diff --git a/study/debug/filesystem/procfs/kptr_restrict/README.md b/study/debug/filesystem/procfs/kptr_restrict/README.md new file mode 100644 index 0000000..d227099 --- /dev/null +++ b/study/debug/filesystem/procfs/kptr_restrict/README.md @@ -0,0 +1,163 @@ +kptr_restrict 向用户空间内核中的指针(/proc/kallsyms-modules显示value全部为0) + +======= + +| CSDN | GitHub | +|:----:|:------:| +| [kptr_restrict 向用户空间内核中的指针(/proc/kallsyms-modules显示value全部为0)](http://blog.csdn.net/gatieme) | [`LinuxDeviceDrivers/study/debug/filesystem/procfs/kptr_restrict`](https://github.com/gatieme/LDD-LinuxDeviceDrivers/tree/master/study/debug/filesystem/procfs/kptr_restrict) | + +
+ +知识共享许可协议 + +本作品采用知识共享署名-非商业性使用-相同方式共享 4.0 国际许可协议进行许可, 转载请注明出处, 谢谢合作 + +因本人技术水平和知识面有限, 内容如有纰漏或者需要修正的地方, 欢迎大家指正, 也欢迎大家提供一些其他好的调试工具以供收录, 鄙人在此谢谢啦 + +
+ +#1 /proc/kallsyms显示value全部为0 +------- + +今天一个同事问我 `cat /proc/kallsyms` 显示 `value` 全部为 `0`. 我在手机端试了一下, 果然如此. + +切换到 `root` 用户运行, 依然是 `0`. 感到十分奇怪, 因为内核发生 `crash` 或者打开 `trace` 的时候, 都是调用的 `sprint_ symbol` 来打印的. 为啥内核可以, 用户态 `cat` 就不行呢? + +后来发现是系统为了保护这些符号地址泄露, 而用的一种保护手段, 从而使除 `root` 用户外的普通用户不能直接查看符号地址. + +#2 kptr_restrict 介绍 +------- + +原因在于内核文件 [`kallsyms.c`](http://elixir.free-electrons.com/linux/v4.13.9/source/kernel/kallsyms.c#L583) 中的显示符号地址命令中做了如下限制. + +```cpp +seq_printf(m, "%pK %c %s\n", (void *)iter->value, iter->type, iter->name); +``` + +只需要把其中的 `%pK` 换成 `%p` 就可以让普通用户查看符号地址了. 很多提权漏洞一般会用到此处的修改来获取符号地址 + +内核提供控制变量 `/proc/sys/kernel/kptr_restrict` 来进行修改. 从内核文档 `Documentation/sysctl/kernel.txt` 中可以看到 `kptr_restrict` 用于控制内核的一些输出打印. + +[`Documentation/printk-formats.txt`](http://elixir.free-electrons.com/linux/v4.13.9/source/Documentation/sysctl/kernel.txt#L371) 有更加详细的描述, 除了我们平时遇到的一些打印格式之外, 还有一些比较特殊的格式(我以前没注意到). + +```cpp +============================================================== + +kptr_restrict: + +This toggle indicates whether restrictions are placed on +exposing kernel addresses via /proc and other interfaces. + +When kptr_restrict is set to (0), the default, there are no restrictions. + +When kptr_restrict is set to (1), kernel pointers printed using the %pK +format specifier will be replaced with 0's unless the user has CAP_SYSLOG +and effective user and group ids are equal to the real ids. This is +because %pK checks are done at read() time rather than open() time, so +if permissions are elevated between the open() and the read() (e.g via +a setuid binary) then %pK will not leak kernel pointers to unprivileged +users. Note, this is a temporary solution only. The correct long-term +solution is to do the permission checks at open() time. Consider removing +world read permissions from files that use %pK, and using dmesg_restrict +to protect against uses of %pK in dmesg(8) if leaking kernel pointer +values to unprivileged users is a concern. + +When kptr_restrict is set to (2), kernel pointers printed using +%pK will be replaced with 0's regardless of privileges. +============================================================== +``` + +| kptr_restrict | 权限描述 | +|:-------------:|:-------:| +| 0 | root和普通用户都可以读取 | +| 1 | root用户有权限读取, 普通用户没有权限 | +| 2 | 内核将符号地址打印为全0, root和普通用户都没有权限 | + + +`kptr_restrict` 值为 `2` 时, 所有用户都无法读取内核符号地址. + + +![`kptr_restrict` 值为 `2` 时](kptr_restrict_2.png) + +`kptr_restrict` 值为 `1` 时, 普通用户都无法读取内核符号地址, `root` 用户可以查看. + + +![`kptr_restrict` 值为 `1` 时](kptr_restrict_1.png) + +`kptr_restrict` 值为 `0` 时, 所有用户都可以读取内核地址. + + +![`kptr_restrict` 值为 `0` 时](kptr_restrict_0.png) + +>注意 `kptr_restrict` 对内核中很多地址和符号表的信息导出都有影响, 比如 `/proc/modules` 等. + +#3 kptr_restrict的设计 +------- + +`kptr_restrict` 在内核 [`commit 455cd5ab--kptr_restrict for hiding kernel pointers from unprivileged users`](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=455cd5ab305c90ffc422dd2e0fb634730942b257), 具体实现源码位于 [`lib/vsprintf.c, line 1708`](http://elixir.free-electrons.com/linux/v4.13.9/source/lib/vsprintf.c#L1708) + + +```cpp +#http://elixir.free-electrons.com/linux/v4.13.9/source/lib/vsprintf.c#L1708 + case 'K': + switch (kptr_restrict) { + case 0: + /* Always print %pK values */ + break; + case 1: { + const struct cred *cred; + + /* + * kptr_restrict==1 cannot be used in IRQ context + * because its test for CAP_SYSLOG would be meaningless. + */ + if (in_irq() || in_serving_softirq() || in_nmi()) { + if (spec.field_width == -1) + spec.field_width = default_width; + return string(buf, end, "pK-error", spec); + } + + /* + * Only print the real pointer value if the current + * process has CAP_SYSLOG and is running with the + * same credentials it started with. This is because + * access to files is checked at open() time, but %pK + * checks permission at read() time. We don't want to + * leak pointer values if a binary opens a file using + * %pK and then elevates privileges before reading it. + */ + cred = current_cred(); + if (!has_capability_noaudit(current, CAP_SYSLOG) || + !uid_eq(cred->euid, cred->uid) || + !gid_eq(cred->egid, cred->gid)) + ptr = NULL; + break; + } + case 2: + default: + /* Always print 0's for %pK */ + ptr = NULL; + break; + } + break; +``` + +#4 参考 +------- + +[Introducing Linux Kernel Symbols](https://onebitbug.me/2011/03/04/introducing-linux-kernel-symbols/) + +[Is there a way to set kptr_restrict to 0?](https://stackoverflow.com/questions/20390601/is-there-a-way-to-set-kptr-restrict-to-0) + +[kptr_restrict for hiding kernel pointers from unprivileged users](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=455cd5ab305c90ffc422dd2e0fb634730942b257) + +[Linux kallsyms 机制分析](http://blog.csdn.net/kehyuanyu/article/details/46346321) + + +
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