Messages in this thread |  | | From | Dmitry Vyukov <> | Date | Tue, 21 Nov 2017 10:09:11 +0100 | Subject | Re: KASAN help, please (Re: [PATCH 00/16] Entry stuff, in decent shape now) |
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On Mon, Nov 20, 2017 at 10:44 PM, Andy Lutomirski <luto@kernel.org> wrote: > On Mon, Nov 20, 2017 at 9:07 AM, Andy Lutomirski <luto@kernel.org> wrote: >> This sets up stack switching, including for SYSCALL. I think it's >> in decent shape. >> >> Known issues: >> - KASAN is likely to be busted. This could be fixed either by teaching >> KASAN that cpu_entry_area contains valid stacks (I have no clue how >> to go about doing this) or by rigging up the IST entry code to switch >> RSP to point to the direct-mapped copy of the stacks before calling >> into non-KASAN-excluded C code. >> > > I tried to fix the KASAN issue, and I'm doing something wrong. I'm > building this tree: > > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry_stack&id=8319677bd04a1ab291ca71fe1da7aa023306e4a9 > > for 64 bits with KASAN on. The relevant commit is: > > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry_stack&id=a4bdb48c3469708b6b51e5ab90d27bf0c859000c > > If I run tools/testing/selftests/single_step_syscall_32, then the > kernel goes into lala land and infinite loops. The root cause seems > to we're hitting do_debug with RSP pointing into the fixmap, > specifically in the cpu_entry_area's exception stack, with a value of > roughly 0xffffffffff1bd108. The KASAN instrumentation in do_debug is > then getting a page fault. I think my KASAN setup code should be > populating the KASAN data there and, indeed, gdb seems to be able to > access the faulting address. So I'm confused.
Hi,
I don't have any great insights.
You have stack instrumentation turned on, right? And the fault happens on stack instrumentation? Stack instrumentation is turned on with gcc7+ I think. And as the result compiler adds redzones on stack and poisons/unpoisons shadow for them in function prologue/epilogue.
The fact that KASAN instrumentation faults, but gdb can access it sounds strange. KASAN instrumentation is no magic, it just does not a normal memory load. Please check exact faulting address. KASAN can do accesses with large offset from RSP.
Does the fault happen before/after kasan_early_init? Before that there is a different bootstrap shadow mapped by kasan_map_early_shadow.
Does the fault happen on read access or write access? Stack instrumentation does write into shadow, but some parts of shadow are mapped with a single read-only page. Can gdb write to that address?
Is it possible that the stack has overflowed? I see that we increase EXCEPTION_STACK_ORDER by order 1 under KASAN (from 4k page to 8k pages), but it may be not enough. Normal stacks are increased from 16k to 32k.
Last stupid question: why is it -1 here: FIX_CPU_ENTRY_AREA_BOTTOM = FIX_CPU_ENTRY_AREA_TOP + (CPU_ENTRY_AREA_PAGES * NR_CPUS) - 1, ? Say CPU_ENTRY_AREA_PAGES=1 (we need only 1 page) and NR_CPUS=1, then the increment will be 0, which looks wrong for any case (must be at least 1, right?).
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