[PATCH] limit max map count to safe value under ELF v2

From: KAMEZAWA Hiroyuki
Date: Mon Jun 22 2009 - 02:38:05 EST


This is a replacement for
elf-reduce-default-max_map_count-to-be-safe-value.patch

I think all necessary info are written in comments..

==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

With ELF, at generating coredump, some more headers other than used vmas
are added.
When, max_map_count == 65536, a core generated by following kinds of
code can be unreadable because the number of ELF's program header is
written in 16bit in Ehdr (please see elf.h) and the number overflows.
==
... = mmap(); (munmap, mprotect, etc...)
if (failed)
abort();
==
This can be happen in mmap/munmap/mprotect/etc...which calls
split_vma().

I think 65536 is not safe as _default_ and reduce it to 65530 is good
for avoiding unexpected corrupted core.

Anyway, max_map_count can be enlarged by sysctl if a user is brave..

Changelog: v1 -> v2
- set limit to 65530
- added more comments for explaining the reason of value.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
fs/binfmt_elf.c | 5 ++++-
include/linux/sched.h | 16 ++++++++++++++--
2 files changed, 18 insertions(+), 3 deletions(-)

Index: linux-2.6.30-git18/include/linux/sched.h
===================================================================
--- linux-2.6.30-git18.orig/include/linux/sched.h
+++ linux-2.6.30-git18/include/linux/sched.h
@@ -349,8 +349,20 @@ extern int mutex_spin_on_owner(struct mu
struct nsproxy;
struct user_namespace;

-/* Maximum number of active map areas.. This is a random (large) number */
-#define DEFAULT_MAX_MAP_COUNT 65536
+/*
+ * Default maximum number of active map areas, this limits the number of vmas
+ * per mm struct. Users can overwrite this number by sysctl but there is a
+ * problem.
+ *
+ * When a program's coredump is generated as ELF format, a section is created
+ * per a vma. In ELF, the number of sections is represented in unsigned short.
+ * This means the number of sections should be smaller than 65535 at coredump.
+ * Because the kernel adds some informative sections to a image of program at
+ * generating coredump, we need some margin. The number of extra sections is
+ * 1-3 now and depends on arch. We use "5" as safe margin, here.
+ */
+#define MAPCOUNT_ELF_CORE_MARGIN (5)
+#define DEFAULT_MAX_MAP_COUNT (USHORT_MAX - MAPCOUNT_ELF_CORE_MARGIN)

extern int sysctl_max_map_count;

Index: linux-2.6.30-git18/fs/binfmt_elf.c
===================================================================
--- linux-2.6.30-git18.orig/fs/binfmt_elf.c
+++ linux-2.6.30-git18/fs/binfmt_elf.c
@@ -1929,7 +1929,10 @@ static int elf_core_dump(long signr, str
elf = kmalloc(sizeof(*elf), GFP_KERNEL);
if (!elf)
goto out;
-
+ /*
+ * The number of segs are recored into ELF header as 16bit value.
+ * Please check DEFAULT_MAX_MAP_COUNT definition when you modify here.
+ */
segs = current->mm->map_count;
#ifdef ELF_CORE_EXTRA_PHDRS
segs += ELF_CORE_EXTRA_PHDRS;

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