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diff --git a/doc/UIDS-GIDS.md b/doc/UIDS-GIDS.md new file mode 100644 index 0000000000..e19cc88162 --- /dev/null +++ b/doc/UIDS-GIDS.md @@ -0,0 +1,243 @@ +# Users, Groups, UIDs and GIDs on `systemd` systems + +Here's a summary of the requirements `systemd` (and Linux) make on UID/GID +assignments and their ranges. + +Note that while in theory UIDs and GIDs are orthogonal concepts they really +aren't IRL. With that in mind, when we discuss UIDs below it should be assumed +that whatever we say about UIDs applies to GIDs in mostly the same way, and all +the special assignments and ranges for UIDs always have mostly the same +validity for GIDs too. + +## Special Linux UIDs + +In theory, the range of the C type `uid_t` is 32bit wide on Linux, +i.e. 0…4294967295. However, four UIDs are special on Linux: + +1. 0 → The `root` super-user + +2. 65534 → The `nobody` UID, also called the "overflow" UID or similar. It's + where various subsystems map unmappable users to, for example file systems + only supporting 16bit UIDs, NFS or user namespacing. (The latter can be + changed with a sysctl during runtime, but that's not supported on + `systemd`. If you do change it you void your warranty.) Because Fedora is a + bit confused the `nobody` user is called `nfsnobody` there (and they have a + different `nobody` user at UID 99). I hope this will be corrected eventually + though. (Also, some distributions call the `nobody` group `nogroup`. I wish + they didn't.) + +3. 4294967295, aka "32bit `(uid_t) -1`" → This UID is not a valid user ID, as + `setresuid()`, `chown()` and friends treat -1 as a special request to not + change the UID of the process/file. This UID is hence not available for + assignment to users in the user database. + +4. 65535, aka "16bit `(uid_t) -1`" → Before Linux kernel 2.4 `uid_t` used to be + 16bit, and programs compiled for that would hence assume that `(uid_t) -1` + is 65535. This UID is hence not usable either. + +The `nss-systemd` glibc NSS module will synthesize user database records for +the UIDs 0 and 65534 if the system user database doesn't list them. This means +that any system where this module is enabled works to some minimal level +without `/etc/passwd`. + +## Special Distribution UID ranges + +Distributions generally split the available UID range in two: + +1. 1…999 → System users. These are users that do not map to actual "human" + users, but are used as security identities for system daemons, to implement + privilege separation and run system daemons with minimal privileges. + +2. 1000…65533 and 65536…4294967294 → Everything else, i.e. regular (human) users. + +Note that most distributions allow changing the boundary between system and +regular users, even during runtime as user configuration. Moreover, some older +systems placed the boundary at 499/500, or even 99/100. In `systemd`, the +boundary is configurable only during compilation time, as this should be a +decision for distribution builders, not for users. Moreover, we strongly +discourage downstreams to change the boundary from the upstream default of +999/1000. + +Also note that programs such as `adduser` tend to allocate from a subset of the +available regular user range only, usually 1000..60000. And it's also usually +user-configurable, too. + +Note that systemd requires that system users and groups are resolvable without +networking available — a requirement that is not made for regular users. This +means regular users may be stored in remote LDAP or NIS databases, but system +users may not (except when there's a consistent local cache kept, that is +available during earliest boot, including in the initial RAM disk). + +## Special `systemd` GIDs + +`systemd` defines no special UIDs beyond what Linux already defines (see +above). However, it does define some special group/GID assignments, which are +primarily used for `systemd-udevd`'s device management. The precise list of the +currently defined groups is found in this `sysusers.d` snippet: +[basic.conf](https://raw.githubusercontent.com/systemd/systemd/master/sysusers.d/basic.conf.in) + +It's strongly recommended that downstream distributions include these groups in +their default group databases. + +Note that the actual GID numbers assigned to these groups do not have to be +constant beyond a specific system. There's one exception however: the `tty` +group must have the GID 5. That's because it must be encoded in the `devpts` +mount parameters during earliest boot, at a time where NSS lookups are not +possible. (Note that the actual GID can be changed during `systemd` build time, +but downstreams are strongly advised against doing that.) + +## Special `systemd` UID ranges + +`systemd` defines a number of special UID ranges: + +1. 