linux-kernel - Re: [PATCH 1/1] mm/madvise: initialize prev pointer in madvise_walk

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Message-ID: <251d9ab2-96ce-47e9-9766-82a2eaf6196b@lucifer.local>
Date: Tue, 17 Jun 2025 10:26:44 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, 21cnbao@...il.com, david@...hat.com,
        Liam.Howlett@...cle.com, vbabka@...e.cz, jannh@...gle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Lance Yang <ioworker0@...il.com>
Subject: Re: [PATCH 1/1] mm/madvise: initialize prev pointer in
 madvise_walk_vmas

On Tue, Jun 17, 2025 at 05:21:03PM +0800, Lance Yang wrote:
>
>
> On 2025/6/17 16:50, Lorenzo Stoakes wrote:
> > Lace - To simplify and not get bogged down in sub-threads am replying at the top
> > level.

Firstly let me profusely apologise for calling you 'Lace' LOL! ;)

Busy morning and typos are happening...

> >
> > TL;DR this fix is incorrect, but the issue is correct :)
> >
> > So the patch at [0] introduced by Barry changed things in a way that _appears_
> > broken but in fact aren't, however we should do something about this, obviously.
> >
> > That patch added:
> >
> > 	if (madv_behavior && madv_behavior->lock_mode == MADVISE_VMA_READ_LOCK) {
> > 		vma = try_vma_read_lock(mm, madv_behavior, start, end);
> > 		if (vma) {
> > 			error = visit(vma, &prev, start, end, arg);
> > 			vma_end_read(vma);
> > 			return error;
> > 		}
> > 	}
> >
> > And the problem is, in this case, we don't initialise prev.
> >
> > In all other cases, we do (under mmap lock):
> >
> > 	vma = find_vma_prev(mm, start, &prev);
> > 	if (vma && start > vma->vm_start)
> > 		prev = vma;
> >
> > The reason this isn't a problem is that the only madvise operation that
> > currently supports this, madvise_dontneed_free() will initialise *prev = vma.
> >
> > BUT we really shouldn't be relying on this, so I attach a fixpatch.
> >
> > Given Barry's patch isn't mainline yet, I think this should just be squashed
> > into that as a fix?
> >
> > It kind of sucks to do this, but it resolves any potential bug.
> >
> > I think a follow up is needed, as there's an implicit assumption it seems that
> > prev is updated immediately for most callers, but of course anon_vma_name is a
> > special snowflake.
> >
> > todos++;
>
> Ah, please keep me in the loop ;)

Will do! Thanks for finding this issue, is appreciated!

>
> >
> > Lance - I suggest you reply to Barry's series with the below fix, or I can if
> > you prefer?
>
> Sure, go ahead!

Thanks will chase up!

Cheers, Lorenzo

>
> Thanks,
> Lance
>
> >
> > [0]: https://lore.kernel.org/linux-mm/20250607220150.2980-1-21cnbao@gmail.com/
> >
> > Thanks!
> >
> > On Tue, Jun 17, 2025 at 10:05:43AM +0800, Lance Yang wrote:
> > > From: Lance Yang <lance.yang@...ux.dev>
> > >
> > > The prev pointer was uninitialized, which could lead to undefined behavior
> > > where its address is taken and passed to the visit() callback without being
> > > assigned a value.
> > >
> > > Initializing it to NULL makes the code safer and prevents potential bugs
> > > if a future callback function attempts to read from it.
> > >
> > > Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> > > ---
> > >   mm/madvise.c | 4 ++--
> > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/mm/madvise.c b/mm/madvise.c
> > > index 267d8e4adf31..c87325000303 100644
> > > --- a/mm/madvise.c
> > > +++ b/mm/madvise.c
> > > @@ -1536,10 +1536,10 @@ int madvise_walk_vmas(struct mm_struct *mm, unsigned long start,
> > >   				   struct vm_area_struct **prev, unsigned long start,
> > >   				   unsigned long end, void *arg))
> > >   {
> > > +	struct vm_area_struct *prev = NULL;
> > >   	struct vm_area_struct *vma;
> > > -	struct vm_area_struct *prev;
> > > -	unsigned long tmp;
> > >   	int unmapped_error = 0;
> > > +	unsigned long tmp;
> > >   	int error;
> > >
> > >   	/*
> > > --
> > > 2.49.0
> > >
> >
> > ----8<----
> >  From c8dc9f5b2929e389cac44b79201fff43e0ab8195 Mon Sep 17 00:00:00 2001
> > From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > Date: Tue, 17 Jun 2025 09:46:27 +0100
> > Subject: [PATCH] fix
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > ---
> >   mm/madvise.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 267d8e4adf31..45ea4588e34e 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -1549,6 +1549,7 @@ int madvise_walk_vmas(struct mm_struct *mm, unsigned long start,
> >   	if (madv_behavior && madv_behavior->lock_mode == MADVISE_VMA_READ_LOCK) {
> >   		vma = try_vma_read_lock(mm, madv_behavior, start, end);
> >   		if (vma) {
> > +			*prev = vma;
> >   			error = visit(vma, &prev, start, end, arg);
> >   			vma_end_read(vma);
> >   			return error;
> > --
> > 2.49.0
>