On Mon, May 01, 2023 at 12:02:00PM -0700, Andy Lutomirski wrote: > On Sun, Apr 30, 2023 at 3:26 PM Lorenzo Stoakes <lstoakes@xxxxxxxxx> wrote: > > > > In order for a F_SEAL_WRITE sealed memfd mapping to have an opportunity to > > clear VM_MAYWRITE, we must be able to invoke the appropriate vm_ops->mmap() > > handler to do so. We would otherwise fail the mapping_map_writable() check > > before we had the opportunity to avoid it. > > Is there any reason this can't go before patch 3? I don't quite understand what you mean by this? I mean sure, we could, but intent was to build to this point and leave the most controversial change for last :) > > If I'm understanding correctly, a comment like the following might > make this a lot more comprehensible: > > > > > This patch moves this check after the call_mmap() invocation. Only memfd > > actively denies write access causing a potential failure here (in > > memfd_add_seals()), so there should be no impact on non-memfd cases. > > > > This patch makes the userland-visible change that MAP_SHARED, PROT_READ > > mappings of an F_SEAL_WRITE sealed memfd mapping will now succeed. > > > > Link: https://bugzilla.kernel.org/show_bug.cgi?id=217238 > > Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx> > > --- > > mm/mmap.c | 12 ++++++------ > > 1 file changed, 6 insertions(+), 6 deletions(-) > > > > diff --git a/mm/mmap.c b/mm/mmap.c > > index 646e34e95a37..1608d7f5a293 100644 > > --- a/mm/mmap.c > > +++ b/mm/mmap.c > > @@ -2642,17 +2642,17 @@ unsigned long mmap_region(struct file *file, unsigned long addr, > > vma->vm_pgoff = pgoff; > > > > if (file) { > > - if (is_shared_maywrite(vm_flags)) { > > - error = mapping_map_writable(file->f_mapping); > > - if (error) > > - goto free_vma; > > - } > > - > > vma->vm_file = get_file(file); > > error = call_mmap(file, vma); > > if (error) > > goto unmap_and_free_vma; > > > > /* vm_ops->mmap() may have changed vma->flags. Check for writability now. */ > Ack, will add on next spin. > > + if (vma_is_shared_maywrite(vma)) { > > + error = mapping_map_writable(file->f_mapping); > > + if (error) > > + goto close_and_free_vma; > > + } > > + > > Alternatively, if anyone is nervous about the change in ordering here, > there could be a whole new vm_op like adjust_vma_flags() that happens > before any of this. Agreed, clearly this change is the most controversial thing here. I did look around and couldn't find any instance where this could cause an issue, since it is purely the mapping_map_writable() that gets run at a different point, but this is certainly an alterative. I have a feeling people might find adding a new op there possibly _more_ nerve-inducing :) but it's an option. > > --Andy
