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

["Mickaël Salaün" <mic@digikod.net>]

On 26/02/2020 21:29, Jann Horn wrote:
> On Mon, Feb 24, 2020 at 5:03 PM Mickaël Salaün <mic@digikod.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.