Date: Tue, 19 Sep 2017 18:34:44 +0200 From: Solar Designer <solar@...nwall.com> To: Salvatore Mesoraca <s.mesoraca16@...il.com> Cc: Kees Cook <keescook@...omium.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Alexander Viro <viro@...iv.linux.org.uk>, "Eric W. Biederman" <ebiederm@...ssion.com>, LKML <linux-kernel@...r.kernel.org>, "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org> Subject: Re: [RFC] Restrict writes into untrusted FIFOs and regular files On Tue, Sep 19, 2017 at 06:06:15PM +0200, Salvatore Mesoraca wrote: > 2017-09-19 2:37 GMT+02:00 Solar Designer <solar@...nwall.com>: > > On Mon, Sep 18, 2017 at 02:00:50PM -0700, Kees Cook wrote: > >> On Fri, Sep 15, 2017 at 1:43 AM, Salvatore Mesoraca <s.mesoraca16@...il.com> wrote: > >> > +protected_regular_files: > >> > + > >> > +This protection is similar to protected_fifos, but it > >> > +avoids writes to an attacker-controlled regular file, where program > >> > +expected to create one. > >> > + > >> > +When set to "0", regular files writing is unrestricted. > >> > + > >> > +When set to "1" don't allow O_CREAT open on regular files that we > >> > +don't own in world writable sticky directories, unless they are > >> > +owned by the owner of the directory. [...] > > Although this is sufficient against attacks (if the kernel's check for > > these properties is not racy; I don't know if it is), for the policy > > enforcement use case and reason we might want to support a simpler mode > > where O_CREAT without O_EXCL would be disallowed in sticky directories > > (only world writable? or also writable by anyone other than us? - e.g., > > it'd catch some unsafe uses of mail spools) regardless of whether a > > file of that name already exists or not. Maybe extra settings: > > > > When set to "2" also don't allow O_CREAT open without O_EXCL in > > world-writable sticky directories (even if the files don't already > > exist, for consistent policy enforcement) > > > > When set to "3" also don't allow O_CREAT open on regular files that we > > don't own in sticky directories writable by anyone else, unless the > > files are owned by the owner of the directory. > > > > When set to "4" also don't allow O_CREAT open without O_EXCL in > > sticky directories writable by anyone else (even if the files don't > > already exist, for consistent policy enforcement) > > > > Or maybe "2" and "4" should be a separate knob, so that "3" could be > > used without the policy enforcement aspect of "2", although enabling > > this separate knob at the highest level would make protected_regular > > redundant. > > > > I could envision further levels for non-sticky world-writable and > > writable-by-others directories, and even for unsafe writes without > > O_CREAT and unsafe reads, but then the protected_regular name would > > become even more misleading as without O_CREAT the program could > > actually intend to work with a non-regular file. > > > > Let's avoid further scope creep for now, but have this in mind. As I > > had mentioned in another thread on kernel-hardening, policy enforcement > > like this implemented in a kernel module helped me find weaknesses in > > old Postfix' privsep implementation, which were promptly patched (that > > was many years ago). Having this generally available and easy to enable > > could result in more findings like this by more people. > > > > A setting similar to "3" above should probably also exist for > > protected_fifos (would be "2" there). > > I think I could add "3" to both protected_fifos and protected_regulars > actually using "2" for both. And then add another sysctl for modes > "2" and "4" for both fifos and regular files. Sounds good to me. The third sysctl (or several) could be introduced with a separate patch, focusing on file access safety policy warnings and enforcement in general rather than on any specific file types. Alexander
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