How to take down a system to the point of requiring a newfs with one line of C (userland)

[Jim Bryant]

FYI: The system assigned kern/120781 to this bug report.

IMHO, a security advisory should be issued ASAP.

Jim Bryant wrote:
> One line summary:
>    Too many files in a top-level UFS-2 filesystem directory will cause 
> a panic on mount.
>
> Kern/Critical/High Priority/SW-Bug
>
> Which FreeBSD Release You Are Using:
>    6.3-STABLE
>
> Environment (output of "uname -a" on the problem machine):
>    FreeBSD wahoo.sd67dfl.org 6.3-STABLE FreeBSD 6.3-STABLE #0: Sun Feb 
> 10 21:13:39 CST 2008     
> jbryant at wahoo.sd67dfl.org:/usr/obj/usr/src/sys/WAHOO-SMP  i386
>
>    Note: I just cvsupped earlier, and no changes have been put into 
> cvsup that would fix this problem.
>
> Full Description:
>    I was doing a reorganization of my filesystems, and since I do 
> offline installs, I keep a local distfiles collection (or did until 
> yesterday when this happened), and in the process, put all of the 
> distfiles on their own filesystem to be mounted under 
> /usr/ports/distfiles.
>
> All was fine until I rebooted.
>
> On rebooting, I got a page fault panic on mount of the new distfiles 
> filesystem.
>
> i booted again, got it again, booted again this time into single-user, 
> and did a fsck on the filesystem, and it only showed as being "dirty", 
> but otherwise had no problems in the eyes of fsck.  booted again, 
> instant panic.
>
> i booted an older 6.2 CD and mounted the filesystem fine.  i then put 
> that filesystem the way it was by mkdir'ing a distfiles dir and mv'ing 
> everything into it, but on reboot it still paniced on mount.
>
> only a newfs was able to enable the filesystem to be mounted.
>
> today i did further research, thinking it had to do with the number of 
> files in the top-level filesystem directory, and found that to be 
> true.  the short c program in the next section (how to repeat the 
> problem) contains this.
>
> a second test shows that, after a newfs, if this done in any 
> subdirectory of that filesystem, the panic is averted, and all is 
> well.  apparently this bug only effects top-level directories of a 
> UFS2 filesystem.
>
> I have not attempted this to a non-UFS2 filesystem.
>
> IMHO, a security advisory should be released, since any user with 
> write access to ANY top level directory of ANY mounted filesystem 
> (most systems have /tmp as a world writable top level filesystem 
> directory) can create a panic situation requiring a newfs of the said 
> filesystem.  A malicious user with root access can do this to /.  
> Either way, on boot, or any attempt to mount said filesystem on a 
> running system, will cause a panic, which of course will cause an 
> unbootable system on reboot.
>
> How to repeat the problem:
>    Compile and run the following as instructed:
>
> #include <stdio.h>
> #include <stdlib.h>
>
> int main(int argc, char **argv) { int i; char buf[1024]; bzero(buf, 
> 1024); for(i = 0; i < 10000; i++) { sprintf(buf, "touch %s%05d\n", 
> argv[1], i); system((const char *)buf);} return(0);}
>
> /* pass a top-level mountpoint directory name of a mounted filesystem, 
> with a trailing slash to the above as argv[1], and run.
>
> This will create 10,000 zero-length files in the specified directory.
>
> umount that filesystem.
>
> perform a shitload of sync's to make sure everything outstanding is 
> flushed to disk on all filesystems.
>
> mount the target filesystem (preferably from a vty or serial console 
> to catch the messages when it panics, which it will as soon as the 
> mount is attempted).
> */
>
> Fix to the problem if known:
>    newfs(8)
>
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