On Fri, Apr 21, 2023 at 11:01:26AM +0200, Jan Kara wrote:
> Hi!
>
> On Mon 03-04-23 23:28:29, Lorenzo Stoakes wrote:
> > This patch series is in two parts:-
> >
> > 1. Currently there are a number of places in the kernel where we assume
> > VM_SHARED implies that a mapping is writable. Let's be slightly less
> > strict and relax this restriction in the case that VM_MAYWRITE is not
> > set.
> >
> > This should have no noticeable impact as the lack of VM_MAYWRITE implies
> > that the mapping can not be made writable via mprotect() or any other
> > means.
> >
> > 2. Align the behaviour of F_SEAL_WRITE and F_SEAL_FUTURE_WRITE on mmap().
> > The latter already clears the VM_MAYWRITE flag for a sealed read-only
> > mapping, we simply extend this to F_SEAL_WRITE too.
> >
> > For this to have effect, we must also invoke call_mmap() before
> > mapping_map_writable().
> >
> > As this is quite a fundamental change on the assumptions around VM_SHARED
> > and since this causes a visible change to userland (in permitting read-only
> > shared mappings on F_SEAL_WRITE mappings), I am putting forward as an RFC
> > to see if there is anything terribly wrong with it.
>
> So what I miss in this series is what the motivation is. Is it that you need
> to map F_SEAL_WRITE read-only? Why?
>
This originated from the discussion in [1], which refers to the bug
reported in [2]. Essentially the user is write-sealing a memfd then trying
to mmap it read-only, but receives an -EPERM error.
F_SEAL_FUTURE_WRITE _does_ explicitly permit this but F_SEAL_WRITE does not.
The fcntl() man page states:
Furthermore, trying to create new shared, writable memory-mappings via
mmap(2) will also fail with EPERM.
So the kernel does not behave as the documentation states.
I took the user-supplied repro and slightly modified it, enclosed
below. After this patch series, this code works correctly.
I think there's definitely a case for the VM_MAYWRITE part of this patch
series even if the memfd bits are not considered useful, as we do seem to
make the implicit assumption that MAP_SHARED == writable even if
!VM_MAYWRITE which seems odd.
Reproducer:-
int main()
{
int fd = memfd_create("test", MFD_ALLOW_SEALING);
if (fd == -1) {
perror("memfd_create");
return EXIT_FAILURE;
}
write(fd, "test", 4);
if (fcntl(fd, F_ADD_SEALS, F_SEAL_WRITE) == -1) {
perror("fcntl");
return EXIT_FAILURE;
}
void *ret = mmap(NULL, 4, PROT_READ, MAP_SHARED, fd, 0);
if (ret == MAP_FAILED) {
perror("mmap");
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}
[1]:https://lore.kernel.org/all/20230324133646.16101dfa666f253c4715d965@xxxxxxxxxxxxxxxxxxxx/
[2]:https://bugzilla.kernel.org/show_bug.cgi?id=217238
> Honza
> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR
