squashfs: add more sanity checks in id lookup

Sysbot has reported a number of "slab-out-of-bounds reads" and
"use-after-free read" errors which has been identified as being caused
by a corrupted index value read from the inode.  This could be because
the metadata block is uncompressed, or because the "compression" bit has
been corrupted (turning a compressed block into an uncompressed block).

This patch adds additional sanity checks to detect this, and the
following corruption.

1. It checks against corruption of the ids count.  This can either
   lead to a larger table to be read, or a smaller than expected
   table to be read.

   In the case of a too large ids count, this would often have been
   trapped by the existing sanity checks, but this patch introduces
   a more exact check, which can identify too small values.

2. It checks the contents of the index table for corruption.

Link: https://lkml.kernel.org/r/20210204130249.4495-3-phillip@squashfs.org.uk


Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Reported-by:  <syzbot+b06d57ba83f604522af2@syzkaller.appspotmail.com>
Reported-by:  <syzbot+c021ba012da41ee9807c@syzkaller.appspotmail.com>
Reported-by:  <syzbot+5024636e8b5fd19f0f19@syzkaller.appspotmail.com>
Reported-by:  <syzbot+bcbc661df46657d0fa4f@syzkaller.appspotmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>