Date: Thu, 21 Sep 2017 16:26:03 +0300 From: Alexander Popov <alex.popov@...ux.com> To: Tycho Andersen <tycho@...ker.com> Cc: Kees Cook <keescook@...omium.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, PaX Team <pageexec@...email.hu>, Brad Spengler <spender@...ecurity.net>, Laura Abbott <labbott@...hat.com>, Mark Rutland <mark.rutland@....com>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, "x86@...nel.org" <x86@...nel.org>, Andy Lutomirski <luto@...capital.net> Subject: Re: [PATCH RFC v3 1/1] gcc-plugins: Add stackleak feature erasing the kernel stack at the end of syscalls On 20.09.2017 17:13, Tycho Andersen wrote: > On Wed, Sep 20, 2017 at 02:27:05PM +0300, Alexander Popov wrote: >>> +/* >>> + * Note that the way this test fails is kind of ugly; it hits the BUG() in >>> + * track_stack, but then the BUG() handler blows the stack and hits the stack >>> + * guard page. >>> + */ >> >> Yes, actually, the reason is deeper. >> >> When there are less than (THREAD_SIZE / 16) bytes left in the kernel stack, the >> BUG() in track_stack() is hit. But do_error_trap(), which handles the invalid >> opcode, has a big stack frame. So it is instrumented by the STACKLEAK gcc plugin >> and itself calls track_stack() at the beginning. Hence we have a recursive >> BUG(), which eventually hits the guard page. >> >> I banned the instrumentation of do_error_trap() in the plugin, but it didn't >> really help, since there are several other instrumented functions called during >> BUG() handling. >> >> So it seems to me that this BUG() in track_stack() is really useless and can be >> dropped. Moreover: >> - it is not a part of the PaX patch; >> - it never worked in Grsecurity kernel because of the error spotted by Tycho. >> >> What do you think about it? > > We'll only have a stack guard page in the case of vmap stack, so maybe Thanks, that's an important aspect. > we can do: > > diff --git a/fs/exec.c b/fs/exec.c > index 8333c4dce59b..8351369cd1e4 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -1960,7 +1960,8 @@ void __used track_stack(void) > current->thread.lowest_stack = sp; > } > > - if (unlikely((sp & (THREAD_SIZE - 1)) < (THREAD_SIZE / 16))) > + if (!IS_ENABLED(CONFIG_VMAP_STACK) && > + unlikely((sp & (THREAD_SIZE - 1)) < (THREAD_SIZE / 16))) > BUG(); > } > EXPORT_SYMBOL(track_stack); In that case the recursive BUG() in track_stack() will happen anyway. You know, I would better make CONFIG_GCC_PLUGIN_STACKLEAK depend on CONFIG_VMAP_STACK. > Anyway, thanks for the reviews, I'll post an updated version shortly. You're welcome. Best regards, Alexander
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