[Bug 246412] Return EISDIR when reading a directory

Thu Jun 4 18:10:45 UTC 2020

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=246412

--- Comment #3 from commit-hook at freebsd.org ---
A commit references this bug:

Author: kevans
Date: Thu Jun  4 18:09:57 UTC 2020
New revision: 361798
URL: https://svnweb.freebsd.org/changeset/base/361798

Log:
  vfs: add restrictions to read(2) of a directory [1/2]

  Historically, we've allowed read() of a directory and some filesystems will
  accommodate (e.g. ufs/ffs, msdosfs). From the history department staffed by
  Warner: <<EOF

  pdp-7 unix seemed to allow reading directories, but they were weird, special
  things there so I'm unsure (my pdp-7 assembler sucks).

  1st Edition's sources are lost, mostly. The kernel allows it. The
  reconstructed sources from 2nd or 3rd edition read it though.

  V6 to V7 changed the filesystem format, and should have been a warning, but
  reading directories weren't materially changed.

  4.1b BSD introduced readdir because of UFS. UFS broke all directory reading
  programs in 1983. ls, du, find, etc all had to be rewritten. readdir() and
  friends were introduced here.

  SysVr3 picked up readdir() in 1987 for the AT&T fork of Unix. SysVr4 updated
  all the directory reading programs in 1988 because different filesystem
  types were introduced.

  In the 90s, these interfaces became completely ubiquitous as PDP-11s running
  V7 faded from view and all the folks that initially started on V7 upgraded
  to SysV. Linux never supported this (though I've not done the software
  archeology to check) because it has always had a pathological diversity of
  filesystems.
  EOF

  Disallowing read(2) on a directory has the side-effect of masking
  application bugs from relying on other implementation's behavior
  (e.g. Linux) of rejecting these with EISDIR across the board, but allowing
  it has been a vector for at least one stack disclosure bug in the past[0].

  By POSIX, this is implementation-defined whether read() handles directories
  or not. Popular implementations have chosen to reject them, and this seems
  sensible: the data you're reading from a directory is not structured in some
  unified way across filesystem implementations like with readdir(2), so it is
  impossible for applications to portably rely on this.

  With this patch, we will reject most read(2) of a dirfd with EISDIR. Users
  that know what they're doing can conscientiously set
  bsd.security.allow_read_dir=1 to allow read(2) of directories, as it has
  proven useful for debugging or recovery. A future commit will further limit
  the sysctl to allow only the system root to read(2) directories, to make it
  at least relatively safe to leave on for longer periods of time.

  While we're adding logic pertaining to directory vnodes to vn_io_fault, an
  additional assertion has also been added to ensure that we're not reaching
  vn_io_fault with any write request on a directory vnode. Such request would
  be a logical error in the kernel, and must be debugged rather than allowing
  it to potentially silently error out.

  Commented out shell aliases have been placed in root's chsrc/shrc to promote
  awareness that grep may become noisy after this change, depending on your
  usage.

  A tentative MFC plan has been put together to try and make it as trivial as
  possible to identify issues and collect reports; note that this will be
  strongly re-evaluated. Tentatively, I will MFC this knob with the default as
  it is in HEAD to improve our odds of actually getting reports. The future
  priv(9) to further restrict the sysctl WILL NOT BE MERGED BACK, so the knob
  will be a faithful reversion on stable/12. We will go into the merge
  acknowledging that the sysctl default may be flipped back to restore
  historical behavior at *any* point if it's warranted.

  [0] https://www.freebsd.org/security/advisories/FreeBSD-SA-19:10.ufs.asc

  PR:           246412
  Reviewed by:  mckusick, kib, emaste, jilles, cy, phk, imp (all previous)
  Reviewed by:  rgrimes (latest version)
  MFC after:    1 month (note the MFC plan mentioned above)
  Relnotes:     absolutely, but will amend previous RELNOTES entry
  Differential Revision:        https://reviews.freebsd.org/D24596

Changes:
  head/bin/csh/dot.cshrc
  head/bin/sh/dot.shrc
  head/lib/libc/sys/read.2
  head/sys/kern/vfs_vnops.c

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