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euid, ruid, suid

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User Identification Variables

  • ruid: The real user ID denotes the user who initiated the process.
  • euid: Known as the effective user ID, it represents the user identity utilized by the system to ascertain process privileges. Generally, euid mirrors ruid, barring instances like a SetUID binary execution, where euid assumes the file owner's identity, thus granting specific operational permissions.
  • suid: This saved user ID is pivotal when a high-privilege process (typically running as root) needs to temporarily relinquish its privileges to perform certain tasks, only to later reclaim its initial elevated status.

Important Note

A process not operating under root can only modify its euid to match the current ruid, euid, or suid.

Understanding set*uid Functions

  • setuid: Contrary to initial assumptions, setuid primarily modifies euid rather than ruid. Specifically, for privileged processes, it aligns ruid, euid, and suid with the specified user, often root, effectively solidifying these IDs due to the overriding suid. Detailed insights can be found in the setuid man page.
  • setreuid and setresuid: These functions allow for the nuanced adjustment of ruid, euid, and suid. However, their capabilities are contingent on the process's privilege level. For non-root processes, modifications are restricted to the current values of ruid, euid, and suid. In contrast, root processes or those with CAP_SETUID capability can assign arbitrary values to these IDs. More information can be gleaned from the setresuid man page and the setreuid man page.

These functionalities are designed not as a security mechanism but to facilitate the intended operational flow, such as when a program adopts another user's identity by altering its effective user ID.

Notably, while setuid might be a common go-to for privilege elevation to root (since it aligns all IDs to root), differentiating between these functions is crucial for understanding and manipulating user ID behaviors in varying scenarios.

Program Execution Mechanisms in Linux

execve System Call

  • Functionality: execve initiates a program, determined by the first argument. It takes two array arguments, argv for arguments and envp for the environment.
  • Behavior: It retains the memory space of the caller but refreshes the stack, heap, and data segments. The program's code is replaced by the new program.
  • User ID Preservation:
    • ruid, euid, and supplementary group IDs remain unaltered.
    • euid might have nuanced changes if the new program has the SetUID bit set.
    • suid gets updated from euid post-execution.
  • Documentation: Detailed information can be found on the execve man page.

system Function

  • Functionality: Unlike execve, system creates a child process using fork and executes a command within that child process using execl.
  • Command Execution: Executes the command via sh with execl("/bin/sh", "sh", "-c", command, (char *) NULL);.
  • Behavior: As execl is a form of execve, it operates similarly but in the context of a new child process.
  • Documentation: Further insights can be obtained from the system man page.

Behavior of bash and sh with SUID

  • bash:
    • Has a -p option influencing how euid and ruid are treated.
    • Without -p, bash sets euid to ruid if they initially differ.
    • With -p, the initial euid is preserved.
    • More details can be found on the bash man page.
  • sh:
    • Does not possess a mechanism similar to -p in bash.
    • The behavior concerning user IDs is not explicitly mentioned, except under the -i option, emphasizing the preservation of euid and ruid equality.
    • Additional information is available on the sh man page.

These mechanisms, distinct in their operation, offer a versatile range of options for executing and transitioning between programs, with specific nuances in how user IDs are managed and preserved.

Testing User ID Behaviors in Executions

Examples taken from https://0xdf.gitlab.io/2022/05/31/setuid-rabbithole.html#testing-on-jail, check it for further information

Case 1: Using setuid with system

Objective: Understanding the effect of setuid in combination with system and bash as sh.

C Code:

#define _GNU_SOURCE
#include <stdlib.h>
#include <unistd.h>

int main(void) {
    setuid(1000);
    system("id");
    return 0;
}

Compilation and Permissions:

oxdf@hacky$ gcc a.c -o /mnt/nfsshare/a;
oxdf@hacky$ chmod 4755 /mnt/nfsshare/a
bash-4.2$ $ ./a
uid=99(nobody) gid=99(nobody) groups=99(nobody) context=system_u:system_r:unconfined_service_t:s0

Analysis:

  • ruid and euid start as 99 (nobody) and 1000 (frank) respectively.
  • setuid aligns both to 1000.
  • system executes /bin/bash -c id due to the symlink from sh to bash.
  • bash, without -p, adjusts euid to match ruid, resulting in both being 99 (nobody).

Case 2: Using setreuid with system

C Code:

#define _GNU_SOURCE
#include <stdlib.h>
#include <unistd.h>

int main(void) {
    setreuid(1000, 1000);
    system("id");
    return 0;
}

Compilation and Permissions:

oxdf@hacky$ gcc b.c -o /mnt/nfsshare/b; chmod 4755 /mnt/nfsshare/b

Execution and Result:

bash-4.2$ $ ./b
uid=1000(frank) gid=99(nobody) groups=99(nobody) context=system_u:system_r:unconfined_service_t:s0

Analysis:

  • setreuid sets both ruid and euid to 1000.
  • system invokes bash, which maintains the user IDs due to their equality, effectively operating as frank.

Case 3: Using setuid with execve

Objective: Exploring the interaction between setuid and execve.

#define _GNU_SOURCE
#include <stdlib.h>
#include <unistd.h>

int main(void) {
    setuid(1000);
    execve("/usr/bin/id", NULL, NULL);
    return 0;
}

Execution and Result:

bash-4.2$ $ ./c
uid=99(nobody) gid=99(nobody) euid=1000(frank) groups=99(nobody) context=system_u:system_r:unconfined_service_t:s0

Analysis:

  • ruid remains 99, but euid is set to 1000, in line with setuid's effect.

C Code Example 2 (Calling Bash):

#define _GNU_SOURCE
#include <stdlib.h>
#include <unistd.h>

int main(void) {
    setuid(1000);
    execve("/bin/bash", NULL, NULL);
    return 0;
}

Execution and Result:

bash-4.2$ $ ./d
bash-4.2$ $ id
uid=99(nobody) gid=99(nobody) groups=99(nobody) context=system_u:system_r:unconfined_service_t:s0

Analysis:

  • Although euid is set to 1000 by setuid, bash resets euid to ruid (99) due to the absence of -p.

C Code Example 3 (Using bash -p):

#define _GNU_SOURCE
#include <stdlib.h>
#include <unistd.h>

int main(void) {
    char *const paramList[10] = {"/bin/bash", "-p", NULL};
    setuid(1000);
    execve(paramList[0], paramList, NULL);
    return 0;
}

Execution and Result:

bash-4.2$ $ ./e
bash-4.2$ $ id
uid=99(nobody) gid=99(nobody) euid=100

References

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