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Date: Thu, 6 Oct 2022 16:13:37 +0200 From: Jann Horn <jannh@...gle.com> To: Christian Brauner <brauner@...nel.org> Cc: Kees Cook <keescook@...omium.org>, Eric Biederman <ebiederm@...ssion.com>, Jorge Merlino <jorge.merlino@...onical.com>, Alexander Viro <viro@...iv.linux.org.uk>, Thomas Gleixner <tglx@...utronix.de>, Andy Lutomirski <luto@...nel.org>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, John Johansen <john.johansen@...onical.com>, Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>, "Serge E. Hallyn" <serge@...lyn.com>, Stephen Smalley <stephen.smalley.work@...il.com>, Eric Paris <eparis@...isplace.org>, Richard Haines <richard_c_haines@...nternet.com>, Casey Schaufler <casey@...aufler-ca.com>, Xin Long <lucien.xin@...il.com>, "David S. Miller" <davem@...emloft.net>, Todd Kjos <tkjos@...gle.com>, Ondrej Mosnacek <omosnace@...hat.com>, Prashanth Prahlad <pprahlad@...hat.com>, Micah Morton <mortonm@...omium.org>, Fenghua Yu <fenghua.yu@...el.com>, Andrei Vagin <avagin@...il.com>, linux-kernel@...r.kernel.org, apparmor@...ts.ubuntu.com, linux-security-module@...r.kernel.org, selinux@...r.kernel.org, linux-hardening@...r.kernel.org Subject: Re: [PATCH 1/2] fs/exec: Explicitly unshare fs_struct on exec On Thu, Oct 6, 2022 at 11:05 AM Christian Brauner <brauner@...nel.org> wrote: > On Thu, Oct 06, 2022 at 01:27:34AM -0700, Kees Cook wrote: > > The check_unsafe_exec() counting of n_fs would not add up under a heavily > > threaded process trying to perform a suid exec, causing the suid portion > > to fail. This counting error appears to be unneeded, but to catch any > > possible conditions, explicitly unshare fs_struct on exec, if it ends up > > Isn't this a potential uapi break? Afaict, before this change a call to > clone{3}(CLONE_FS) followed by an exec in the child would have the > parent and child share fs information. So if the child e.g., changes the > working directory post exec it would also affect the parent. But after > this change here this would no longer be true. So a child changing a > workding directoro would not affect the parent anymore. IOW, an exec is > accompanied by an unshare(CLONE_FS). Might still be worth trying ofc but > it seems like a non-trivial uapi change but there might be few users > that do clone{3}(CLONE_FS) followed by an exec. I believe the following code in Chromium explicitly relies on this behavior, but I'm not sure whether this code is in active use anymore: https://source.chromium.org/chromium/chromium/src/+/main:sandbox/linux/suid/sandbox.c;l=101?q=CLONE_FS&sq=&ss=chromium
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