linux-kernel - Re: [PATCH] s390/mm: Fix memory leak in add

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Message-ID: <CAH-r-ZG8vP=6qH42ew26BMBL9dRB3OtLUeFmMmKXzp1tnKvkxQ@mail.gmail.com>
Date: Mon, 27 Oct 2025 19:53:25 +0800
From: 林妙倩 <linmq006@...il.com>
To: Heiko Carstens <hca@...ux.ibm.com>
Cc: Alexander Gordeev <agordeev@...ux.ibm.com>, Gerald Schaefer <gerald.schaefer@...ux.ibm.com>, 
	Vasily Gorbik <gor@...ux.ibm.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Sven Schnelle <svens@...ux.ibm.com>, linux-s390@...r.kernel.org, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] s390/mm: Fix memory leak in add_marker() when kvrealloc fails

Hi, Heiko

Thank you for the feedback.

Heiko Carstens <hca@...ux.ibm.com> 于2025年10月27日周一 18:15写道：
>
> On Sun, Oct 26, 2025 at 05:13:51PM +0800, Miaoqian Lin wrote:
> > When kvrealloc() fails, the original markers memory is leaked
> > because the function directly assigns the NULL to the markers pointer,
> > losing the reference to the original memory.
> >
> > As a result, the kvfree() in pt_dump_init() ends up freeing NULL instead
> > of the previously allocated memory.
> >
> > Fix this by using a temporary variable to store kvrealloc()'s return
> > value and only update the markers pointer on success.
> >
> > Found via static anlaysis and this is similar to commit 42378a9ca553
> > ("bpf, verifier: Fix memory leak in array reallocation for stack state")
> >
> > Fixes: d0e7915d2ad3 ("s390/mm/ptdump: Generate address marker array dynamically")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Miaoqian Lin <linmq006@...il.com>
> > ---
> >  arch/s390/mm/dump_pagetables.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/s390/mm/dump_pagetables.c b/arch/s390/mm/dump_pagetables.c
> > index 9af2aae0a515..0f2e0c93a1e0 100644
> > --- a/arch/s390/mm/dump_pagetables.c
> > +++ b/arch/s390/mm/dump_pagetables.c
> > @@ -291,16 +291,19 @@ static int ptdump_cmp(const void *a, const void *b)
> >
> >  static int add_marker(unsigned long start, unsigned long end, const char *name)
> >  {
> > +     struct addr_marker *new_markers;
> >       size_t oldsize, newsize;
> >
> >       oldsize = markers_cnt * sizeof(*markers);
> >       newsize = oldsize + 2 * sizeof(*markers);
> >       if (!oldsize)
> > -             markers = kvmalloc(newsize, GFP_KERNEL);
> > +             new_markers = kvmalloc(newsize, GFP_KERNEL);
> >       else
> > -             markers = kvrealloc(markers, newsize, GFP_KERNEL);
> > -     if (!markers)
> > +             new_markers = kvrealloc(markers, newsize, GFP_KERNEL);
> > +     if (!new_markers)
> >               goto error;
> > +
> > +     markers = new_markers;
>
> This is not better to the situation before. If the allocation fails,
> markers_cnt will be set to zero, but the old valid markers pointer will stay,
> which means that the next call to add_marker() will allocate a new area via
> kvmalloc() instead of kvrealloc(), and thus leaking the old area too.
>
> add_marker() needs to changes to return in a manner that both marker and
> marker_cnt correlate with each other. And I guess it is also easily possible
> to get rid of the two different allocation paths.
>
> Care to send a new version?

I'm not sure if I can make it right.
Do you think this way can fix the leak correctly? Thanks.

```diff
static int add_marker(unsigned long start, unsigned long end, const char *name)
 {
-       size_t oldsize, newsize;
-
-       oldsize = markers_cnt * sizeof(*markers);
-       newsize = oldsize + 2 * sizeof(*markers);
-       if (!oldsize)
-               markers = kvmalloc(newsize, GFP_KERNEL);
-       else
-               markers = kvrealloc(markers, newsize, GFP_KERNEL);
-       if (!markers)
-               goto error;
+       struct addr_marker *new_markers;
+       size_t newsize;
+
+       newsize = (markers_cnt + 2) * sizeof(*markers);
+       new_markers = kvrealloc(markers, newsize, GFP_KERNEL);
+       if (!new_markers)
+               return -ENOMEM;
+
+       markers = new_markers;
        markers[markers_cnt].is_start = 1;
        markers[markers_cnt].start_address = start;
        markers[markers_cnt].size = end - start;
@@ -312,9 +311,6 @@ static int add_marker(unsigned long start,
unsigned long end, const char *name)
        markers[markers_cnt].name = name;
        markers_cnt++;
        return 0;
-error:
-       markers_cnt = 0;
-       return -ENOMEM;
 }