Date: Tue, 14 Nov 2017 18:36:31 +0300 From: Alexander Popov <alex.popov@...ux.com> To: Andy Lutomirski <luto@...nel.org> Cc: kernel-hardening@...ts.openwall.com, keescook@...omium.org, pageexec@...email.hu, spender@...ecurity.net, Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 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@...nel.org Subject: Re: [PATCH RFC v5 2/5] gcc-plugins: Add STACKLEAK plugin for tracking the kernel stack 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. 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? 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? Best regards, Alexander
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