Date: Fri, 29 Sep 2017 00:17:25 +0300 From: Alexander Popov <alex.popov@...ux.com> To: Tycho Andersen <tycho@...ker.com>, Kees Cook <keescook@...omium.org> Cc: "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 21.09.2017 16:26, Alexander Popov wrote: > 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. That turned out to be a bad idea. Unfortunately, VMAP_STACK is not available on x86_32, but STACKLEAK works on that platform. So I'll put the check behind #ifdef. Maybe having it is better than having a silent stack overflow. Best regards, Alexander
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