linux-kernel - Re: [PATCH hotfix 6.12 v2 4/8] mm: resolve faulty mmap

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Message-ID: <bea02efe-a695-49e0-b15c-2270a82cadbf@lucifer.local>
Date: Mon, 28 Oct 2024 20:17:39 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mark Brown <broonie@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Peter Xu <peterx@...hat.com>, linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Aishwarya TCV <Aishwarya.TCV@....com>
Subject: Re: [PATCH hotfix 6.12 v2 4/8] mm: resolve faulty mmap_region()
 error path behaviour

On Mon, Oct 28, 2024 at 04:00:29PM -0400, Liam R. Howlett wrote:
> * Liam R. Howlett <Liam.Howlett@...cle.com> [241028 15:50]:
> > * Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [241028 15:14]:
> > > On Mon, Oct 28, 2024 at 09:05:44AM -1000, Linus Torvalds wrote:
> > > > On Mon, 28 Oct 2024 at 08:57, Lorenzo Stoakes
> > > > <lorenzo.stoakes@...cle.com> wrote:
> > > > >
> > > > > 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?
> >
> > It is probably fine?  We would see a mapping appear then disappear.
> > We'd have a (benign) race with rmap for truncating the PTEs (but it's
> > safe).  Page faults would be stopped though.
> >
> > Unfortunately, we'd have to write to the vma tree so that we could...
> > write to the vma tree.  We'd have to somehow ensure munmap() is done
> > with a gfp flag to ensure no failures as well...
> >
> > Maybe we should just call close on the vma again (and do whatever
> > call_mmap() needs to undo)?
>
> I take it back, that won't work.
>
> >
> > >
> > > 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.

I'm genuinely not opposed to a horrible, awful:

#ifdef CONFIG_ARM64
	if (file && file->f_ops == shmem_file_operations)
		vm_flags |= VM_MTE_ALLOWED;
#endif

Early in the operation prior to the arch_validate_flags() check.

Just to get this over the finish line (original idea credit to Vlastimil, insane
ugliness credit to me).

Could be in a __arch_workarounds() type function heavily commented...

I mean this is pretty gross but...