Re: [6.2.0-rc7] BUG: KASAN: slab-out-of-bounds in hop_cmp+0x26/0x110

+ Kees Cook [off-list ref]
+ Miguel Ojeda [off-list ref]
+ Nick Desaulniers [off-list ref]

On Wed, Feb 15, 2023 at 09:24:52AM +0100, Bruno Goncalves wrote:

On Tue, 14 Feb 2023 at 15:32, Yury Norov [off-list ref] wrote:

quoted

On Tue, Feb 14, 2023 at 02:23:06PM +0100, Bruno Goncalves wrote:

quoted

Hello,

recently when testing kernel with debug options set from net-next [1]
and bpf-next [2] the following call trace happens:

Hi Bruno,

Thanks for report.

This looks weird, because the hop_cmp() spent for 3 month in -next till
now. Anyways, can you please share your NUMA configuration so I'll try
to reproduce the bug locally? What 'numactl -H' outputs?

Here is the output:

numactl -H
available: 4 nodes (0-3)
node 0 cpus: 0 1 2 3 4 5 6 7 32 33 34 35 36 37 38 39
node 0 size: 32063 MB
node 0 free: 31610 MB
node 1 cpus: 8 9 10 11 12 13 14 15 40 41 42 43 44 45 46 47
node 1 size: 32248 MB
node 1 free: 31909 MB
node 2 cpus: 16 17 18 19 20 21 22 23 48 49 50 51 52 53 54 55
node 2 size: 32248 MB
node 2 free: 31551 MB
node 3 cpus: 24 25 26 27 28 29 30 31 56 57 58 59 60 61 62 63
node 3 size: 32239 MB
node 3 free: 31468 MB
node distances:
node   0   1   2   3
  0:  10  21  31  21
  1:  21  10  21  31
  2:  31  21  10  21
  3:  21  31  21  10

Bruno

So, I was able to reproduce it, and it seems like a compiler issue.

The problem is that hop_cmp() calculates pointer to a previous hop
object unconditionally at the beginning of the function:

       struct cpumask **prev_hop = *((struct cpumask ***)b - 1);
 
Obviously, for the first hop, there's no such thing like a previous
one, and later in the code 'prev_hop' is used conditionally on that:

       k->w = (b == k->masks) ? 0 : cpumask_weight_and(k->cpus, prev_hop[k->node]);

To me the code above looks like it instructs the compiler to dereference
'b - 1' only if b != k->masks, i.e. when b is not the first hop. But GCC
does that unconditionally, which looks wrong.

If I defer dereferencing manually like in the snippet below, the kasan
warning goes away.

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 48838a05c008..5f297f81c574 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c

@@ -2081,14 +2081,14 @@ struct __cmp_key {

 static int hop_cmp(const void *a, const void *b)
 {
-       struct cpumask **prev_hop = *((struct cpumask ***)b - 1);
        struct cpumask **cur_hop = *(struct cpumask ***)b;
        struct __cmp_key *k = (struct __cmp_key *)a;

        if (cpumask_weight_and(k->cpus, cur_hop[k->node]) <= k->cpu)
                return 1;

-       k->w = (b == k->masks) ? 0 : cpumask_weight_and(k->cpus, prev_hop[k->node]);
+       k->w = (b == k->masks) ? 0 :
+               cpumask_weight_and(k->cpus, (*((struct cpumask ***)b - 1))[k->node]);
        if (k->w <= k->cpu)
                return 0;

I don't understand why GCC doesn't optimize out unneeded dereferencing.
It does that even if I replace ternary operator with if-else construction.
To me it looks like a compiler bug.

However, I acknowledge that I'm not a great expert in C standard, so
it's quite possible that there may be some rule that prevents from
doing such optimizations, even for non-volatile variables.

Adding compiler people. Guys, could you please clarify on that?
If it's my fault, I'll submit fix shortly.

Thanks,
Yury