Messages in this thread | | | Date | Wed, 15 Feb 2023 17:51:39 -0800 | From | Kees Cook <> | Subject | Re: [6.2.0-rc7] BUG: KASAN: slab-out-of-bounds in hop_cmp+0x26/0x110 |
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On February 15, 2023 4:31:32 PM PST, Yury Norov <yury.norov@gmail.com> wrote: >+ Kees Cook <keescook@chromium.org> >+ Miguel Ojeda <ojeda@kernel.org> >+ Nick Desaulniers <ndesaulniers@google.com> > >On Wed, Feb 15, 2023 at 09:24:52AM +0100, Bruno Goncalves wrote: >> On Tue, 14 Feb 2023 at 15:32, Yury Norov <yury.norov@gmail.com> wrote: >> > >> > On Tue, Feb 14, 2023 at 02:23:06PM +0100, Bruno Goncalves wrote: >> > > 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.
My understanding is that without getting inlined, the compiler cannot evaluate "b == k->masks" at compile time (if it can at all). So since the dereference is part of variable initialization, it's not executed later: it's executed at function entry.
Regardless, this whole function looks kind of hard to read. Why not fully expand it with the if/else logic and put any needed variables into the respective clauses? Then humans can read it and the compiler will optimize it down just as efficiently.
-Kees
-- Kees Cook
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