The 2.1 kernel has a problem with setuid programs that try to write a file
on NFS mounted fs. It has nothing to do with root access to NFS fs. It's
really a bug and it has been introduced in the 2.1 kernel.
If I am correct both Alan C. and Bill H. are keeping an eye on NFS related
issues. Perhaps they know what to do.
Here is a shell script to quickly show the effect. You need a NFS mounted
filesystem (for example /mnt/kernel). It doesn't matter if you use
localhost as the server. The script uses 'su' as the setuid program. First a
file gets opened by yourself, and then (in this case 'date') writes into
the file as a different user. Normal UNIX behaviour allows this, and it
works on 2.0.x kernels, as well as other unices. It also works fine if you
write a non-NFS file.
#!/bin/sh
#
file1=${1-/mnt/kernel/temp}
user=hanneke
# Now execute a suid program by some other user.
su $user -c date > $file1
ls -l $file1
ls -l $file1
ls -l $file1
ls -l $file1
sleep 4
ls -l $file1
# Cleanup afterwards
rm -f $file1
Note that recently a change was made in sunrpc (2.1.88) to fix another
problem in this area. However the above problem still exists.
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