Messages in this thread | | | Date | Tue, 14 Jun 2022 11:06:24 -0500 | Subject | Re: [PATCH v3] cred: Propagate security_prepare_creds() error code | From | Frederick Lawler <> |
| |
On 6/13/22 11:44 PM, Eric W. Biederman wrote: > Frederick Lawler <fred@cloudflare.com> writes: > >> Hi Eric, >> >> On 6/13/22 12:04 PM, Eric W. Biederman wrote: >>> Frederick Lawler <fred@cloudflare.com> writes: >>> >>>> While experimenting with the security_prepare_creds() LSM hook, we >>>> noticed that our EPERM error code was not propagated up the callstack. >>>> Instead ENOMEM is always returned. As a result, some tools may send a >>>> confusing error message to the user: >>>> >>>> $ unshare -rU >>>> unshare: unshare failed: Cannot allocate memory >>>> >>>> A user would think that the system didn't have enough memory, when >>>> instead the action was denied. >>>> >>>> This problem occurs because prepare_creds() and prepare_kernel_cred() >>>> return NULL when security_prepare_creds() returns an error code. Later, >>>> functions calling prepare_creds() and prepare_kernel_cred() return >>>> ENOMEM because they assume that a NULL meant there was no memory >>>> allocated. >>>> >>>> Fix this by propagating an error code from security_prepare_creds() up >>>> the callstack. >>> Why would it make sense for security_prepare_creds to return an error >>> code other than ENOMEM? >>> > That seems a bit of a violation of what that function is supposed to do >>> >> >> The API allows LSM authors to decide what error code is returned from the >> cred_prepare hook. security_task_alloc() is a similar hook, and has its return >> code propagated. > > It is not an api. It is an implementation detail of the linux kernel. > It is a set of convenient functions that do a job. > > The general rule is we don't support cases without an in-tree user. I > don't see an in-tree user. > >> I'm proposing we follow security_task_allocs() pattern, and add visibility for >> failure cases in prepare_creds(). > > I am asking why we would want to. Especially as it is not an API, and I > don't see any good reason for anything but an -ENOMEM failure to be > supported. > We're writing a LSM BPF policy, and not a new LSM. Our policy aims to solve unprivileged unshare, similar to Debian's patch [1]. We're in a position such that we can't use that patch because we can't block _all_ of our applications from performing an unshare. We prefer a granular approach. LSM BPF seems like a good choice.
Because LSM BPF exposes these hooks, we should probably treat them as an API. From that perspective, userspace expects unshare to return a EPERM when the call is denied permissions.
> Without an in-tree user that cares it is probably better to go the > opposite direction and remove the possibility of return anything but > memory allocation failure. That will make it clearer to implementors > that a general error code is not supported and this is not a location > to implement policy, this is only a hook to allocate state for the LSM. >
That's a good point, and it's possible we're using the wrong hook for the policy. Do you know of other hooks we can look into?
>>> I have probably missed a very interesting discussion where that was >>> mentioned but I don't see link to the discussion or anything explaining >>> why we want to do that in this change. >>> >> >> AFAIK, this is the start of the discussion. > > You were on v3 and had an out of tree piece of code so I assumed someone > had at least thought about why you want to implement policy in a piece > of code whose only purpose is to allocate memory to store state. >
No worries.
> Eric > > >
Links: 1: https://sources.debian.org/patches/linux/3.16.56-1+deb8u1/debian/add-sysctl-to-disallow-unprivileged-CLONE_NEWUSER-by-default.patch/
| |