linux-kernel - Re: [PATCH v7 5/6] x86/signal: Detect and prevent an alternate signal stack overflow

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Date:   Tue, 16 Mar 2021 12:52:48 +0100
From:   Borislav Petkov <bp@...e.de>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>
Cc:     tglx@...utronix.de, mingo@...nel.org, luto@...nel.org,
        x86@...nel.org, len.brown@...el.com, dave.hansen@...el.com,
        hjl.tools@...il.com, Dave.Martin@....com, jannh@...gle.com,
        mpe@...erman.id.au, carlos@...hat.com, tony.luck@...el.com,
        ravi.v.shankar@...el.com, libc-alpha@...rceware.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 5/6] x86/signal: Detect and prevent an alternate
 signal stack overflow

On Mon, Mar 15, 2021 at 11:52:14PM -0700, Chang S. Bae wrote:
> @@ -272,7 +275,8 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
>  	 * If we are on the alternate signal stack and would overflow it, don't.
>  	 * Return an always-bogus address instead so we will die with SIGSEGV.
>  	 */
> -	if (onsigstack && !likely(on_sig_stack(sp)))
> +	if (onsigstack && unlikely(sp <= current->sas_ss_sp ||
> +				   sp - current->sas_ss_sp > current->sas_ss_size))
>  		return (void __user *)-1L;

So clearly I'm missing something because trying to trigger the test case
in the bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=153531

on current tip/master doesn't work. Runs with MY_MINSIGSTKSZ under 2048
fail with:

tst-minsigstksz-2: sigaltstack: Cannot allocate memory

and above 2048 don't overwrite bytes below the stack.

So something else is missing. How did you test this patch?

Thx.

-- 
Regards/Gruss,
    Boris.

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