'Signing' a compressed file system for app verification
A friend asked me, "How do you know that someone didn't replace your KNOPPIX compressed file system with a tainted one containing malware and such?"
I do verify portions of systems by appending a MD5 hash (plus a bit of arcania) to the end of some executables so that I can later verify they are the ones I supplied. The executables don't mind the extra 32 bytes,
But the clooped file system DOES seem to mind. I only hash as much as is needed to detect changes. I then append the MD5 hash and a time stamp to the end of the file.
I have been through the sourcecode in "cloop-2"; and not found anything that explicitly compares the end of compressed data with the file size. And I have compressed FSs with odd byte sizes, so there is no specific size multiple. So why the failures?
BillS
This is the result of loading and mounting a 'signed' compressed file.
Code:
root@lp2:/tmp# losetup /dev/cloop2 /tmp/KNOPPIX_NOV_29_04
root@lp2:/tmp# losetup -a
root@lp2:/tmp# head -4 KNOPPIX_NOV_29_04
#!/bin/sh
#V2.0 Format
insmod cloop.o file=$0 && mount -r -t iso9660 /dev/cloop $1
exit $?
root@lp2:/tmp# mount -r -t iso9660 /dev/cloop2 /mnt/test
mount: wrong fs type, bad option, bad superblock on /dev/cloop2,
or too many mounted file systems
And then the same file without the signature (simply lacking the 32 bytes):
Code:
root@lp2:/tmp# losetup /dev/cloop /extra/CompressedFS/KNOPPIX_NOV_29_04
root@lp2:/tmp# losetup -a
root@lp2:/tmp# mount -r -t iso9660 /dev/cloop /mnt/test
root@lp2:/tmp# losetup -a
root@lp2:/tmp# ls /mnt/test
bin cdrom etc home mnt proc sbin tmp var
boot dev floppy lib none root sys usr vmlinuz
root@lp2:/tmp# cmp KNOPPIX_NOV_29_04 /extra/CompressedFS/KNOPPIX_NOV_29_04
cmp: EOF on /extra/CompressedFS/KNOPPIX_NOV_29_04
'Signing' a compressed file system for app verification
'cloop-2' code does 'lseek()' calls for either current position only, or to an absolute position using an offset value already stored in the compressed file index. And it is curious that the failure is in the mount (unless the losetup failed and didn't produce an error message.)
BillS