The permission model's security guarantees fall apart in the presence of
relative symbolic links. When an application attempts to create a
relative symlink, the permission model currently resolves the relative
path into an absolute path based on the process's current working
directory, checks whether the process has the relevant permissions, and
then creates the symlink using the absolute target path. This behavior
is plainly incorrect for two reasons:
1. The target path should never be resolved relative to the current
working directory. If anything, it should be resolved relative to the
symlink's location. (Of course, there is one insane exception to this
rule: on Windows, each process has a current working directory per
drive, and symlinks can be created with a target path relative to the
current working directory of a specific drive. In that case, the
relative path will be resolved relative to the current working
directory for the respective drive, and the symlink will be created
on disk with the resulting absolute path. Other relative symlinks
will be stored as-is.)
2. Silently creating an absolute symlink when the user requested a
relative symlink is wrong. The user may (or may not) rely on the
symlink being relative. For example, npm heavily relies on relative
symbolic links such that node_modules directories can be moved around
without breaking.
Because we don't know the user's intentions, we don't know if creating
an absolute symlink instead of a relative symlink is acceptable. This
patch prevents the faulty behavior by not (incorrectly) resolving
relative symlink targets when the permission model is enabled, and by
instead simply refusing the create any relative symlinks.
The fs APIs accept Uint8Array objects for paths to be able to handle
arbitrary file name charsets, however, checking whether such an object
represents a relative part in a reliable and portable manner is tricky.
Other parts of the permission model incorrectly convert such objects to
strings and then back to an Uint8Array (see 1f64147eb6),
however, for now, this bug fix will simply throw on non-string symlink
targets when the permission model is enabled. (The permission model
already breaks existing applications in various ways, so this shouldn't
be too dramatic.)
PR-URL: https://github.com/nodejs/node/pull/49156
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
