Re: [RFC][PATCH] vfs: always protect diretory file->fpos with inodemutex

[Miao Xie]

On tue, 19 Feb 2013 09:22:40 +0800, Li Zefan wrote:
> There's a long long-standing bug...As long as I don't know when it dates
> from.
> 
> I've written and attached a simple program to reproduce this bug, and it can
> immediately trigger the bug in my box. It uses two threads, one keeps calling
> read(), and the other calling readdir(), both on the same directory fd.
> 
> When I ran it on ext3 (can be replaced with ext2/ext4) which has _dir_index_
> feature disabled, I got this:
> 
> EXT3-fs error (device loop1): ext3_readdir: bad entry in directory #34817: rec_len is smaller than minimal - offset=993, inode=0, rec_len=0, name_len=0
> EXT3-fs error (device loop1): ext3_readdir: bad entry in directory #34817: rec_len is smaller than minimal - offset=1009, inode=0, rec_len=0, name_len=0
> EXT3-fs error (device loop1): ext3_readdir: bad entry in directory #34817: rec_len is smaller than minimal - offset=993, inode=0, rec_len=0, name_len=0
> EXT3-fs error (device loop1): ext3_readdir: bad entry in directory #34817: rec_len is smaller than minimal - offset=1009, inode=0, rec_len=0, name_len=0
> ...
> 
> If we configured errors=remount-ro, the filesystem will become read-only.
> 
> SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)
> {
> 	...
> 		loff_t pos = file_pos_read(file);
> 		ret = vfs_read(file, buf, count, &pos);
> 		file_pos_write(file, pos);
> 		fput_light(file, fput_needed);
> 	...
> }
> 
> While readdir() is protected with i_mutex, f_pos can be changed without any locking
> in various read()/write() syscalls, which leads to this bug.
> 
> What makes things worse is Andi removed i_mutex from generic_file_llseek, so you
> can trigger the same bug by replacing read() with lseek() in the test program.
> 
> commit ef3d0fd27e90f67e35da516dafc1482c82939a60
> Author: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Date:   Thu Sep 15 16:06:48 2011 -0700
> 
>     vfs: do (nearly) lockless generic_file_llseek
> 
> I've tested ext3 with dir_index enabled and btrfs, nothing bad happened, but there
> should be some other vulnerabilities. For example, running the test program on /sys
> for a few minutes triggered this warning:
> 
> [  917.994600] ------------[ cut here ]------------
> [  917.994614] WARNING: at fs/sysfs/sysfs.h:195 sysfs_readdir+0x24c/0x260()
> [  917.994621] Hardware name: Tecal RH2285
> ...
> [  917.994725] Pid: 8754, comm: a.out Not tainted 3.8.0-rc2-tj-0.7-default+ #69
> [  917.994731] Call Trace:
> [  917.994736]  [<ffffffff81205c6c>] ? sysfs_readdir+0x24c/0x260
> [  917.994743]  [<ffffffff81205c6c>] ? sysfs_readdir+0x24c/0x260
> [  917.994752]  [<ffffffff81041fff>] warn_slowpath_common+0x7f/0xc0
> [  917.994759]  [<ffffffff8104205a>] warn_slowpath_null+0x1a/0x20
> [  917.994766]  [<ffffffff81205c6c>] sysfs_readdir+0x24c/0x260
> [  917.994774]  [<ffffffff8119cbd0>] ? sys_ioctl+0x90/0x90
> [  917.994780]  [<ffffffff8119cbd0>] ? sys_ioctl+0x90/0x90
> [  917.994787]  [<ffffffff8119cfc1>] vfs_readdir+0xb1/0xd0
> [  917.994794]  [<ffffffff8119d07b>] sys_getdents64+0x9b/0x110
> [  917.994803]  [<ffffffff814a45d9>] system_call_fastpath+0x16/0x1b
> [  917.994809] ---[ end trace 6efe15a65b89022a ]---
> [  917.994816] ida_remove called for id=13073 which is not allocated.
> 
> 
> We can fix this bug in each filesystem, but can't we just make sure i_mutex is
> acquired in lseek(), read(), write() and readdir() for directory file operations?

I think it is unnecessary to acquire i_mutex in lseek(), read() and write(), because
we can be aware of the change of f_pos, and then get and tune the value in readdir(),
just like ext3 with dir_index enabled.

Thanks
Miao
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