Date: Fri, 6 Jan 2012 01:58:08 -0800 From: Andrew Morton <akpm@...ux-foundation.org> To: Ingo Molnar <mingo@...e.hu> Cc: Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org, Alexander Viro <viro@...iv.linux.org.uk>, Rik van Riel <riel@...hat.com>, Federica Teodori <federica.teodori@...glemail.com>, Lucian Adrian Grijincu <lucian.grijincu@...il.com>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Eric Paris <eparis@...hat.com>, Randy Dunlap <rdunlap@...otime.net>, Dan Rosenberg <drosenberg@...curity.com>, linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org, kernel-hardening@...ts.openwall.com Subject: Re: [PATCH v2012.1] fs: symlink restrictions on sticky directories On Fri, 6 Jan 2012 10:43:40 +0100 Ingo Molnar <mingo@...e.hu> wrote: > > * Andrew Morton <akpm@...ux-foundation.org> wrote: > > > > +config PROTECTED_STICKY_SYMLINKS > > > + bool "Protect symlink following in sticky world-writable directories" > > > + default y > > > + help > > > + A long-standing class of security issues is the symlink-based > > > + time-of-check-time-of-use race, most commonly seen in > > > + world-writable directories like /tmp. The common method of > > > + exploitation of this flaw is to cross privilege boundaries > > > + when following a given symlink (i.e. a root process follows > > > + a malicious symlink belonging to another user). > > > + > > > + Enabling this solves the problem by permitting symlinks to be > > > + followed only when outside a sticky world-writable directory, > > > + or when the uid of the symlink and follower match, or when > > > + the directory and symlink owners match. > > > > This is all quite misleading. One would expect that > > CONFIG_PROTECTED_STICKY_SYMLINKS turns the entire feature on > > or off permanently. ie, it controls whether the code is > > generated into vmlinux in the usual fashion. But it's not > > that at all - the user gets the feature whether or not he > > wants it, and this variable only sets the initial value. > > > > Why are we forcing the user to have the feature if he doesn't > > want it, btw? > > Basing on the (not yet fully confirmed) assertion that no apps > are broken by this change but exploits, I'd argue that this is > actually the sane and correct semantics for symlinks - i.e. we > want this to be the default Linux behavior - not just a > 'feature'. > > That way the configuration knobs are compat settings in essence > - in case some app cares. > > If people disagree and want it default off and as a separate > feature then it has to be modularized out some more. There's > notable silence from VFS folks on all this so Kees made an > educated guess. It might be wrong. Maybe true for a general purpose computer, but someone who is making a single-purpose device such as a digital TV or a wifi router won't want it. > > And we appear to be enabling the feature if CONFIG_PROC_FS=n, > > which might not be terribly useful? > > It can still be useful if it's default on - just cannot be > turned off via /proc/sys/, right? > > The combination that is not so useful is when it's off and > there's !PROC_FS. If it's a compat feature then i wouldnt bother > about it. If it's a separated out modular feature in a separate > .c file then it can all be properly shaped via Kconfig > dependencies. Spose so. I'd have thought the way to configure this feature would be to have CONFIG_PROTECTED_STICKY_SYMLINKS to control the code generation then a 0 or 1 CONFIG_PROTECTED_STICKY_SYMLINKS_ENABLED to control the initial setting.
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