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 1:08 PM Alex Bennée <alex.bennee@linaro.org> wrote:

>
> Mahmoud Mandour <ma.mandourr@gmail.com> writes:
>
> > 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.
>
> I haven't managed to trigger it with testthread but of course my
> libcache is trying to to defend against it.
>

https://pastebin.com/X4Xazemh
That's a minimal program that reproduces the bug consistently for me (to my
observation, a simple
program that creates a bunch of threads that immediately exit does not
produce the bug as frequently)

You can also make hotblocks produce a similar crash (but make sure to add a
g_hash_table_destroy(hotblocks)
at the end of plugin_exit.)


>
> >
> >  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)
>
> We have mechanisms for safely unloading plugins during running so I
> think we should be able to do something cleanly here. I'll cook up an
> RFC.
>

I'll be waiting for it. Note that as I think I mentioned in the cover
letter, it's that plugin_uninstall
is probably problematic since it unregisters callbacks but already-fired
callbacks will continue executing.
So calling plugin_uninstall at the end of plugin_exit does not relieve the
bug...


>
> >
> >  > 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
>
>
> --
> Alex Bennée
>

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