does nosuid work on bind-mounts to suid partitions?

Question

if i have /target mounted with suid and then make a bind-mount on /bound with mount -o bind,nosuid /target /bound, will nosuid take effect on /bound ?

(imo it should take effect but i'd still like a definite answer, and nobody else had asked yet here or so it seems)

@JimL. nosuid works when i try it, check the the answer below for details ^^ — hanshenrik, Dec 12 '19 at 08:18
@ctrl-alt-delor yup that's a better name, feel free to edit the posts ^^ — hanshenrik, Jul 31 '22 at 03:10

score 2 · Accepted Answer · answered Dec 12 '19 at 08:05

yes, bind is capable of enforcing nosuid even if the target has suid. here's a test i ran:

C source code of a.out:

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
int main(){
uid_t uid=getuid(), euid=geteuid();
printf("uid: %u, euid: %u\n",uid,euid);
return 0;
}

and then

root@ratma:/# mount -o bind,nosuid /target /bound
root@ratma:/# su hans
hans@ratma:/$ stat /target/a.out
  File: /target/a.out
  Size: 16712           Blocks: 40         IO Block: 4096   regular file
Device: 18h/24d Inode: 194454      Links: 1
Access: (6755/-rwsr-sr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2019-12-12 07:52:45.132465174 +0000
Modify: 2019-12-12 07:52:45.132465174 +0000
Change: 2019-12-12 07:53:24.720322010 +0000
 Birth: -
hans@ratma:/$ stat /bound/a.out
  File: /bound/a.out
  Size: 16712           Blocks: 40         IO Block: 4096   regular file
Device: 18h/24d Inode: 194454      Links: 1
Access: (6755/-rwsr-sr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2019-12-12 07:52:45.132465174 +0000
Modify: 2019-12-12 07:52:45.132465174 +0000
Change: 2019-12-12 07:53:24.720322010 +0000
 Birth: -
hans@ratma:/$ id
uid=1000(hans) gid=1000(hans) groups=1000(hans),24(cdrom),25(floppy),27(sudo),29(audio),30(dip),44(video),46(plugdev),109(netdev)
hans@ratma:/$ /target/a.out
uid: 1000, euid: 0
hans@ratma:/$ /bound/a.out
uid: 1000, euid: 1000

success. if it didn't work, it would say "euid: 0" on /bound/a.out :)

does nosuid work on bind-mounts to suid partitions?

1 Answers1