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1be7107fbe
Stack guard page is a useful feature to reduce a risk of stack smashing into a different mapping. We have been using a single page gap which is sufficient to prevent having stack adjacent to a different mapping. But this seems to be insufficient in the light of the stack usage in userspace. E.g. glibc uses as large as 64kB alloca() in many commonly used functions. Others use constructs liks gid_t buffer[NGROUPS_MAX] which is 256kB or stack strings with MAX_ARG_STRLEN. This will become especially dangerous for suid binaries and the default no limit for the stack size limit because those applications can be tricked to consume a large portion of the stack and a single glibc call could jump over the guard page. These attacks are not theoretical, unfortunatelly. Make those attacks less probable by increasing the stack guard gap to 1MB (on systems with 4k pages; but make it depend on the page size because systems with larger base pages might cap stack allocations in the PAGE_SIZE units) which should cover larger alloca() and VLA stack allocations. It is obviously not a full fix because the problem is somehow inherent, but it should reduce attack space a lot. One could argue that the gap size should be configurable from userspace, but that can be done later when somebody finds that the new 1MB is wrong for some special case applications. For now, add a kernel command line option (stack_guard_gap) to specify the stack gap size (in page units). Implementation wise, first delete all the old code for stack guard page: because although we could get away with accounting one extra page in a stack vma, accounting a larger gap can break userspace - case in point, a program run with "ulimit -S -v 20000" failed when the 1MB gap was counted for RLIMIT_AS; similar problems could come with RLIMIT_MLOCK and strict non-overcommit mode. Instead of keeping gap inside the stack vma, maintain the stack guard gap as a gap between vmas: using vm_start_gap() in place of vm_start (or vm_end_gap() in place of vm_end if VM_GROWSUP) in just those few places which need to respect the gap - mainly arch_get_unmapped_area(), and and the vma tree's subtree_gap support for that. Original-patch-by: Oleg Nesterov <oleg@redhat.com> Original-patch-by: Michal Hocko <mhocko@suse.com> Signed-off-by: Hugh Dickins <hughd@google.com> Acked-by: Michal Hocko <mhocko@suse.com> Tested-by: Helge Deller <deller@gmx.de> # parisc Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
115 lines
3.2 KiB
C
115 lines
3.2 KiB
C
/* elf-fdpic.c: ELF FDPIC memory layout management
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*
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* Copyright (C) 2004 Red Hat, Inc. All Rights Reserved.
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* Written by David Howells (dhowells@redhat.com)
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version
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* 2 of the License, or (at your option) any later version.
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*/
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#include <linux/sched.h>
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#include <linux/sched/mm.h>
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#include <linux/mm.h>
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#include <linux/fs.h>
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#include <linux/elf-fdpic.h>
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#include <asm/mman.h>
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/*****************************************************************************/
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/*
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* lay out the userspace VM according to our grand design
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*/
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#ifdef CONFIG_MMU
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void elf_fdpic_arch_lay_out_mm(struct elf_fdpic_params *exec_params,
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struct elf_fdpic_params *interp_params,
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unsigned long *start_stack,
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unsigned long *start_brk)
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{
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*start_stack = 0x02200000UL;
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/* if the only executable is a shared object, assume that it is an interpreter rather than
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* a true executable, and map it such that "ld.so --list" comes out right
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*/
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if (!(interp_params->flags & ELF_FDPIC_FLAG_PRESENT) &&
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exec_params->hdr.e_type != ET_EXEC
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) {
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exec_params->load_addr = PAGE_SIZE;
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*start_brk = 0x80000000UL;
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}
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else {
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exec_params->load_addr = 0x02200000UL;
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if ((exec_params->flags & ELF_FDPIC_FLAG_ARRANGEMENT) ==
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ELF_FDPIC_FLAG_INDEPENDENT
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) {
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exec_params->flags &= ~ELF_FDPIC_FLAG_ARRANGEMENT;
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exec_params->flags |= ELF_FDPIC_FLAG_CONSTDISP;
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}
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}
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} /* end elf_fdpic_arch_lay_out_mm() */
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#endif
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/*****************************************************************************/
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/*
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* place non-fixed mmaps firstly in the bottom part of memory, working up, and then in the top part
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* of memory, working down
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*/
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unsigned long arch_get_unmapped_area(struct file *filp, unsigned long addr, unsigned long len,
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unsigned long pgoff, unsigned long flags)
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{
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struct vm_area_struct *vma;
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struct vm_unmapped_area_info info;
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if (len > TASK_SIZE)
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return -ENOMEM;
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/* handle MAP_FIXED */
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if (flags & MAP_FIXED)
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return addr;
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/* only honour a hint if we're not going to clobber something doing so */
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if (addr) {
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addr = PAGE_ALIGN(addr);
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vma = find_vma(current->mm, addr);
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if (TASK_SIZE - len >= addr &&
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(!vma || addr + len <= vm_start_gap(vma)))
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goto success;
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}
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/* search between the bottom of user VM and the stack grow area */
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info.flags = 0;
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info.length = len;
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info.low_limit = PAGE_SIZE;
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info.high_limit = (current->mm->start_stack - 0x00200000);
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info.align_mask = 0;
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info.align_offset = 0;
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addr = vm_unmapped_area(&info);
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if (!(addr & ~PAGE_MASK))
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goto success;
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VM_BUG_ON(addr != -ENOMEM);
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/* search from just above the WorkRAM area to the top of memory */
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info.low_limit = PAGE_ALIGN(0x80000000);
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info.high_limit = TASK_SIZE;
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addr = vm_unmapped_area(&info);
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if (!(addr & ~PAGE_MASK))
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goto success;
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VM_BUG_ON(addr != -ENOMEM);
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#if 0
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printk("[area] l=%lx (ENOMEM) f='%s'\n",
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len, filp ? filp->f_path.dentry->d_name.name : "");
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#endif
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return -ENOMEM;
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success:
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#if 0
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printk("[area] l=%lx ad=%lx f='%s'\n",
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len, addr, filp ? filp->f_path.dentry->d_name.name : "");
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#endif
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return addr;
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} /* end arch_get_unmapped_area() */
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