Practice · Protection and Security: From File Permissions to Zero-Trust Edge

Q1

What is the difference between protection and security in OS terminology?

A Protection is about networks; security is about files. B They are identical. C Protection is the set of internal mechanisms controlling access to resources; security is the broader goal of defending the system against threats, including external attackers. D Security is hardware; protection is software.

Q2

A web server process runs as root "because it was easier". Which principle is violated, and why does it matter?

A Least privilege; if the server is compromised, the attacker gets full control of the machine B None C Defense in depth; it makes the server slower D Availability; root processes crash more

Q3

An access control list stores:

A The password hashes B For each object, the domains that can access it and their rights C For each domain, the objects it can access D The page table

Q4

Why is revocation easier with ACLs than with capabilities?

A ACLs are encrypted. B The rights are stored with the object, so removing an entry revokes access immediately; capabilities may have been copied to many holders. C ACLs are smaller. D Capabilities cannot be revoked at all.

Q5

After fd = open("grades.txt", O_RDONLY) succeeds, the file's permissions are changed to 000. Can the process still read through fd?

A Yes: permissions are checked at open time; the file descriptor acts like a capability for the rights granted then. B Only if it is root. C Only the first read succeeds. D No, every read re-checks the permissions.

Q6

What is the octal form of rwxr-x---?

A 770 B 750 C 640 D 754

Q7

A file is owned by ana (group staff) with mode 640. Ben is in group staff. Which operations can Ben perform?

A Nothing B Write only C Read only D Read and write

Q8

A file owned by ana has mode 044 (---r--r--). Can ana read it?

A Yes, the others bits allow it. B Yes, owners can always read. C Only if ana is in the group. D No: ana is the owner, so only the owner bits (---) are checked.

Q9

With umask 027, what permissions does a newly created directory get?

A 755 B 750 (rwxr-x---) C 640 D 027

Q10

Why is a setuid-root program a security risk?

A It disables ASLR. B It runs slower. C It runs with root privileges whatever user starts it, so any exploitable bug in it gives an ordinary user root access. D It cannot be executed.

Q11

A program only needs to listen on port 443. What is the least-privilege way to allow it on Linux?

A Make it setuid root B Disable the firewall C Run it as root D Grant it the CAP_NET_BIND_SERVICE capability and run it as a normal user

Q12

Bell–LaPadula levels: Public < Confidential < Secret. A Confidential analyst tries to write into a Public brochure. Allowed?

A Yes, writing down is harmless. B Only with the owner's permission. C Yes, because the analyst can read it. D No: "no write down" prevents leaking confidential information to a lower level.

Q13

In the same Bell–LaPadula system, can the Confidential analyst read the Secret exams?

A Only by writing first B Yes C Only on weekends D No: no read up

Q14

Under the Biba integrity model, which access is denied?

A An Untrusted browser writes a System file. B An Untrusted browser reads a System file. C A System installer writes System files. D A System process reads a System file.

Q15

What distinguishes mandatory access control (SELinux) from discretionary access control (Unix permissions)?

A Under MAC a system-wide policy decides and users (even owners, even root processes in a confined domain) cannot override it; under DAC, owners decide who gets access. B DAC only applies to networks. C There is no difference. D MAC is faster.

Q16

Why does each stored password hash use a random salt?

A So identical passwords have different hashes and precomputed tables (rainbow tables) cannot be reused across users or systems B To make hashing faster C To shorten the hash D To encrypt the password reversibly

Q17

How many possible 8-character passwords made of lowercase letters exist, and how long does an offline attacker at 10¹⁰ guesses/s need on average?

A 26 × 8 guesses; instantly B 10⁸; 1 day C 8²⁶; thousands of years D 26⁸ ≈ 2.1 × 10¹¹; about 10 seconds

Q18

Why do systems use deliberately slow password hashes (bcrypt, scrypt, Argon2)?

A Because fast hashes are insecure for files B A legitimate login computes one hash (a few ms is fine), while an attacker must compute billions; slowing each hash by 10⁶ slows the attack by 10⁶ C To save energy D To annoy users

Q19

Which OS/hardware defense makes injected code on the stack impossible to execute?

A NX/DEP (non-executable pages, enforced by a page-table bit) B Stack canaries C ASLR D umask

Q20

What does ASLR randomize, and why?

A The scheduling priorities B The order of disk requests C The addresses of the stack, heap and libraries in each process, so an attacker cannot predict where code or data lies D Passwords, to make them stronger

Q21

Which is the most effective protection against losing data to ransomware?

A A longer password on the file server only B Regular, tested backups that the infected machine cannot modify (offline or immutable copies) C RAID 6 D Faster disks

Q22

What does seccomp do for a sandboxed process?

A Gives it more CPU B Encrypts its memory C Restricts which system calls it may invoke, shrinking the kernel attack surface D Randomizes its addresses

Q23

Why are secure boot and a TPM especially important for edge servers?

A They replace passwords. B They are needed for Wi-Fi. C Edge servers are physically accessible; secure boot refuses tampered bootloaders/kernels and the TPM can attest the boot state before the server receives workloads or secrets. D They make servers boot faster.

Q24

What is the core idea of zero trust?

A Trust nobody, so allow no access B Trust only the cloud provider C Do not trust requests because of their network location; authenticate, authorize and encrypt every request, with least-privilege identities and continuous monitoring D Remove all passwords

Q25

Which authentication method best resists phishing and password-database theft?

A Passkeys / hardware security keys (public-key credentials bound to the device and to the website) B SMS codes only C Security questions D A longer password with special characters

Q26

What do trusted execution environments (Intel SGX/TDX, AMD SEV) protect against in the cloud?

A Weak passwords B A compromised OS/hypervisor or a curious cloud operator reading the protected code and data, which stay encrypted in memory C Network outages D Disk failures

Q27

Why do memory-safe languages (Java, Rust, Go) reduce OS-level security incidents?

A They encrypt all data automatically. B They disable the network. C They run only in the kernel. D They prevent buffer overflows and use-after-free bugs by construction (bounds checks, ownership or garbage collection), eliminating a major class of exploitable vulnerabilities.

Q28 Short answer

Compute, for each case, the octal mode or the decision: (a) rwxr-xr-- in octal; (b) the mode of a new file and a new directory with umask 022 and with umask 077; (c) ana (groups staff) writes a file ana:staff mode 640; (d) ben (groups staff) writes the same file; (e) eve (groups guests) reads it; (f) ana reads a file ana:staff mode 044. Explain (f).

Q29 Short answer

EdgeCampus stores student-account password hashes. An attacker steals the database. Compare (a) unsalted MD5 and (b) salted bcrypt (cost 12) in terms of what the attacker can do. Then compute the average cracking time for random 8-character passwords over 62 symbols with 10¹⁰ MD5 guesses/s and 10⁴ bcrypt guesses/s, and propose a password and authentication policy.

Q30 Short answer

Build a short threat model for EdgeCampus (phones with CampusAR, building edge servers, public cloud). Identify at least four threats (using STRIDE: Spoofing, Tampering, Repudiation, Information disclosure, Denial of service, Elevation of privilege) and, for each, an OS- or system-level countermeasure from this course.