On Wed, 14 Oct 2020 at 22:44, Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote: > This patchset is not complete (see particular TODOs in the last patch), > and I haven't performed any benchmarking yet, but I would like to start the > discussion now and hear people's opinions regarding the questions mentioned > below. > > === Overview > > This patchset adopts the existing hardware tag-based KASAN mode [1] for > use in production as a memory corruption mitigation. Hardware tag-based > KASAN relies on arm64 Memory Tagging Extension (MTE) [2] to perform memory > and pointer tagging. Please see [3] and [4] for detailed analysis of how > MTE helps to fight memory safety problems. > > The current plan is reuse CONFIG_KASAN_HW_TAGS for production, but add a > boot time switch, that allows to choose between a debugging mode, that > includes all KASAN features as they are, and a production mode, that only > includes the essentials like tag checking. > > It is essential that switching between these modes doesn't require > rebuilding the kernel with different configs, as this is required by the > Android GKI initiative [5]. > > The last patch of this series adds a new boot time parameter called > kasan_mode, which can have the following values: > > - "kasan_mode=on" - only production features > - "kasan_mode=debug" - all debug features > - "kasan_mode=off" - no checks at all (not implemented yet) > > Currently outlined differences between "on" and "debug": > > - "on" doesn't keep track of alloc/free stacks, and therefore doesn't > require the additional memory to store those > - "on" uses asyncronous tag checking (not implemented yet) > > === Questions > > The intention with this kind of a high level switch is to hide the > implementation details. Arguably, we could add multiple switches that allow > to separately control each KASAN or MTE feature, but I'm not sure there's > much value in that. > > Does this make sense? Any preference regarding the name of the parameter > and its values? KASAN itself used to be a debugging tool only. So introducing an "on" mode which no longer follows this convention may be confusing. Instead, maybe the following might be less confusing: "full" - current "debug", normal KASAN, all debugging help available. "opt" - current "on", optimized mode for production. "on" - automatic selection => chooses "full" if CONFIG_DEBUG_KERNEL, "opt" otherwise. "off" - as before. Also, if there is no other kernel boot parameter named "kasan" yet, maybe it could just be "kasan=..." ? > What should be the default when the parameter is not specified? I would > argue that it should be "debug" (for hardware that supports MTE, otherwise > "off"), as it's the implied default for all other KASAN modes. Perhaps we could make this dependent on CONFIG_DEBUG_KERNEL as above. I do not think that having the full/debug KASAN enabled on production kernels adds any value because for it to be useful requires somebody to actually look at the stacktraces; I think that choice should be made explicitly if it's a production kernel. My guess is that we'll save explaining performance differences and resulting headaches for ourselves and others that way. > Should we somehow control whether to panic the kernel on a tag fault? > Another boot time parameter perhaps? It already respects panic_on_warn, correct? > Any ideas as to how properly estimate the slowdown? As there's no > MTE-enabled hardware yet, the only way to test these patches is use an > emulator (like QEMU). The delay that is added by the emulator (for setting > and checking the tags) is different from the hardware delay, and this skews > the results. > > A question to KASAN maintainers: what would be the best way to support the > "off" mode? I see two potential approaches: add a check into each kasan > callback (easier to implement, but we still call kasan callbacks, even > though they immediately return), or add inline header wrappers that do the > same. [...] Thanks, -- Marco
