Bug 45066 - process looses access to files via the
process looses access to files via the
Product: Red Hat Linux
Classification: Retired
Component: kernel (Show other bugs)
i386 Linux
medium Severity medium
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Assigned To: Arjan van de Ven
Aaron Brown
Depends On:
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Reported: 2001-06-19 20:23 EDT by Need Real Name
Modified: 2007-04-18 12:33 EDT (History)
0 users

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Fixed In Version:
Doc Type: Bug Fix
Doc Text:
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Last Closed: 2001-06-25 04:21:19 EDT
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Description Need Real Name 2001-06-19 20:23:22 EDT
From Bugzilla Helper:
User-Agent: Mozilla/4.72 [en] (X11; U; Linux 2.2.14-5.0 i686)

Description of problem:
Files that are normally accessable via the "other" permission bits are no
longer accessable to applications that switch from superuser to a specific
user id via the setuid, seteuid, and setreuid function calls. 

How reproducible:

Steps to Reproduce:
1.  Write a c program that looks like this:
(modify the uid/gid ids to that of a valid non-privileged user)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char **argv)
    int fd = 0;


    if (argc < 2)
        return EXIT_FAILURE;

    if ((fd = open(argv[1], O_RDWR)) == -1)
        printf("Error: %d: %s\n", errno, strerror(errno));

    sleep(10); // to verify with ps that the process id has switched.

    return EXIT_SUCCESS;

2. touch a file (as root) (# touch afile)
3. chmod 006 afile
4. run the program as root (# openTest afile)
5. verify process is owned by the user id specified with ps -ef 
6. program outputs an error message indicating permission has been denied.

Actual Results:  File cannot be opened by the program, EACCES error is in

Expected Results:  File should be opened via the "other" permissions.

Program works fine if you comment out the setuid/setgid functions and run
program as root or the non-privileged user. 

Additional info:

No apparent work around.
Comment 1 Jakub Jelinek 2001-06-25 03:43:44 EDT
This has nothing to do with glibc, the relevant function calls are stright
system calls. So I'll let kernel folks to explain this.
Comment 2 Arjan van de Ven 2001-06-25 03:51:10 EDT
Which kernel is this exactly ?
Comment 3 Arjan van de Ven 2001-06-25 04:18:45 EDT
"Other" permission bits are irrelevant if GID of file is one of your
supplementary groups. setgid(2) doesn't alter them.
Comment 4 Arjan van de Ven 2001-06-25 04:21:15 EDT
man setgroups

gives info on how to manipulate this.
Comment 5 Need Real Name 2001-06-25 12:19:43 EDT
I've made the following modifications to my test program:

insert these two lines before the setgid() call, (which probably isn't 
necessary anymore.):

const gid_t groupList[] = {501};
setgroups((sizeof(groupList) / sizeof(gid_t)), &groupList);

And now the program can access the file via the other bits, as a result of 
losing group 0. 

BTW, it is kernel 2.2.14.

Comment 6 Need Real Name 2001-06-25 12:28:00 EDT
Opps there was a bug in my code:

add this line:
#include <grp.h>

and  change the segroups() line to this:

setgroups((sizeof(groupList)/sizeof(gid_t)), groupList);

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