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internal/as_user: drop supplementary groups #2301

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internal/as_user: drop supplementary groups
JasonColapietro Aug 21, 2026
c6e73c9
Merge branch 'main' into as-user/drop-supplementary-groups
JasonColapietro Aug 31, 2026
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2 changes: 2 additions & 0 deletions docs/release-notes.md
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,8 @@ nav_order: 9

### Bug fixes

- Drop supplementary groups when dropping privileges to write files as a user ([#2242](https://github.com/coreos/ignition/issues/2242))


## Ignition 2.27.0 (2026-08-26)

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13 changes: 12 additions & 1 deletion internal/as_user/as_user.c
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Expand Up @@ -15,6 +15,7 @@
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <grp.h>
#include <sched.h>
#include <signal.h>
#include <stdio.h>
Expand Down Expand Up @@ -54,7 +55,9 @@ typedef struct au_thread_ctxt {
int err;
} au_thread_ctxt_t;

/* set_eids() sets the effective gid and uid of the calling thread to ids */
/* set_eids() drops the supplementary groups and sets the effective gid and
* uid of the calling thread to ids
*/
static int set_eids(au_ids_t *ids) {
uid_t cu;
gid_t cg;
Expand All @@ -64,6 +67,14 @@ static int set_eids(au_ids_t *ids) {
cu = geteuid();
cg = getegid();

/* Drop the supplementary groups inherited from the caller before
* relinquishing the gid and uid, while we still have the privilege
* required to do so. Otherwise the thread would keep root's group
* memberships while acting as the target user.
*/
if(setgroups(0, NULL) == -1)
return -1;

if(cg != ids->gid && setregid(-1, ids->gid) == -1)
return -1;

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