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.