61184…65519 → UIDs for dynamic users are allocated from this range (see the + `DynamicUser=` documentation in + [`systemd.exec(5)`](https://www.freedesktop.org/software/systemd/man/systemd.exec.html)). This + range has been chosen so that it is below the 16bit boundary (i.e. below + 65535), in order to provide compatibility with container environments that + assign a 64K range of UIDs to containers using user namespacing. This range + is above the 60000 boundary, so that its allocations are unlikely to be + affected by `adduser` allocations (see above). And we leave some room + upwards for other purposes. (And if you wonder why precisely these numbers: + if you write them in hexadecimal, they might make more sense: 0xEF00 and + 0xFFEF). The `nss-systemd` module will synthesize user records implicitly + for all currently allocated dynamic users from this range. Thus, NSS-based + user record resolving works correctly without those users being in + `/etc/passwd`. + +2. 524288…1879048191 → UID range for `systemd-nspawn`'s automatic allocation of + per-container UID ranges. When the `--private-users=pick` switch is used (or + `-U`) then it will automatically find a so far unused 16bit subrange of this + range and assign it to the container. The range is picked so that the upper + 16bit of the 32bit UIDs are constant for all users of the container, while + the lower 16bit directly encode the 65536 UIDs assigned to the + container. This mode of allocation means that the upper 16bit of any UID + assigned to a container are kind of a "container ID", while the lower 16bit + directly expose the container's own UID numbers. If you wonder why precisely + these numbers, consider them in hexadecimal: 0x00080000…0x6FFFFFFF. This + range is above the 16bit boundary. Moreover it's below the 31bit boundary, + as some broken code (specifically: the kernel's `devpts` file system) + erroneously considers UIDs signed integers, and hence can't deal with values + above 2^31. The `nss-mymachines` glibc NSS module will synthesize user + database records for all UIDs assigned to a running container from this + range. + +Note for both allocation ranges: when an UID allocation takes place NSS is +checked for collisions first, and a different UID is picked if an entry is +found. Thus, the user database is used as synchronization mechanism to ensure +exclusive ownership of UIDs and UID ranges. To ensure compatibility with other +subsystems allocating from the same ranges it is hence essential that they +ensure that whatever they pick shows up in the user/group databases, either by +providing an NSS module, or by adding entries directly to `/etc/passwd` and +`/etc/group`. For performance reasons, do note that `systemd-nspawn` will only +do an NSS check for the first UID of the range it allocates, not all 65536 of +them. Also note that while the allocation logic is operating, the glibc +`lckpwdf()` user database lock is taken, in order to make this logic race-free. + +## Figuring out the system's UID boundaries + +The most important boundaries of the local system may be queried with +`pkg-config`: + +``` +$ pkg-config --variable=systemuidmax systemd +999 +$ pkg-config --variable=dynamicuidmin systemd +61184 +$ pkg-config --variable=dynamicuidmax systemd +65519 +$ pkg-config --variable=containeruidbasemin systemd +524288 +$ pkg-config --variable=containeruidbasemax systemd +1878982656 +``` + +(Note that the latter encodes the maximum UID *base* `systemd-nspawn` might +pick — given that 64K UIDs are assigned to each container according to this +allocation logic, the maximum UID used for this range is hence +1878982656+65535=1879048191.) + +Note that systemd does not make any of these values runtime-configurable. All +these boundaries are chosen during build time. That said, the system UID/GID +boundary is traditionally configured in /etc/login.defs, though systemd won't +look there during runtime. + +## Considerations for container managers + +If you hack on a container manager, and wonder how and how many UIDs best to +assign to your