Check primary group memberships as well as supplementary group

memberships, and only allow group-writability by groups with exactly one
member, as zero-member groups are typically used by setgid binaries
rather than being user-private groups (closes: #581697).

1 files changed, 33 insertions, 8 deletions

diff --git a/debian/patches/user-group-modes.patch b/debian/patches/user-group-modes.patch
index 8cf862049..164b8ec81 100644
--- a/debian/patches/user-group-modes.patch
+++ b/debian/patches/user-group-modes.patch
@@ -109,7 +109,7 @@ Index: b/misc.c
  #ifdef SSH_TUN_OPENBSD
  #include <net/if.h>
  #endif
-@@ -638,6 +639,30 @@
+@@ -638,6 +639,55 @@
  }
  
  int
@@ -117,22 +117,47 @@ Index: b/misc.c
 +{
 +       if (st->st_uid != 0 && st->st_uid != uid)
 +               return 0;
++       if ((st->st_mode & 002) != 0)
++               return 0;
 +       if ((st->st_mode & 020) != 0) {
 +               /* If the file is group-writable, the group in question must
-+                * have at most one member, namely the file's owner.
++                * have exactly one member, namely the file's owner.
++                * (Zero-member groups are typically used by setgid
++                * binaries, and are unlikely to be suitable.)
 +                */
-+               struct passwd *pw = getpwuid(st->st_uid);
-+               struct group *gr = getgrgid(st->st_gid);
-+               if (!pw || !gr)
++               struct passwd *pw;
++               struct group *gr;
++               int members = 0;
++
++               gr = getgrgid(st->st_gid);
++               if (!gr)
++                       return 0;
++
++               /* Check primary group memberships. */
++               while ((pw = getpwent()) != NULL) {
++                       if (pw->pw_gid == gr->gr_gid) {
++                               ++members;
++                               if (pw->pw_uid != uid)
++                                       return 0;
++                       }
++               }
++               endpwent();
++
++               pw = getpwuid(st->st_uid);
++               if (!pw)
 +                       return 0;
-+               else if (gr->gr_mem[0]) {
++
++               /* Check supplementary group memberships. */
++               if (gr->gr_mem[0]) {
++                       ++members;
 +                       if (strcmp(pw->pw_name, gr->gr_mem[0]) ||
 +                           gr->gr_mem[1])
 +                               return 0;
 +               }
++
++               if (!members)
++                       return 0;
 +       }
-+       if ((st->st_mode & 002) != 0)
-+               return 0;
 +       return 1;
 +}
 +