On 26/02/2020 21:29, Jann Horn wrote:

On Mon, Feb 24, 2020 at 5:03 PM Mickaël Salaün [off-list ref] wrote:

quoted

+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.