linux-kernel - Re: [PATCH hotfix 6.12 2/8] mm: unconditionally close VMAs on error

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Message-ID: <eae8bc95-8da6-4af4-baef-a678e27f4665@lucifer.local>
Date: Wed, 23 Oct 2024 17:58:05 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Jann Horn <jannh@...gle.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH hotfix 6.12 2/8] mm: unconditionally close VMAs on error

On Wed, Oct 23, 2024 at 11:24:40AM +0200, Vlastimil Babka wrote:
> On 10/22/24 22:40, Lorenzo Stoakes wrote:
> > Incorrect invocation of VMA callbacks when the VMA is no longer in a
> > consistent state is bug prone and risky to perform.
> >
> > With regards to the important vm_ops->close() callback We have gone to
> > great lengths to try to track whether or not we ought to close VMAs.
> >
> > Rather than doing so and risking making a mistake somewhere, instead
> > unconditionally close and reset vma->vm_ops to an empty dummy operations
> > set with a NULL .close operator.
> >
> > We introduce a new function to do so - vma_close() - and simplify existing
> > vms logic which tracked whether we needed to close or not.
> >
> > This simplifies the logic, avoids incorrect double-calling of the .close()
> > callback and allows us to update error paths to simply call vma_close()
> > unconditionally - making VMA closure idempotent.
> >
> > Reported-by: Jann Horn <jannh@...gle.com>
> > Fixes: deb0f6562884 ("mm/mmap: undo ->mmap() when arch_validate_flags() fails")
> > Cc: stable <stable@...nel.org>
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> Nice simplification. Nit below.

Thanks!

>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
>
> > +/*
> > + * Unconditionally close the VMA if it has a close hook and prevent hooks from
> > + * being invoked after close. VMA hooks are mutated.
> > + */
> > +static inline void vma_close(struct vm_area_struct *vma)
> > +{
> > +	if (vma->vm_ops && vma->vm_ops->close) {
> > +		vma->vm_ops->close(vma);
> > +
> > +		/*
> > +		 * The mapping is in an inconsistent state, and no further hooks
> > +		 * may be invoked upon it.
> > +		 */
> > +		vma->vm_ops = &vma_dummy_vm_ops;
> > +	}
>
> Nit: if we want to "prevent hooks" as in "any hooks" then we should be
> replacing existing vm_ops even if it has no close hook? If it's enough to
> prevent further close() hooks (as commit log suggests) then the
> implementation is fine but the comment might be misleading.

We prevent hooks _after close_, if it has no close, then no, but I'll update the
comment to be crystal clear.

>
> > +}
> > +
> >  #ifdef CONFIG_MMU
> >
> >  /* Flags for folio_pte_batch(). */
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 10f4ccaf491b..d55c58e99a54 100644