lkml.org 
[lkml]   [2020]   [Feb]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RFC PATCH v14 05/10] fs,landlock: Support filesystem access-control
From
Date


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.

\
 
 \ /
  Last update: 2020-02-27 17:53    [W:0.086 / U:24.032 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site