Re: [PATCH 3/6] plugins/cache: Fixed a use-after-free bug with multithreaded usermode

[Mahmoud Mandour <ma.mandourr@gmail.com>]

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On Mon, Jul 19, 2021 at 11:48 AM Alex Bennée <alex.bennee@linaro.org> wrote:

>
> Mahmoud Mandour <ma.mandourr@gmail.com> writes:
>
> > Since callbacks may be interleaved because of multithreaded execution,
> > we should not make assumptions about `plugin_exit` either. The problem
> > with `plugin_exit` is that it frees shared data structures (caches and
> > `miss_ht` hash table). It should not be assumed that callbacks will not
> > be called after it and hence use already-freed data structures.
>
> What was your test case for this because I wonder if it would be worth
> coming up with one for check-tcg?


I think just any ad-hoc multithreaded execution will evoke the race pretty
much
consistently.


> From what I remember the race is
> in between preexit_cleanup and the eventual _exit/exit_group which nixes
> all other child threads. Maybe we should be triggering a more graceful
> unload here?
>

I think so. This remedies the bug for this particular plugin and I think
there
would be a better solution of course. However, I just can't ever get
plugin_exit
callback to be called more than once so I think that's probably not the
problem?

The problem is that we *use* the data in translation/mem_access/exec
callbacks
after a plugin_exit call is already called (this can be easily verified by
having a
boolean set to true once plugin_exit is called and then g_assert this
boolean is
false in the callbacks)


> > This is mitigated in this commit by synchronizing the call to
> > `plugin_exit` through locking to ensure execlusive access to data
> > structures, and NULL-ifying those data structures so that subsequent
> > callbacks can check whether the data strucutres are freed, and if so,
> > immediately exit.
> >
> > It's okay to immediately exit and don't account for those callbacks
> > since they won't be accounted for anyway since `plugin_exit` is already
> > called once and reported the statistics.
> >
> > Signed-off-by: Mahmoud Mandour <ma.mandourr@gmail.com>
> > ---
> >  contrib/plugins/cache.c | 31 ++++++++++++++++++++++++++++++-
> >  1 file changed, 30 insertions(+), 1 deletion(-)
> >
> > diff --git a/contrib/plugins/cache.c b/contrib/plugins/cache.c
> > index 695fb969dc..a452aba01c 100644
> > --- a/contrib/plugins/cache.c
> > +++ b/contrib/plugins/cache.c
> > @@ -363,6 +363,11 @@ static void vcpu_mem_access(unsigned int
> vcpu_index, qemu_plugin_meminfo_t info,
> >      effective_addr = hwaddr ? qemu_plugin_hwaddr_phys_addr(hwaddr) :
> vaddr;
> >
> >      g_mutex_lock(&mtx);
> > +    if (dcache == NULL) {
> > +        g_mutex_unlock(&mtx);
> > +        return;
> > +    }
> > +
> >      if (!access_cache(dcache, effective_addr)) {
> >          insn = (InsnData *) userdata;
> >          insn->dmisses++;
> > @@ -380,6 +385,11 @@ static void vcpu_insn_exec(unsigned int vcpu_index,
> void *userdata)
> >      g_mutex_lock(&mtx);
> >      insn_addr = ((InsnData *) userdata)->addr;
> >
> > +    if (icache == NULL) {
> > +        g_mutex_unlock(&mtx);
> > +        return;
> > +    }
> > +
> >      if (!access_cache(icache, insn_addr)) {
> >          insn = (InsnData *) userdata;
> >          insn->imisses++;
> > @@ -406,12 +416,24 @@ static void vcpu_tb_trans(qemu_plugin_id_t id,
> struct qemu_plugin_tb *tb)
> >              effective_addr = (uint64_t) qemu_plugin_insn_vaddr(insn);
> >          }
> >
> > +        g_mutex_lock(&mtx);
> > +
> > +        /*
> > +         * is the plugin_exit callback called? If so, any further
> callback
> > +         * registration is useless as it won't get accounted for after
> calling
> > +         * plugin_exit once already, and also will use miss_ht after
> it's freed
> > +         */
> > +        if (miss_ht == NULL) {
> > +            g_mutex_unlock(&mtx);
> > +            return;
> > +        }
> > +
> >          /*
> >           * Instructions might get translated multiple times, we do not
> create
> >           * new entries for those instructions. Instead, we fetch the
> same
> >           * entry from the hash table and register it for the callback
> again.
> >           */
> > -        g_mutex_lock(&mtx);
> > +
> >          data = g_hash_table_lookup(miss_ht,
> GUINT_TO_POINTER(effective_addr));
> >          if (data == NULL) {
> >              data = g_new0(InsnData, 1);
> > @@ -527,13 +549,20 @@ static void log_top_insns()
> >
> >  static void plugin_exit(qemu_plugin_id_t id, void *p)
> >  {
> > +    g_mutex_lock(&mtx);
> >      log_stats();
> >      log_top_insns();
> >
> >      cache_free(dcache);
> > +    dcache = NULL;
> > +
> >      cache_free(icache);
> > +    icache = NULL;
> >
> >      g_hash_table_destroy(miss_ht);
> > +    miss_ht = NULL;
> > +
> > +    g_mutex_unlock(&mtx);
> >  }
> >
> >  static void policy_init()
>
>
> --
> Alex Bennée
>

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