Re: What's wrong with this really simple function?

[linux-os (Dick Johnson)]

On Mon, 28 Nov 2005, Nikita Danilov wrote:

> Juergen Quade writes:
> > On Sun, Nov 27, 2005 at 12:57:47PM -0600, Mohamed El Dawy wrote:
> > > Hi,
> > >  I have created this 5-liner system call, which basically opens a
> > > file, write "Hello World" to it, and then returns. That's all.
> > >
> > > Now, when I actually call it, it creates the file successfully but
> > > writes nothing to it. The file is created and is only zero bytes. So,
> > > either write didn't write, or close didn't close. Any help would be
> > > greatly appreciated.
> > > ...
> >
> > The following (module-) code will create and write a file from
> > inside a kernel. Ok -- you know -- you should not use it
> > without really good reasons ...
> >
> >           Juergen.
> >
> > #include <linux/module.h>
> > #include <linux/moduleparam.h>
> > #include <linux/fs.h>
> > #include <asm/uaccess.h>
> >
> > static char filename[255];
> > module_param_string( filename, filename, sizeof(filename), 666 );
> > struct file *log_file;
> >
> > static int __init mod_init(void)
> > {
> > 	mm_segment_t oldfs;
> >
> > 	if( filename[0]=='\0' )
> > 		strncpy( filename, "/tmp/kernel_file", sizeof(filename) );
> > 	printk("opening filename: %s\n", filename);
> > 	log_file = filp_open( filename, O_WRONLY | O_CREAT, 0644 );
> > 	printk("log_file: %p\n", log_file );
> > 	if( IS_ERR( log_file ) )
> > 		return -EIO;
>
> This code is trivially exploitable: /tmp is usually a world-writable
> directory, and everybody can do
>
> $ ln -s /etc/shadow /tmp/kernel_file
>
> or even
>
> $ ln /etc/shadow /tmp/kernel_file
>
> provided that /tmp and /etc are on the same file system.
>
> Please do not use it.
>
> Nikita.

Also, this has become a FAQ. One needs a context with which to
associate file descriptors and even file buffers. Whose did he
steal? When he finds out, he will find the buffer that never
got written to the file.

Again, to read/write files from inside the kernel either create
a kernel thread, steal init's context, or don't.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.55 BogoMips).
Warning : 98.36% of all statistics are fiction.
.

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