From 341c87bf346f57748230628c5ad6ee69219250e8 Mon Sep 17 00:00:00 2001 From: KAMEZAWA Hiroyuki Date: Tue, 30 Jun 2009 11:41:23 -0700 Subject: [PATCH] elf: limit max map count to safe value 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 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.. Signed-off-by: KAMEZAWA Hiroyuki Cc: Hugh Dickins Cc: Jakub Jelinek Acked-by: Roland McGrath Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- fs/binfmt_elf.c | 5 ++++- include/linux/sched.h | 16 ++++++++++++++-- 2 files changed, 18 insertions(+), 3 deletions(-) diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c index 9fa212b014a5..f1867900e459 100644 --- a/fs/binfmt_elf.c +++ b/fs/binfmt_elf.c @@ -1929,7 +1929,10 @@ static int elf_core_dump(long signr, struct pt_regs *regs, struct file *file, un 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; diff --git a/include/linux/sched.h b/include/linux/sched.h index 4d0754269884..0085d758d645 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -349,8 +349,20 @@ extern int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner); 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; -- 2.30.2