Date: Tue, 14 Nov 2017 08:13:43 -0800 From: Andy Lutomirski <luto@...nel.org> To: Alexander Popov <alex.popov@...ux.com> Cc: Andy Lutomirski <luto@...nel.org>, "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 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. > > 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)); Or you could force a double-fault. > > 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. > > Best regards, > Alexander
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