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

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Date:   Thu, 25 Mar 2021 17:21:04 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Borislav Petkov <bp@...e.de>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "Dave.Martin@....com" <Dave.Martin@....com>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "carlos@...hat.com" <carlos@...hat.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "libc-alpha@...rceware.org" <libc-alpha@...rceware.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-kernel@...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 Mar 25, 2021, at 09:20, Borislav Petkov <bp@...e.de> wrote:
> 
> $ gcc tst-minsigstksz-2.c -DMY_MINSIGSTKSZ=3453 -o tst-minsigstksz-2
> $ ./tst-minsigstksz-2
> tst-minsigstksz-2: changed byte 50 bytes below configured stack
> 
> Whoops.
> 
> And the debug print said:
> 
> [ 5395.252884] signal: get_sigframe: sp: 0x7f54ec39e7b8, sas_ss_sp: 0x7f54ec39e6ce, sas_ss_size 0xd7d
> 
> which tells me that, AFAICT, your check whether we have enough alt stack
> doesn't seem to work in this case.

Yes, in this case.

tst-minsigstksz-2.c has this code:

static void
handler (int signo)
{
  /* Clear a bit of on-stack memory.  */
  volatile char buffer[256];
  for (size_t i = 0; i < sizeof (buffer); ++i)
    buffer[i] = 0;
  handler_run = 1;
}
…

  if (handler_run != 1)
    errx (1, "handler did not run");

  for (void *p = stack_buffer; p < stack_bottom; ++p)
    if (*(unsigned char *) p != 0xCC)
      errx (1, "changed byte %zd bytes below configured stack\n",
            stack_bottom - p);
…

I think the message comes from the handler’s overwriting, not from the kernel.

The patch's check is to detect and prevent the kernel-induced overflow --
whether alt stack enough for signal delivery itself.  The stack is possibly
not enough for the signal handler's use as the kernel does not know for it.

Thanks,
Chang