On Wed, May 6, 2020 at 2:01 PM Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote: > > On Wed, 2020-05-06 at 11:37 +0200, 'Dmitry Vyukov' via kasan-dev wrote: > > On Wed, May 6, 2020 at 8:23 AM Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote: > > > > > This patchset improves KASAN reports by making them to have > > > > > call_rcu() call stack information. It is helpful for programmers > > > > > to solve use-after-free or double-free memory issue. > > > > > > > > > > The KASAN report was as follows(cleaned up slightly): > > > > > > > > > > BUG: KASAN: use-after-free in kasan_rcu_reclaim+0x58/0x60 > > > > > > > > > > Freed by task 0: > > > > > save_stack+0x24/0x50 > > > > > __kasan_slab_free+0x110/0x178 > > > > > kasan_slab_free+0x10/0x18 > > > > > kfree+0x98/0x270 > > > > > kasan_rcu_reclaim+0x1c/0x60 > > > > > rcu_core+0x8b4/0x10f8 > > > > > rcu_core_si+0xc/0x18 > > > > > efi_header_end+0x238/0xa6c > > > > > > > > > > First call_rcu() call stack: > > > > > save_stack+0x24/0x50 > > > > > kasan_record_callrcu+0xc8/0xd8 > > > > > call_rcu+0x190/0x580 > > > > > kasan_rcu_uaf+0x1d8/0x278 > > > > > > > > > > Last call_rcu() call stack: > > > > > (stack is not available) > > > > > > > > > > > > > > > Add new CONFIG option to record first and last call_rcu() call stack > > > > > and KASAN report prints two call_rcu() call stack. > > > > > > > > > > This option doesn't increase the cost of memory consumption. It is > > > > > only suitable for generic KASAN. > > > > > > > > I don’t understand why this needs to be a Kconfig option at all. If call_rcu() stacks are useful in general, then just always gather those information. How do developers judge if they need to select this option or not? > > > > > > Because we don't want to increase slub meta-data size, so enabling this > > > option can print call_rcu() stacks, but the in-use slub object doesn't > > > print free stack. So if have out-of-bound issue, then it will not print > > > free stack. It is a trade-off, see [1]. > > > > > > [1] https://bugzilla.kernel.org/show_bug.cgi?id=198437 > > > > Hi Walter, > > > > Great you are tackling this! > > > > I have the same general sentiment as Qian. I would enable this > > unconditionally because: > > > > 1. We still can't get both rcu stack and free stack. I would assume > > most kernel testing systems need to enable this (we definitely enable > > on syzbot). This means we do not have free stack for allocation > > objects in any reports coming from testing systems. Which greatly > > diminishes the value of the other mode. > > > > 2. Kernel is undertested. Introducing any additional configuration > > options is a problem in such context. Chances are that some of the > > modes are not working or will break in future. > > > > 3. That free stack actually causes lots of confusion and I never found > > it useful: > > https://bugzilla.kernel.org/show_bug.cgi?id=198425 > > If it's a very delayed UAF, either one may get another report for the > > same bug with not so delayed UAF, or if it's way too delayed, then the > > previous free stack is wrong as well. > > > > 4. Most users don't care that much about debugging tools to learn > > every bit of every debugging tool and spend time fine-tuning it for > > their context. Most KASAN users won't even be aware of this choice, > > and they will just use whatever is the default. > > > > 5. Each configuration option increases implementation complexity. > > > > What would have value is if we figure out how to make both of them > > work at the same time without increasing memory consumption. But I > > don't see any way to do this. > > > > I propose to make this the only mode. I am sure lots of users will > > find this additional stack useful, whereas the free stack is even > > frequently confusing. > > > > Ok. > If we want to have a default enabling it, but it should only work in > generic KASAN, because we need to get object status(allocation or > freeing) from shadow memory, tag-based KASAN can't do it. So we should > have a default enabling it in generic KASAN? Yes, let's do generic KASAN always memorizes rcu stack; tags KASAN never memorizes rcu stacks. No new configurations.
