardly be used by daemonless container
engines like Podman.
And, /* sorry for repeating.. */ the performance overhead of notifiers
is not close to ebpf, which prevents use cases that require native
performance.
quoted
quoted
eBPF doesn't really have a privilege model yet. There was a long and
disappointing thread about this awhile back.
The idea is that “seccomp-eBPF does not make life easier for an
adversary”. Any attack an adversary could potentially utilize
seccomp-eBPF, they can do the same with other eBPF features, i.e. it
would be an issue with eBPF in general rather than specifically
seccomp’s use of eBPF.
Here it is referring to the helpers goes to the base
bpf_base_func_proto if the caller is unprivileged (!bpf_capable ||
!perfmon_capable). In this case, if the adversary would utilize eBPF
helpers to perform an attack, they could do it via another
unprivileged prog type.
That said, there are a few additional helpers this patchset is adding:
* get_current_uid_gid
* get_current_pid_tgid
These two provide public information (are namespaces a concern?). I
have no idea what kind of exploit it could add unless the adversary
somehow side-channels the task_struct? But in that case, how is the
reading of task_struct different from how the rest of the kernel is
reading task_struct?
Yes, namespaces are a concern. This idea got mostly shot down for kdbus
(what ever happened to that?), and it likely has the same problems for
seccomp.
quoted
quoted
What is this for?
Memory reading opens up lots of use cases. For example, logging what
files are being opened without imposing too much performance penalty
from strace. Or as an accelerator for user notify emulation, where
syscalls can be rejected on a fast path if we know the memory contents
does not satisfy certain conditions that user notify will check.
This has all kinds of race conditions.
I hate to be a party pooper, but this patchset is going to very high bar
to acceptance. Right now, seccomp has a couple of excellent properties:
First, while it has limited expressiveness, it is simple enough that the
implementation can be easily understood and the scope for
vulnerabilities that fall through the cracks of the seccomp sandbox
model is low. Compare this to Windows' low-integrity/high-integrity
sandbox system: there is a never ending string of sandbox escapes due to
token misuse, unexpected things at various integrity levels, etc.
Seccomp doesn't have tokens or integrity levels, and these bugs don't
happen.
Second, seccomp works, almost unchanged, in a completely unprivileged
context. The last time making eBPF work sensibly in a less- or
-unprivileged context, the maintainers mostly rejected the idea of
developing/debugging a permission model for maps, cleaning up the bpf
object id system, etc. You are going to have a very hard time
convincing the seccomp maintainers to let any of these mechanism
interact with seccomp until the underlying permission model is in place.
--Andy
Thanks for pointing out the tradeoff between expressiveness vs. simplicity.
Note that we are _not_ proposing to replace cbpf, but propose to also
support ebpf filters. There certainly are use cases where cbpf is
sufficient, but there are also important use cases ebpf could make
life much easier.
Most importantly, we strongly believe that ebpf filters can be
supported without reducing security.
No worries about “party pooping” and we appreciate the feedback. We’d
love to hear concerns and collect feedback so we can address them to
hit that very high bar.
~t
--
Tianyin Xu
University of Illinois at Urbana-Champaign
https://tianyin.github.io/
On Sun, May 16, 2021 at 03:38:00AM -0500, Tianyin Xu wrote:
On Sat, May 15, 2021 at 10:49 AM Andy Lutomirski [off-list ref] wrote:
quoted
On 5/10/21 10:21 PM, YiFei Zhu wrote:
quoted
On Mon, May 10, 2021 at 12:47 PM Andy Lutomirski [off-list ref] wrote:
quoted
On Mon, May 10, 2021 at 10:22 AM YiFei Zhu [off-list ref] wrote:
quoted
From: YiFei Zhu <redacted>
Based on: https://urldefense.com/v3/__https://lists.linux-foundation.org/pipermail/containers/2018-February/038571.html__;!!DZ3fjg!thbAoRgmCeWjlv0qPDndNZW1j6Y2Kl_huVyUffr4wVbISf-aUiULaWHwkKJrNJyo$
This patchset enables seccomp filters to be written in eBPF.
Supporting eBPF filters has been proposed a few times in the past.
The main concerns were (1) use cases and (2) security. We have
identified many use cases that can benefit from advanced eBPF
filters, such as:
I haven't reviewed this carefully, but I think we need to distinguish
a few things:
1. Using the eBPF *language*.
2. Allowing the use of stateful / non-pure eBPF features.
3. Allowing the eBPF programs to read the target process' memory.
I'm generally in favor of (1). I'm not at all sure about (2), and I'm
even less convinced by (3).
quoted
* exec-only-once filter / apply filter after exec
This is (2). I'm not sure it's a good idea.
The basic idea is that for a container runtime it may wait to execute
a program in a container without that program being able to execve
another program, stopping any attack that involves loading another
binary. The container runtime can block any syscall but execve in the
exec-ed process by using only cBPF.
The use case is suggested by Andrea Arcangeli and Giuseppe Scrivano.
@Andrea and @Giuseppe, could you clarify more in case I missed
something?
We've discussed having a notifier-using filter be able to replace its
filter. This would allow this and other use cases without any
additional eBPF or cBPF code.
A notifier is not always a solution (even ignoring its perf overhead).
One problem, pointed out by Andrea Arcangeli, is that notifiers need
userspace daemons. So, it can hardly be used by daemonless container
engines like Podman.
I'm not sure I buy this argument. Podman already has a conmon instance
for each container, this could be a child of that conmon process, or
live inside conmon itself.