containers, here are a few recommendations: + +1. Definitely, don't assign less than 65536 UIDs/GIDs. After all the `nobody` +user has magic properties, and hence should be available in your container, and +given that it's assigned the UID 65534, you should really cover the full 16bit +range in your container. Note that systemd will — as mentioned — synthesize +user records for the `nobody` user, and assumes its availability in various +other parts of its codebase, too, hence assigning fewer users means you lose +compatibility with running systemd code inside your container. And most likely +other packages make similar restrictions. + +2. While it's fine to assign more than 65536 UIDs/GIDs to a container, there's +most likely not much value in doing so, as Linux distributions won't use the +higher ranges by default (as mentioned neither `adduser` nor `systemd`'s +dynamic user concept allocate from above the 16bit range). Unless you actively +care for nested containers, it's hence probably a good idea to allocate exactly +65536 UIDs per container, and neither less nor more. A pretty side-effect is +that by doing so, you expose the same number of UIDs per container as Linux 2.2 +supported for the whole system, back in the days. + +3. Consider allocating UID ranges for containers so that the first UID you +assign has the lower 16bits all set to zero. That way, the upper 16bits become +a container ID of some kind, while the lower 16bits directly encode the +internal container UID. This is the way `systemd-nspawn` allocates UID ranges +(see above). Following this allocation logic ensures best compability with +`systemd-nspawn` and all other container managers following the scheme, as it +is sufficient then to check NSS for the first UID you pick regarding conflicts, +as that's what they do, too. Moreover, it makes `chown()`ing container file +system trees nicely robust to interruptions: as the external UID encodes the +internal UID in a fixed way, it's very easy to adjust the container's base UID +without the need to know the original base UID: to change the container base, +just mask away the upper 16bit, and insert the upper 16bit of the new container +base instead. Here are the easy conversions to derive the internal UID, the +external UID, and the container base UID from each other: + + ``` + INTERNAL_UID = EXTERNAL_UID & 0x0000FFFF + CONTAINER_BASE_UID = EXTERNAL_UID & 0xFFFF0000 + EXTERNAL_UID = INTERNAL_UID | CONTAINER_BASE_UID + ``` + +4. When picking a UID range for containers, make sure to check NSS first, with +a simple `getpwuid()` call: if there's already a user record for the first UID +you want to pick, then it's already in use: pick a different one. Wrap that +call in a `lckpwdf()` + `ulckpwdf()` pair, to make allocation +race-free. Provide an NSS module that makes all UIDs you end up taking show up +in the user database, and make sure that the NSS module returns up-to-date +information before you release the lock, so that other system components can +safely use the NSS user database as allocation check, too. Note that if you +follow this scheme no changes to `/etc/passwd` need to be made, thus minimizing +the artifacts the container manager persistently leaves in the system. + +## Summary + +| UID/GID | Purpose | Defined By | Listed in | +|-----------------------|-----------------------|---------------|-------------------------------| +| 0 | `root` user | Linux | `/etc/passwd` + `nss-systemd` | +| 1…4 | System users | Distributions | `/etc/passwd` | +| 5 | `tty` group | `systemd` | `/etc/passwd` | +| 6…999 | System users | Distributions | `/etc/passwd` | +| 1000…60000 | Regular users | Distributions | `/etc/passwd` + LDAP/NIS/… | +| 60001…61183 | Unused | | | +| 61184…65519 | Dynamic service users | `systemd` | `nss-systemd` | +| 65520…65533 | Unused | | | +| 65534 | `nobody` user | Linux | `/etc/passwd` + `nss-systemd` | +| 65535 | 16bit `(uid_t) -1` | Linux | | +| 65536…524287 | Unused | | | +| 524288…1879048191 | Container UID ranges | `systemd` | `nss-mymachines` | +| 1879048192…4294967294 | Unused | | | +| 4294967295 | 32bit `(uid_t) -1` | Linux | | + +Note that "Unused" in the table above doesn't meant that these ranges are +really unused. It just means that these ranges have no well-established +pre-defined purposes between Linux, generic low-level distributions and +`systemd`. There might very well be other packages that allocate from these +ranges. |