linux-kernel - [PATCH 0/2] binfmt_elf: fix calculations for bss padding

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Date:	Fri,  8 Jul 2016 14:48:12 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Kees Cook <keescook@...omium.org>,
	Hector Marco-Gisbert <hecmargi@....es>,
	Ismael Ripoll Ripoll <iripoll@....es>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Chen Gang <gang.chen.5i5j@...il.com>,
	Michal Hocko <mhocko@...e.com>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 0/2] binfmt_elf: fix calculations for bss padding

This fixes a double-bug in ELF loading, as noticed by Hector
Marco-Gisbert. To quote his original email:

 The size of the bss section for some interpreters is not correctly
 calculated resulting in unnecessary calls to vm_brk() with enormous size
 values.

 The bug appears when loading some interpreters with a small bss size. Once
 the last loadable segment has been loaded, the bss section is zeroed up to
 the page boundary and the elf_bss variable is updated to this new page
 boundary.  Because of this update (alignment), the last_bss could be less
 than elf_bss and the subtraction "last_bss - elf_bss" value could overflow.
 ...
 [e.g.] The size value requested to the vm_brk() call (last_bss - elf_bss) is
 0xfffffffffffff938 and internally this size is page aligned in the do_brk()
 function resulting in a 0 length request.

This series takes a slightly different approach to fixing it and updates
vm_brk to refuse bad allocation sizes.

-Kees