Re: [PATCH linux-next] alloc_tag: Fix boot failure due to NULL pointer dereference

[Suren Baghdasaryan]

On Fri, Sep 26, 2025 at 5:48 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 9/26/25 10:06, ranxiaokai627@xxxxxxx wrote:
> > From: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
> >
> > There is a boot failure when both CONFIG_DEBUG_KMEMLEAK and
> > CONFIG_MEM_ALLOC_PROFILING are enabled.
> >
> > BUG: kernel NULL pointer dereference, address: 0000000000000000
> > RIP: 0010:__alloc_tagging_slab_alloc_hook+0x181/0x2f0
> > Call Trace:
> >  kmem_cache_alloc_noprof+0x1c8/0x5c0
> >  __alloc_object+0x2f/0x290
> >  __create_object+0x22/0x80
> >  kmemleak_init+0x122/0x190
> >  mm_core_init+0xb6/0x160
> >  start_kernel+0x39f/0x920
> >  x86_64_start_reservations+0x18/0x30
> >  x86_64_start_kernel+0x104/0x120
> >  common_startup_64+0x12c/0x138
> >
> > In kmemleak, mem_pool_alloc() directly calls kmem_cache_alloc_noprof(),
> > as a result, the alloc_tag structure associated with object_cache is not
> > defined neither initialized. So current->alloc_tag is NULL,
> > leading to a null pointer dereference.
>
> Agree with Harry. This should be enough:
>
> "as a result, current->alloc_tag is NULL, leading to a null pointer
> dereference."

Yes, that's much more clear.

>
> > Move the checks for SLAB_NO_OBJ_EXT, SLAB_NOLEAKTRACE, and
> > __GFP_NO_OBJ_EXT to the parent function __alloc_tagging_slab_alloc_hook()
> > to fix this.
> >
> > Also this distinguishes the SLAB_NOLEAKTRACE case between the actual memory
> > allocation failures case, make CODETAG_FLAG_INACCURATE more accurate.

Awsome!

>
> Good point.
>
> > Fixes: b9e2f58ffb84 ("alloc_tag: mark inaccurate allocation counters in /proc/allocinfo output")
>
> That's in mm-stable so the fix should go there (probably too late to fold
> now) if it's to be in the merge window PR.
>
> > Signed-off-by: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

Thanks for the fix!

>
> > ---
> >  mm/slub.c | 18 +++++++++---------
> >  1 file changed, 9 insertions(+), 9 deletions(-)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 867a07260acf..09cbe580842c 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -2197,15 +2197,6 @@ prepare_slab_obj_exts_hook(struct kmem_cache *s, gfp_t flags, void *p)
> >  {
> >       struct slab *slab;
> >
> > -     if (!p)
> > -             return NULL;
> > -
> > -     if (s->flags & (SLAB_NO_OBJ_EXT | SLAB_NOLEAKTRACE))
> > -             return NULL;
> > -
> > -     if (flags & __GFP_NO_OBJ_EXT)
> > -             return NULL;
> > -
> >       slab = virt_to_slab(p);
> >       if (!slab_obj_exts(slab) &&
> >           alloc_slab_obj_exts(slab, s, flags, false)) {
> > @@ -2223,6 +2214,15 @@ __alloc_tagging_slab_alloc_hook(struct kmem_cache *s, void *object, gfp_t flags)
> >  {
> >       struct slabobj_ext *obj_exts;
> >
> > +     if (!object)
> > +             return;
> > +
> > +     if (s->flags & (SLAB_NO_OBJ_EXT | SLAB_NOLEAKTRACE))
> > +             return;
> > +
> > +     if (flags & __GFP_NO_OBJ_EXT)
> > +             return;
> > +
> >       obj_exts = prepare_slab_obj_exts_hook(s, flags, object);
> >       /*
> >        * Currently obj_exts is used only for allocation profiling.
>