Re: [PATCH hotfix 6.12 v2 4/8] mm: resolve faulty mmap_region() error path behaviour

On Mon, Oct 28, 2024 at 09:05:44AM -1000, Linus Torvalds wrote:

On Mon, 28 Oct 2024 at 08:57, Lorenzo Stoakes
[off-list ref] wrote:

quoted

So likely hook on your mapping changes flags to set VM_MTE | VM_MTE_ALLOWED and
expects this to be checked after (ugh).

Gaah. Yes. mm/shmem.c: shmem_mmap() does

        /* arm64 - allow memory tagging on RAM-based files */
        vm_flags_set(vma, VM_MTE_ALLOWED);

and while I found the equivalent hack for the VM_SPARC_ADI case, I
hadn't noticed that MTE thing.

How very annoying.

So the arch_validate_flags() case does need to be done after the ->mmap() call.

How about just finalizing everything, and then doing a regular
munmap() afterwards and returning an error (all still holding the mmap
semaphore, of course).

That still avoids the whole "partially completed mmap" case.

             Linus

Yeah I was thinking the same... just bite the bullet, go through the whole damn
process and revert if arch_validate_flags() chokes. It also removes the ugly
#ifdef CONFIG_SPARC64 hack...

This will litearlly only be applicable for these two cases and (hopefully) most
of the time you'd not fail it.

I mean by then it'll be added into the rmap and such but nothing will be
populated yet and we shouldn't be able to fault as vma_start_write() should have
incremented the vma lock seqnum.

Any issues from the RCU visibility stuff Liam?

Any security problems Jann...?

It'd suck to have to bring back a partial complete case. Though I do note we
handle errors from mmap_file() ok so we could still potentially handle that
there, but would sort of semi-undo some of the point of the series.