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Date: Thu, 14 May 2020 11:14:04 +0300
From: "Lev R. Oshvang ." <levonshe@...il.com>
To: Kees Cook <keescook@...omium.org>
Cc: Mickaël Salaün <mic@...ikod.net>,
linux-kernel@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>,
Alexei Starovoitov <ast@...nel.org>, Al Viro <viro@...iv.linux.org.uk>,
Andy Lutomirski <luto@...nel.org>, Christian Heimes <christian@...hon.org>,
Daniel Borkmann <daniel@...earbox.net>, Deven Bowers <deven.desai@...ux.microsoft.com>,
Eric Chiang <ericchiang@...gle.com>, Florian Weimer <fweimer@...hat.com>,
James Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>, Jann Horn <jannh@...gle.com>,
Jonathan Corbet <corbet@....net>, Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
Matthew Garrett <mjg59@...gle.com>, Matthew Wilcox <willy@...radead.org>,
Michael Kerrisk <mtk.manpages@...il.com>, Mickaël Salaün <mickael.salaun@....gouv.fr>,
Mimi Zohar <zohar@...ux.ibm.com>,
Philippe Trébuchet <philippe.trebuchet@....gouv.fr>,
Scott Shell <scottsh@...rosoft.com>,
Sean Christopherson <sean.j.christopherson@...el.com>, Shuah Khan <shuah@...nel.org>,
Steve Dower <steve.dower@...hon.org>, Steve Grubb <sgrubb@...hat.com>,
Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
Vincent Strubel <vincent.strubel@....gouv.fr>, kernel-hardening@...ts.openwall.com,
linux-api@...r.kernel.org, linux-integrity@...r.kernel.org,
LSM List <linux-security-module@...r.kernel.org>, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v5 2/6] fs: Add a MAY_EXECMOUNT flag to infer the noexec
mount property
On Wed, May 13, 2020 at 12:09 AM Kees Cook <keescook@...omium.org> wrote:
>
> On Tue, May 05, 2020 at 05:31:52PM +0200, Mickaël Salaün wrote:
> > This new MAY_EXECMOUNT flag enables to check if the underlying mount
> > point of an inode is marked as executable. This is useful to implement
> > a security policy taking advantage of the noexec mount option.
> >
Security policy is expressed by sysadmin by mount -noexec very clear,
I don't think there is a need
in sysctl, wish is system-wide
> > This flag is set according to path_noexec(), which checks if a mount
> > point is mounted with MNT_NOEXEC or if the underlying superblock is
> > SB_I_NOEXEC.
> >
> > Signed-off-by: Mickaël Salaün <mic@...ikod.net>
> > Reviewed-by: Philippe Trébuchet <philippe.trebuchet@....gouv.fr>
> > Reviewed-by: Thibaut Sautereau <thibaut.sautereau@....gouv.fr>
> > Cc: Aleksa Sarai <cyphar@...har.com>
> > Cc: Al Viro <viro@...iv.linux.org.uk>
> > Cc: Kees Cook <keescook@...omium.org>
> > ---
> > fs/namei.c | 2 ++
> > include/linux/fs.h | 2 ++
> > 2 files changed, 4 insertions(+)
> >
> > diff --git a/fs/namei.c b/fs/namei.c
> > index a320371899cf..33b6d372e74a 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -2849,6 +2849,8 @@ static int may_open(const struct path *path, int acc_mode, int flag)
> > break;
> > }
> >
> > + /* Pass the mount point executability. */
> > + acc_mode |= path_noexec(path) ? 0 : MAY_EXECMOUNT;
> > error = inode_permission(inode, MAY_OPEN | acc_mode);
> > if (error)
> > return error;
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 313c934de9ee..79435fca6c3e 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -103,6 +103,8 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
> > #define MAY_NOT_BLOCK 0x00000080
> > /* the inode is opened with O_MAYEXEC */
> > #define MAY_OPENEXEC 0x00000100
> > +/* the mount point is marked as executable */
> > +#define MAY_EXECMOUNT 0x00000200
> >
> > /*
> > * flags in file.f_mode. Note that FMODE_READ and FMODE_WRITE must correspond
>
> I find this name unintuitive, but I cannot think of anything better,
> since I think my problem is that "MAY" doesn't map to the language I
> want to use to describe what this flag is indicating.
>
> Reviewed-by: Kees Cook <keescook@...omium.org>
>
> --
> Kees Cook
I think that the original patch was perfect, I quite it again
@@ -3167,6 +3167,14 @@ static int may_open(struct path *path, int
acc_mode, int flag)
+
+ if ((acc_mode & MAY_OPENEXEC)
+ && (S_ISREG(inode->i_mode) || S_ISDIR(inode->i_mode))
+ && (path->mnt && (path->mnt->mnt_flags & MNT_NOEXEC)))
+ return -EACCES;
+
+
+
error = inode_permission(inode, MAY_OPEN | acc_mode);
As I said in the inline comment above, sysadmin had already express
security policy in a very clear way,
mount -noexec !
I would only check inside inode_permission() whether the file mode is
any ---x permission and deny such
open when file is opened with O_MAYEXEC under MNT_NOEXEC mount point
New sysctl is indeed required to allow userspace that places scripts
or libs under noexec mounts.
fs.mnt_noexec_strict =0 (allow, e) , 1 (deny any file with --x
permission), 2 (deny when O_MAYEXEC absent), for any file with ---x
permissions)
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