Subject: 28.2; bookmark context strings in encrypted files

From: Gustavo Barros <[email-masked]>
Date: Fri, 16 Sep 2022 08:08:25 -0300

Hi All,

I guess this one is midway between a bug report and a feature request. 
I don't see that this is anyway against expected/documented behavior of 
involved libraries (bookmark.el, epg.el), but it is arguably a bad 
corner case of interaction between the two, which represents a (small) 
potential security issue.

Currently (Emacs 28.2), when setting a bookmark in a gpg encrypted file, 
part of the buffer is stored unencrypted as `front-context-string' and 
`rear-context-string' in the `bookmark-default-file' whenever 
`bookmark-search-size' is larger than 0, which by default is 16.

It could be argued that it is unwise to set a bookmark in this context. 
But, well, users do all kind of stuff.  Besides, Emacs provides no hint 
that this may be risky (as far as I can tell).  So it would be nice if 
Emacs would be a little more conservative here, and locally set 
`bookmark-search-size' to 0 in buffers visiting encrypted files.

I think it'd be overkill to provide a full reproduction recipe, since 
most of it would just be to set up environment (key etc.) for GPG.  But 
anyone who already has a setup and an encrypted file can reproduce the 
following simple steps (which I have tested in an .org.gpg file with 
`emacs -Q'):

Visit the encrypted file.  Set a bookmark with `bookmark-set' ("C-x r 
m") somewhere near a non-empty part of the buffer.  Save bookmarks with 
`bookmark-save'.  Inspect `bookmark-default-file' (by default 
"~/.emacs.d/bookmarks"), particularly `front-context-string' and 
`rear-context-string' of the pertinent bookmark, to find part of the 
original encrypted file stored there unencrypted.

Best regards,
Gustavo.

In GNU Emacs 28.2 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.20, 
cairo version 1.16.0)
 of 2022-09-12 built on gusbrs-laptop
Windowing system distributor 'The X.Org Foundation', version 
11.0.12013000
System Description: Linux Mint 20.3

Configured using:
 'configure --with-mailutils --with-xwidgets --with-native-compilation
 --without-compress-install'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES
NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF
TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM XWIDGETS GTK3 ZLIB

Important settings:
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Major mode: Lisp Interaction

Minor modes in effect:
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Memory information:
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 (symbols 48 20306 0)
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