[glibc/release/2.40/master] support: Pick group in support_capture_subprogram_self_sgid if UID == 0

[Florian Weimer]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=23a02e382c8ffebfed00a082d8898f1aa468b5da

commit 23a02e382c8ffebfed00a082d8898f1aa468b5da
Author: Florian Weimer <fweimer@redhat.com>
Date:   Wed May 21 16:47:34 2025 +0200

    support: Pick group in support_capture_subprogram_self_sgid if UID == 0
    
    When running as root, it is likely that we can run under any group.
    Pick a harmless group from /etc/group in this case.
    
    Reviewed-by: Carlos O'Donell <carlos@redhat.com>
    (cherry picked from commit 2f769cec448d84a62b7dd0d4ff56978fe22c0cd6)

Diff:
---
 support/support_capture_subprocess.c | 58 +++++++++++++++++++++++++++++++++---
 1 file changed, 54 insertions(+), 4 deletions(-)

diff --git a/support/support_capture_subprocess.c b/support/support_capture_subprocess.c
index 2383481911..1cb344eb04 100644
--- a/support/support_capture_subprocess.c
+++ b/support/support_capture_subprocess.c
@@ -21,7 +21,11 @@
 
 #include <errno.h>
 #include <fcntl.h>
+#include <grp.h>
+#include <scratch_buffer.h>
+#include <stdio_ext.h>
 #include <stdlib.h>
+#include <string.h>
 #include <support/check.h>
 #include <support/xunistd.h>
 #include <support/xsocket.h>
@@ -210,10 +214,48 @@ err:
   return status;
 }
 
+/* Returns true if a group with NAME has been found, and writes its
+   GID to *TARGET.  */
+static bool
+find_sgid_group (gid_t *target, const char *name)
+{
+  /* Do not use getgrname_r because it does not work in statically
+     linked binaries if the system libc is different.  */
+  FILE *fp = fopen ("/etc/group", "rce");
+  if (fp == NULL)
+    return false;
+  __fsetlocking (fp, FSETLOCKING_BYCALLER);
+
+  bool ok = false;
+  struct scratch_buffer buf;
+  scratch_buffer_init (&buf);
+  while (true)
+    {
+      struct group grp;
+      struct group *result = NULL;
+      int status = fgetgrent_r (fp, &grp, buf.data, buf.length, &result);
+      if (status == 0 && result != NULL)
+	{
+	  if (strcmp (result->gr_name, name) == 0)
+	    {
+	      *target = result->gr_gid;
+	      ok = true;
+	      break;
+	    }
+	}
+      else if (errno != ERANGE)
+	break;
+      else if (!scratch_buffer_grow (&buf))
+	break;
+    }
+  scratch_buffer_free (&buf);
+  fclose (fp);
+  return ok;
+}
+
 int
 support_capture_subprogram_self_sgid (const char *child_id)
 {
-  gid_t target = 0;
   const int count = 64;
   gid_t groups[count];
 
@@ -225,6 +267,7 @@ support_capture_subprogram_self_sgid (const char *child_id)
 		     (intmax_t) getuid ());
 
   gid_t current = getgid ();
+  gid_t target = current;
   for (int i = 0; i < ret; ++i)
     {
       if (groups[i] != current)
@@ -234,9 +277,16 @@ support_capture_subprogram_self_sgid (const char *child_id)
 	}
     }
 
-  if (target == 0)
-    FAIL_UNSUPPORTED("Could not find a suitable GID for user %jd\n",
-		     (intmax_t) getuid ());
+  if (target == current)
+    {
+      /* If running as root, try to find a harmless group for SGID.  */
+      if (getuid () != 0
+	  || (!find_sgid_group (&target, "nogroup")
+	      && !find_sgid_group (&target, "bin")
+	      && !find_sgid_group (&target, "daemon")))
+	FAIL_UNSUPPORTED("Could not find a suitable GID for user %jd\n",
+			 (intmax_t) getuid ());
+    }
 
   return copy_and_spawn_sgid (child_id, target);
 }