Date: Wed, 15 Nov 2017 00:50:36 +0300 From: Alexander Popov <alex.popov@...ux.com> To: Andy Lutomirski <luto@...nel.org> Cc: "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Kees Cook <keescook@...omium.org>, PaX Team <pageexec@...email.hu>, Brad Spengler <spender@...ecurity.net>, Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Tycho Andersen <tycho@...ker.com>, Laura Abbott <labbott@...hat.com>, Mark Rutland <mark.rutland@....com>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, Borislav Petkov <bp@...en8.de>, Thomas Gleixner <tglx@...utronix.de>, "H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org> Subject: Re: [PATCH RFC v5 2/5] gcc-plugins: Add STACKLEAK plugin for tracking the kernel stack Hello Andy, Thanks for your prompt reply! On 14.11.2017 19:13, Andy Lutomirski wrote: > On Tue, Nov 14, 2017 at 7:36 AM, Alexander Popov <alex.popov@...ux.com> wrote: >> On 30.10.2017 21:06, Alexander Popov wrote: >>> On 30.10.2017 20:32, Peter Zijlstra wrote: >>>> On Mon, Oct 30, 2017 at 07:51:33PM +0300, Alexander Popov wrote: >>>>> When the thread stack is exhausted, this BUG() is hit. But do_error_trap(), >>>>> which handles the exception, calls track_stack() itself again (since it is >>>>> instrumented by the gcc plugin). So this recursion proceeds with exhausting the >>>>> thread stack. >>>> >>>> Add a __attribute__((nostacktrack)) on it? >>> >>> Yes, I already tried some blacklisting in the plugin, but it didn't really help, >>> because: >>> >>> 1. there are other (more than 5) instrumented functions, that are called during >>> BUG() handling too; >>> >>> 2. decreasing CONFIG_STACKLEAK_TRACK_MIN_SIZE would add more instrumented >>> functions, which should be manually blacklisted (not good). >>> >>> I guess handling BUG() in another stack would be a solution. For example, Andy >>> Lutomirski calls handle_stack_overflow in the DOUBLEFAULT_STACK >>> (arch/x86/mm/fault.c). Should I do something similar? >> >> Hello Andy! May I ask your advice? >> >> When CONFIG_VMAP_STACK is disabled and STACKLEAK is enabled (for example, on >> x86_32), we need another way to detect stack depth overflow. That is the reason >> of having this BUG() in track_stack(). But it turns out to be recursive since >> track_stack() will be called again during BUG() handling. > > What does the STEAKLACK plugin actually do? I haven't followed this enough. I've just replied to Mark's explanation. >> We can avoid that recursion by handling oops in another stack. It looks similar >> to the way you call handle_stack_overflow() in arch/x86/mm/fault.c. But it seems >> that I can't reuse that code, am I right? > > You'd probably have to make 32-bit compatible, which means making a > 32-bit variant of this thingy: > > asm volatile ("movq %[stack], %%rsp\n\t" > "call handle_stack_overflow\n\t" > "1: jmp 1b" > : ASM_CALL_CONSTRAINT > : "D" ("kernel stack overflow (page fault)"), > "S" (regs), "d" (address), > [stack] "rm" (stack)); Hm, I don't have these pt_regs in track_stack(). That is why I think I can't reuse your handle_stack_overflow(). I guess manually crafting pt_regs will not be good-looking. > Or you could force a double-fault. Could you elaborate on that? The goal is to have a verbose oops message and kill the offending process (if we work on behalf of a process). Can I do that? >> How should I do it properly? >> >> By the way, you wrote that you have some entry code changes which conflict with >> STACKLEAK. May I ask for more details? > > It's this thing: > > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/log/?h=x86/entry_stack.wip > > and I'll probably drop the ".wip" from the name shortly. Wow, it's big. I'll look into it and maybe return with questions. Best regards, Alexander
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