Tycho
On Sun, May 16, 2021 at 1:39 AM Tianyin Xu [off-list ref] wrote:
On Sat, May 15, 2021 at 10:49 AM Andy Lutomirski [off-list ref] wrote:
quoted
On 5/10/21 10:21 PM, YiFei Zhu wrote:
quoted
On Mon, May 10, 2021 at 12:47 PM Andy Lutomirski [off-list ref] wrote:
quoted
On Mon, May 10, 2021 at 10:22 AM YiFei Zhu [off-list ref] wrote:
quoted
From: YiFei Zhu <redacted>
Based on: https://urldefense.com/v3/__https://lists.linux-foundation.org/pipermail/containers/2018-February/038571.html__;!!DZ3fjg!thbAoRgmCeWjlv0qPDndNZW1j6Y2Kl_huVyUffr4wVbISf-aUiULaWHwkKJrNJyo$
This patchset enables seccomp filters to be written in eBPF.
Supporting eBPF filters has been proposed a few times in the past.
The main concerns were (1) use cases and (2) security. We have
identified many use cases that can benefit from advanced eBPF
filters, such as:
I haven't reviewed this carefully, but I think we need to distinguish
a few things:
1. Using the eBPF *language*.
2. Allowing the use of stateful / non-pure eBPF features.
3. Allowing the eBPF programs to read the target process' memory.
I'm generally in favor of (1). I'm not at all sure about (2), and I'm
even less convinced by (3).
quoted
* exec-only-once filter / apply filter after exec
This is (2). I'm not sure it's a good idea.
The basic idea is that for a container runtime it may wait to execute
a program in a container without that program being able to execve
another program, stopping any attack that involves loading another
binary. The container runtime can block any syscall but execve in the
exec-ed process by using only cBPF.
The use case is suggested by Andrea Arcangeli and Giuseppe Scrivano.
@Andrea and @Giuseppe, could you clarify more in case I missed
something?
We've discussed having a notifier-using filter be able to replace its
filter. This would allow this and other use cases without any
additional eBPF or cBPF code.
A notifier is not always a solution (even ignoring its perf overhead).
One problem, pointed out by Andrea Arcangeli, is that notifiers need
userspace daemons. So, it can hardly be used by daemonless container
engines like Podman.
And, /* sorry for repeating.. */ the performance overhead of notifiers
is not close to ebpf, which prevents use cases that require native
performance.
While I agree with you that this is the case right now, there's no reason it
has to be the case. There's a variety of mechanisms that can be employed
to significantly speed up the performance of the notifier. For example, right
now the notifier is behind one large per-filter lock. That could be removed
allowing for better concurrency. There are a large number of mechanisms
that scale O(n) with the outstanding notifications -- again, something
that could be improved.
The other big improvement that could be made is being able to use something
like io_uring with the notifier interface, but it would require a
fairly significant
user API change -- and a move away from ioctl. I'm not sure if people are
excited about that idea at the moment.
quoted
quoted
quoted
eBPF doesn't really have a privilege model yet. There was a long and
disappointing thread about this awhile back.
The idea is that “seccomp-eBPF does not make life easier for an
adversary”. Any attack an adversary could potentially utilize
seccomp-eBPF, they can do the same with other eBPF features, i.e. it
would be an issue with eBPF in general rather than specifically
seccomp’s use of eBPF.
Here it is referring to the helpers goes to the base
bpf_base_func_proto if the caller is unprivileged (!bpf_capable ||
!perfmon_capable). In this case, if the adversary would utilize eBPF
helpers to perform an attack, they could do it via another
unprivileged prog type.
That said, there are a few additional helpers this patchset is adding:
* get_current_uid_gid
* get_current_pid_tgid
These two provide public information (are namespaces a concern?). I
have no idea what kind of exploit it could add unless the adversary
somehow side-channels the task_struct? But in that case, how is the
reading of task_struct different from how the rest of the kernel is
reading task_struct?
Yes, namespaces are a concern. This idea got mostly shot down for kdbus
(what ever happened to that?), and it likely has the same problems for
seccomp.
quoted
quoted
What is this for?
Memory reading opens up lots of use cases. For example, logging what
files are being opened without imposing too much performance penalty
from strace. Or as an accelerator for user notify emulation, where
syscalls can be rejected on a fast path if we know the memory contents
does not satisfy certain conditions that user notify will check.
This has all kinds of race conditions.
I hate to be a party pooper, but this patchset is going to very high bar
to acceptance. Right now, seccomp has a couple of excellent properties:
First, while it has limited expressiveness, it is simple enough that the
implementation can be easily understood and the scope for
vulnerabilities that fall through the cracks of the seccomp sandbox
model is low. Compare this to Windows' low-integrity/high-integrity
sandbox system: there is a never ending string of sandbox escapes due to
token misuse, unexpected things at various integrity levels, etc.
Seccomp doesn't have tokens or integrity levels, and these bugs don't
happen.
Second, seccomp works, almost unchanged, in a completely unprivileged
context. The last time making eBPF work sensibly in a less- or
-unprivileged context, the maintainers mostly rejected the idea of
developing/debugging a permission model for maps, cleaning up the bpf
object id system, etc. You are going to have a very hard time
convincing the seccomp maintainers to let any of these mechanism
interact with seccomp until the underlying permission model is in place.
--Andy
Thanks for pointing out the tradeoff between expressiveness vs. simplicity.
Note that we are _not_ proposing to replace cbpf, but propose to also
support ebpf filters. There certainly are use cases where cbpf is
sufficient, but there are also important use cases ebpf could make
life much easier.
Most importantly, we strongly believe that ebpf filters can be
supported without reducing security.
No worries about “party pooping” and we appreciate the feedback. We’d
love to hear concerns and collect feedback so we can address them to
hit that very high bar.
~t
--
Tianyin Xu
University of Illinois at Urbana-Champaign
https://tianyin.github.io/