linux-kernel - Re: [RFC PATCH v14 05/10] fs,landlock: Support filesystem access-control

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Date:   Thu, 27 Feb 2020 17:50:31 +0100
From:   Mickaël Salaün <mic@...ikod.net>
To:     Jann Horn <jannh@...gle.com>
Cc:     kernel list <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...capital.net>,
        Arnd Bergmann <arnd@...db.de>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        James Morris <jmorris@...ei.org>, Jann Horn <jann@...jh.net>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Mickaël Salaün <mickael.salaun@....gouv.fr>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Shuah Khan <shuah@...nel.org>,
        Vincent Dagonneau <vincent.dagonneau@....gouv.fr>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-doc@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [RFC PATCH v14 05/10] fs,landlock: Support filesystem
 access-control



On 26/02/2020 21:29, Jann Horn wrote:
> On Mon, Feb 24, 2020 at 5:03 PM Mickaël Salaün <mic@...ikod.net> wrote:
>> +static inline u32 get_mem_access(unsigned long prot, bool private)
>> +{
>> +       u32 access = LANDLOCK_ACCESS_FS_MAP;
>> +
>> +       /* Private mapping do not write to files. */
>> +       if (!private && (prot & PROT_WRITE))
>> +               access |= LANDLOCK_ACCESS_FS_WRITE;
>> +       if (prot & PROT_READ)
>> +               access |= LANDLOCK_ACCESS_FS_READ;
>> +       if (prot & PROT_EXEC)
>> +               access |= LANDLOCK_ACCESS_FS_EXECUTE;
>> +       return access;
>> +}
> 
> When I do the following, is landlock going to detect that the mmap()
> is a read access, or is it incorrectly going to think that it's
> neither read nor write?
> 
> $ cat write-only.c
> #include <fcntl.h>
> #include <sys/mman.h>
> #include <stdio.h>
> int main(void) {
>   int fd = open("/etc/passwd", O_RDONLY);
>   char *ptr = mmap(NULL, 0x1000, PROT_WRITE, MAP_PRIVATE, fd, 0);
>   printf("'%.*s'\n", 4, ptr);
> }
> $ gcc -o write-only write-only.c -Wall
> $ ./write-only
> 'root'
> $
> 

Thanks to the "if (!private && (prot & PROT_WRITE))", Landlock allows
this private mmap (as intended) even if there is no write access to this
file, but not with a shared mmap (and a file opened with O_RDWR). I just
added a test for this to be sure.

However, I'm not sure this hook is useful for now. Indeed, the process
still need to have a file descriptor open with the right accesses.