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From: Florian Weimer <fw@deneb.enyo.de>
To: Szabolcs Nagy <szabolcs.nagy@arm.com>
Cc: libc-alpha@sourceware.org,  nd@arm.com
Subject: Re: [PATCH] libio: Disable vtable validation in case of interposition [BZ #23313]
References: <E1fVDN1-0003cW-8d@mid.deneb.enyo.de>
	<773dfddc-66ed-730f-d8b3-a0c9392cc5de@arm.com>
	<87in6f5858.fsf@mid.deneb.enyo.de> <87h8ly3805.fsf@mid.deneb.enyo.de>
	<87woutzyvb.fsf@mid.deneb.enyo.de>
Date: Thu, 21 Jun 2018 21:20:00 -0000
In-Reply-To: <87woutzyvb.fsf@mid.deneb.enyo.de> (Florian Weimer's message of
	"Wed, 20 Jun 2018 09:43:52 +0200")
Message-ID: <87bmc3bzvq.fsf@mid.deneb.enyo.de>
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X-SW-Source: 2018-06/txt/msg00695.txt.bz2

* Florian Weimer:

> libio: Detect foreign vtables in interposed standard streams [BZ #23313]
>
> 2018-06-20  Florian Weimer  <fweimer@redhat.com>
>
> 	[BZ #23313]
> 	* libio/vtables.c (check_stdfiles_vtables): New ELF constructor.
>
> diff --git a/libio/vtables.c b/libio/vtables.c
> index 9fd4ccf642..5b1b581984 100644
> --- a/libio/vtables.c
> +++ b/libio/vtables.c
> @@ -71,3 +71,17 @@ _IO_vtable_check (void)
>  
>    __libc_fatal ("Fatal error: glibc detected an invalid stdio handle\n");
>  }
> +
> +/* Some variants of libstdc++ interpose _IO_2_1_stdin_ etc. and
> +   install their own vtables directly, without calling _IO_init or
> +   other functions.  Detect this by looking at the vtables values
> +   during startup, and disable vtable validation in this case.  */
> +__attribute__ ((constructor))
> +static void
> +check_stdfiles_vtables (void)
> +{
> +  if (_IO_2_1_stdin_.vtable != &_IO_file_jumps
> +      || _IO_2_1_stdout_.vtable != &_IO_file_jumps
> +      || _IO_2_1_stderr_.vtable != &_IO_file_jumps)
> +    IO_set_accept_foreign_vtables (&_IO_vtable_check);
> +}

Any comments on this?