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Date: Tue, 12 May 2020 14:09:41 -0700
From: Kees Cook <keescook@...omium.org>
To: Mickaël Salaün <mic@...ikod.net>
Cc: 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,
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 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.
>
> 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
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