Re: [PATCH v2] alloc_tag: fix use-after-free in /proc/allocinfo after module unload

[Hao Ge]

Hi Suren


On 2026/6/5 03:24, Suren Baghdasaryan wrote:
> On Thu, Jun 4, 2026 at 12:00 AM Hao Ge <hao.ge@xxxxxxxxx> wrote:
> > allocinfo_start() only reinitializes the codetag iterator at position 0.
> > For subsequent reads (position > 0), it reuses cached iterator state from
> > the previous batch.  allocinfo_stop() drops mod_lock between read batches,
> > which allows module unload to complete and free the module memory that the
> > cached iterator still references:
> >
> >    CPU0 (read)                        CPU1 (rmmod)
> >    ----                               ----
> >    allocinfo_start(pos=0)
> >      down_read(mod_lock)
> >      allocinfo_show()
> >      ...
> >    allocinfo_stop()
> >      up_read(mod_lock)
> >                                       codetag_unload_module()
> >                                         kfree(cmod)
> >                                         release_module_tags()
> >                                       ...
> >                                       free_mod_mem()
> >    allocinfo_start(pos=N)
> >      down_read(mod_lock)
> >      // reuses cached iter, skips re-init
> >    allocinfo_show()
> >      ct->filename   <-- UAF
> >
> > After free_mod_mem() frees the module's .rodata, allocinfo_show()
> > dereferences ct->filename, ct->function which point there.
> >
> > Save the iterator state in allocinfo_next() and resume from it in
> > allocinfo_start() with codetag_next_ct(), which detects module removal
> > via idr_find() returning NULL and skips to the next module.
> >
> > Fixes: 9f44df50fee4 ("alloc_tag: keep codetag iterator active between read()")
> > Suggested-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > Signed-off-by: Hao Ge <hao.ge@xxxxxxxxx>
> Acked-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
>
> Thanks!
>
> > ---
> > v2:
> > - save the iterator state in allocinfo_next() and resume from it in
> >    allocinfo_start() with codetag_next_ct(), which detects module removal
> >    via idr_find() returning NULL and skips to the next module (Suggested
> >    by Suren).
> >    v1 link: https://lore.kernel.org/all/20260525072117.112779-1-hao.ge@xxxxxxxxx/
> > ---
> >   lib/alloc_tag.c | 9 +++++++--
> >   1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
> > index f2f574bcf383..551cc14bb1fd 100644
> > --- a/lib/alloc_tag.c
> > +++ b/lib/alloc_tag.c
> > @@ -45,6 +45,7 @@ int alloc_tag_ref_offs;
> >
> >   struct allocinfo_private {
> >          struct codetag_iterator iter;
> > +       struct codetag_iterator reported_iter;
> nit: I would call it maybe prev_iter since it's not yet reported (you
> store it in the allocinfo_next(), not in allocinfo_show()) but that's
> a minor detail.
I see...
> >          bool print_header;
> >   };
> >
> > @@ -58,16 +59,20 @@ static void *allocinfo_start(struct seq_file *m, loff_t *pos)
> >          if (node == 0) {
> >                  priv->print_header = true;
> >                  priv->iter = codetag_get_ct_iter(alloc_tag_cttype);
> > -               codetag_next_ct(&priv->iter);
> > +       } else {
> > +               priv->iter = priv->reported_iter;
> >          }
> > +       codetag_next_ct(&priv->iter);
> >          return priv->iter.ct ? priv : NULL;
> >   }
> >
> >   static void *allocinfo_next(struct seq_file *m, void *arg, loff_t *pos)
> >   {
> >          struct allocinfo_private *priv = (struct allocinfo_private *)arg;
> > -       struct codetag *ct = codetag_next_ct(&priv->iter);
> > +       struct codetag *ct;
> >
> > +       priv->reported_iter = priv->iter;
> > +       ct = codetag_next_ct(&priv->iter);

AFAIK, in the seq_file call flow (start -> show -> next -> show -> next 
-> ...):

start(0): iter = get_ct_iter(); next_ct() -> iter.ct = A

show():   dump entry with iter.ct = A

next():   reported_iter = iter(A); next_ct() -> iter.ct = B

show():   dump entry with iter.ct = B

next():   reported_iter = iter(B); next_ct() -> iter.ct = C

show():   dump entry with iter.ct = C

The same holds for the overflow case in the Fill loop:

next():   reported_iter = iter(C); next_ct() -> iter.ct = D

show():   overflow, output rolled back, break

stop()

on the next read:

start():  iter = reported_iter(C); next_ct() -> iter.ct = D

show():   dump entry with iter.ct = D

So reported_iter always points to the last element that has been

shown to the user.

I'm fine with prev_iter as well,  What do you think?


Thanks

Best Regards

Hao


> >          (*pos)++;
> >          if (!ct)
> >                  return NULL;
> > --
> > 2.25.1