linux-kernel - [PATCH 2/2] fs, elf: drop MAP_FIXED from initial ET

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Date:   Mon, 16 Oct 2017 15:44:46 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Al Viro <viro@...iv.linux.org.uk>,
        Oleg Nesterov <oleg@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Baoquan He <bhe@...hat.com>,
        Michal Hocko <mhocko@...e.com>
Subject: [PATCH 2/2] fs, elf: drop MAP_FIXED from initial ET_DYN segment

From: Michal Hocko <mhocko@...e.com>

eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE") has added
MAP_FIXED flag to the initial ET_DYN segment mapping which defines the
randomized base for the PIE ELF segments. The thing is that MAP_FIXED
shouldn't be really needed because the address is essentially random
anyway. All other segments are mapped relatively to this base. elf_map
makes sure that all segments will fit into the address space by
enforcing total_mapping_size initial map.

Why do we want to drop MAP_FIXED in the first place? Because it is error
prone. If we happen to have an existing mapping in the requested range
then we do not want to corrupt it silently. Without MAP_FIXED vm_mmap
will simply fallback to another range. In reality there shouldn't be
any conflicting mappings at this early exec stage so the mmap should
succeed even without MAP_FIXED but subtle changes to the randomization
can break this assumption so we should rather be careful here.

Fixes: eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE")
Signed-off-by: Michal Hocko <mhocko@...e.com>
---
 fs/binfmt_elf.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 09456e2add18..244cc30dfa24 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -988,7 +988,6 @@ static int load_elf_binary(struct linux_binprm *bprm)
 				load_bias = ELF_ET_DYN_BASE;
 				if (current->flags & PF_RANDOMIZE)
 					load_bias += arch_mmap_rnd();
-				elf_flags |= MAP_FIXED;
 			} else
 				load_bias = 0;

-- 
2.14.2