linux-kernel - Re: [PATCH v5 2/6] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount property

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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