On Fri, 28 Jul 2023 at 15:14, Ondrej Mosnacek <omosnace@xxxxxxxxxx> wrote: > > On Fri, Jul 28, 2023 at 1:52 PM Stephen Smalley > <stephen.smalley.work@xxxxxxxxx> wrote: > > > > On Fri, Jul 28, 2023 at 7:36 AM Ondrej Mosnacek <omosnace@xxxxxxxxxx> wrote: > > > > > > On Fri, Jul 28, 2023 at 4:12 AM Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote: > > > > > > > > Ondrej Mosnacek <omosnace@xxxxxxxxxx> writes: > > > > > Currently, SELinux doesn't allow distinguishing between kernel threads > > > > > and userspace processes that are started before the policy is first > > > > > loaded - both get the label corresponding to the kernel SID. The only > > > > > way a process that persists from early boot can get a meaningful label > > > > > is by doing a voluntary dyntransition or re-executing itself. > > > > > > > > Hi, > > > > > > > > This commit breaks login for me when booting linux-next kernels with old > > > > userspace, specifically Ubuntu 16.04 on ppc64le. 18.04 is OK. > > > > > > > > The symptom is that login never accepts the root password, it just > > > > always says "Login incorrect". > > > > > > > > Bisect points to this commit. > > > > > > > > Reverting this commit on top of next-20230726, fixes the problem > > > > (ie. login works again). > > > > > > > > Booting with selinux=0 also fixes the problem. > > > > > > > > Is this expected? The change log below suggests backward compatibility > > > > was considered, is 16.04 just too old? > > > > > > Hi Michael, > > > > > > I can reproduce it on Fedora 38 when I boot with SELINUX=disabled in > > > /etc/selinux/config (+ a kernel including that commit), so it likely > > > isn't caused by the userspace being old. Can you check what you have > > > in /etc/selinux/config (or if it exists at all)? > > > > > > We have deprecated and removed the "runtime disable" functionality in > > > SELinux recently [1], which was used to implement "disabling" SELinux > > > via the /etc/selinux/config file, so now the situation (selinux=0 + > > > SELINUX=disabled in /etc/selinux/config) leads to a state where > > > SELinux is enabled, but no policy is loaded (and no enforcement is > > > done). Such a state mostly behaves as if SElinux was truly disabled > > > (via kernel command line), but there are some subtle differences and I > > > believe we don't officially support it (Paul might clarify). With > > > latest kernels it is recommended to either disable SELinux via the > > > kernel command line (or Kconfig[2]) or to boot it in Enforcing or > > > Permissive mode with a valid/usable policy installed. > > > > > > So I wonder if Ubuntu ships by default with the bad configuration or > > > if it's just a result of using the custom-built linux-next kernel (or > > > some changes on your part). If Ubuntu's stock kernel is configured to > > > boot with SELinux enabled by default, they should also by default ship > > > a usable policy and SELINUX=permissive/enforcing in > > > /etc/selinux/config (or configure the kernel[2] or bootloader to boot > > > with SELinux disabled by default). (Although if they ship a pre-[1] > > > kernel, they may continue to rely on the runtime disable > > > functionality, but it means people will have to be careful when > > > booting newer or custom kernels.) > > > > > > That said, I'd like to get to the bottom of why the commit causes the > > > login to fail and fix it somehow. I presume something in PAM chokes on > > > the fact that userspace tasks now have "init" instead of "kernel" as > > > the pre-policy-load security context, but so far I haven't been able > > > to pinpoint the problem. I'll keep digging... > > > > > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f22f9aaf6c3d92ebd5ad9e67acc03afebaaeb289 > > > [2] via CONFIG_LSM (or CONFIG_SECURITY_SELINUX_BOOTPARAM_VALUE on older kernels) > > > > Prior to selinux userspace commit > > 685f4aeeadc0b60f3770404d4f149610d656e3c8 ("libselinux: > > is_selinux_enabled(): drop no-policy-loaded test.") libselinux was > > checking the result of reading /proc/self/attr/current to see if it > > returned the "kernel" string as a means of detecting a system with > > SELinux enabled but no policy loaded, and treated that as if SELinux > > were disabled. Hence, this does break old userspace. Not sure though > > why you'd see the same behavior with modern libselinux. > > Hm... now I tried booting the stock Fedora kernel (without the early > boot initial SID commit) and I got the same failure to login as with > the new kernel. So if Ubuntu 16.04 ships with pre-685f4aeeadc0 > libselinux (quite possible), then it seems that the scenario with > terminal login + SELinux enabled + policy not loaded only works with > pre-685f4aeeadc0 libselinux and pre-5b0eea835d4e kernel, the other > combinations are broken. With pre-685f4aeeadc0 libselinux + > post-5b0eea835d4e kernel it is expected as you say (and probably > inevitable barring some hack on the kernel side), but it's not clear > why also only updating libselinux seems to break it... /sys/fs/selinux > is not mounted in my scenario, so there must be something else coming > into play. > > > -- > Ondrej Mosnacek > Senior Software Engineer, Linux Security - SELinux kernel > Red Hat, Inc. > Completely untested: diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c index 2c5be06fbada..1ed275bd4551 100644 --- a/security/selinux/ss/services.c +++ b/security/selinux/ss/services.c @@ -1322,8 +1322,19 @@ static int security_sid_to_context_core(u32 sid, char **scontext, if (!selinux_initialized()) { if (sid <= SECINITSID_NUM) { char *scontextp; - const char *s = initial_sid_to_string[sid]; + const char *s; + /* + * Hide the context split of kernel threads and + * userspace threads from userspace before the first + * policy is loaded. Userspace, e.g. libselinux prior + * to v2.6 or systemd, depends on the context being + * "kernel". + */ + if (sid == SECINITSID_INIT) + sid = SECINITSID_KERNEL; + + s = initial_sid_to_string[sid]; if (!s) return -EINVAL; *scontext_len = strlen(s) + 1;
