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Message-ID: <Y+15ZIVyiOWNnTZ8@yury-laptop> Date: Wed, 15 Feb 2023 16:31:32 -0800 From: Yury Norov <yury.norov@...il.com> To: Bruno Goncalves <bgoncalv@...hat.com> Cc: Networking <netdev@...r.kernel.org>, alan.maguire@...cle.com, Jakub Kicinski <kuba@...nel.org>, LKML <linux-kernel@...r.kernel.org>, CKI Project <cki-project@...hat.com>, Kees Cook <keescook@...omium.org>, Miguel Ojeda <ojeda@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com> Subject: Re: [6.2.0-rc7] BUG: KASAN: slab-out-of-bounds in hop_cmp+0x26/0x110 + Kees Cook <keescook@...omium.org> + Miguel Ojeda <ojeda@...nel.org> + Nick Desaulniers <ndesaulniers@...gle.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@...il.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. Thanks, Yury
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