Re: [PATCH bpf v2] bpf: fix potential race in tail call compatibility check

[Lorenzo Bianconi <lorenzo.bianconi@redhat.com>]

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> On 10/25/21 4:28 PM, Lorenzo Bianconi wrote:
> > > Lorenzo noticed that the code testing for program type compatibility of
> > > tail call maps is potentially racy in that two threads could encounter a
> > > map with an unset type simultaneously and both return true even though they
> > > are inserting incompatible programs.
> > > 
> > > The race window is quite small, but artificially enlarging it by adding a
> > > usleep_range() inside the check in bpf_prog_array_compatible() makes it
> > > trivial to trigger from userspace with a program that does, essentially:
> > > 
> > >          map_fd = bpf_create_map(BPF_MAP_TYPE_PROG_ARRAY, 4, 4, 2, 0);
> > >          pid = fork();
> > >          if (pid) {
> > >                  key = 0;
> > >                  value = xdp_fd;
> > >          } else {
> > >                  key = 1;
> > >                  value = tc_fd;
> > >          }
> > >          err = bpf_map_update_elem(map_fd, &key, &value, 0);
> > > 
> > > While the race window is small, it has potentially serious ramifications in
> > > that triggering it would allow a BPF program to tail call to a program of a
> > > different type. So let's get rid of it by protecting the update with a
> > > spinlock. The commit in the Fixes tag is the last commit that touches the
> > > code in question.
> > > 
> > > v2:
> > > - Use a spinlock instead of an atomic variable and cmpxchg() (Alexei)
> > > 
> > > Fixes: 3324b584b6f6 ("ebpf: misc core cleanup")
> > > Reported-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
> > > Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> > > ---
> > >   include/linux/bpf.h   |  1 +
> > >   kernel/bpf/arraymap.c |  1 +
> > >   kernel/bpf/core.c     | 14 ++++++++++----
> > >   kernel/bpf/syscall.c  |  2 ++
> > >   4 files changed, 14 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > > index 020a7d5bf470..98d906176d89 100644
> > > --- a/include/linux/bpf.h
> > > +++ b/include/linux/bpf.h
> > > @@ -929,6 +929,7 @@ struct bpf_array_aux {
> > >   	 * stored in the map to make sure that all callers and callees have
> > >   	 * the same prog type and JITed flag.
> > >   	 */
> > > +	spinlock_t type_check_lock;
> > 
> > I was wondering if we can use a mutex instead of a spinlock here since it is
> > run from a syscall AFAIU. The only downside is mutex_lock is run inside
> > aux->used_maps_mutex critical section. Am I missing something?
> 
> Hm, potentially it could work, but then it's also 32 vs 4 extra bytes. There's
> also poke_mutex or freeze_mutex, but feels to hacky to 'generalize for reuse',
> so I think the spinlock in bpf_array_aux is fine.

I was wondering if in the future we would need to protect something not supported
by a spinlock but it is probably not the case. I am fine with the spinlock :)

Regards,
Lorenzo

> 
> > >   	enum bpf_prog_type type;
> > >   	bool jited;
> > >   	/* Programs with direct jumps into programs part of this array. */
> > > diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> > > index cebd4fb06d19..da9b1e96cadc 100644
> > > --- a/kernel/bpf/arraymap.c
> > > +++ b/kernel/bpf/arraymap.c
> > > @@ -1072,6 +1072,7 @@ static struct bpf_map *prog_array_map_alloc(union bpf_attr *attr)
> > >   	INIT_WORK(&aux->work, prog_array_map_clear_deferred);
> > >   	INIT_LIST_HEAD(&aux->poke_progs);
> > >   	mutex_init(&aux->poke_mutex);
> > > +	spin_lock_init(&aux->type_check_lock);
> 
> Just as a tiny nit, I would probably name it slightly different, since type_check_lock
> mainly refers to the type property but there's also jit vs non-jit and as pointed out
> there could be other extensions that need checking in future as well. Maybe 'compat_lock'
> would be a more generic one or just:
> 
>         struct {
>                 enum bpf_prog_type type;
>                 bool jited;
>                 spinlock_t lock;
>         } owner;
> 
> > >   	map = array_map_alloc(attr);
> > >   	if (IS_ERR(map)) {
> > > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > > index c1e7eb3f1876..9439c839d279 100644
> > > --- a/kernel/bpf/core.c
> > > +++ b/kernel/bpf/core.c
> > > @@ -1823,20 +1823,26 @@ static unsigned int __bpf_prog_ret0_warn(const void *ctx,
> > >   bool bpf_prog_array_compatible(struct bpf_array *array,
> > >   			       const struct bpf_prog *fp)
> > >   {
> > > +	bool ret;
> > > +
> > >   	if (fp->kprobe_override)
> > >   		return false;
> > > +	spin_lock(&array->aux->type_check_lock);
> > > +
> > >   	if (!array->aux->type) {
> > >   		/* There's no owner yet where we could check for
> > >   		 * compatibility.
> > >   		 */
> > >   		array->aux->type  = fp->type;
> > >   		array->aux->jited = fp->jited;
> > > -		return true;
> > > +		ret = true;
> > > +	} else {
> > > +		ret = array->aux->type  == fp->type &&
> > > +		      array->aux->jited == fp->jited;
> > >   	}
> > > -
> > > -	return array->aux->type  == fp->type &&
> > > -	       array->aux->jited == fp->jited;
> > > +	spin_unlock(&array->aux->type_check_lock);
> > > +	return ret;
> > >   }
> > >   static int bpf_check_tail_call(const struct bpf_prog *fp)
> > > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > > index 4e50c0bfdb7d..955011c7df29 100644
> > > --- a/kernel/bpf/syscall.c
> > > +++ b/kernel/bpf/syscall.c
> > > @@ -543,8 +543,10 @@ static void bpf_map_show_fdinfo(struct seq_file *m, struct file *filp)
> > >   	if (map->map_type == BPF_MAP_TYPE_PROG_ARRAY) {
> > >   		array = container_of(map, struct bpf_array, map);
> > > +		spin_lock(&array->aux->type_check_lock);
> > >   		type  = array->aux->type;
> > >   		jited = array->aux->jited;
> > > +		spin_unlock(&array->aux->type_check_lock);
> > >   	}
> > >   	seq_printf(m,
> > > -- 
> > > 2.33.0
> > > 
> 

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