nodejs/node

fs.lchmod opens file with O_WRONLY unnecessarily, fails on directories and non-writable files

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#23,736 opened on Oct 18, 2018

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fshelp wanted

Description

  • Version: v10.12.0
  • Platform: Darwin geegaw.local 18.0.0 Darwin Kernel Version 18.0.0: Wed Aug 22 20:13:40 PDT 2018; root:xnu-4903.201.2~1/RELEASE_X86_64 x86_64
  • Subsystem: fs

https://github.com/isaacs/chmodr/pull/20

Node's fs.lchmod implementation opens the file in write-only mode. (On Darwin, at least.)

This is unnecessary, and fails for read-only files.

Additionally, this approach (open and then use fchmod) fails for directories. Is there a reason why native lchmod isn't being used? Systems that have O_SYMLINK also have lchmod(3), don't they?

This is important because lchmod is the only way to avoid a (minor) security vulnerability when doing recursive mode setting on directories. If we have to restrict the use of lchmod to only symlinks, then we're back in TOCTOU territory.

Contributor guide

Tech stack
javascriptcppnodejs
Domain
backendsecurity
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Node.js fs moduleC++ basicsUnix file permissions
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Examine Node.js source files lib/fs.js and src/node file.cc to understand current lchmod implementation. Investigate if native lchmod syscall is available on the platform. Consider modifying the code to open with O RDONLY or use lchmod directly. Validate fix on directories and read only files.