Re: [PATCH] ufs: reject malformed cylinder summary geometry

I confirmed the overflow this patch closes. ufs_read_cylinder_structures()
in fs/ufs/super.c derives blks from (s_cssize + s_fsize - 1) >> s_fshift and
then copies a full s_fsize bytes per iteration into a kmalloc(s_cssize)
buffer, so once s_fshift is inconsistent with s_fsize, or s_cssize is not a
fragment multiple, the loop writes past the allocation.

The two new checks cover both cases. Requiring s_fshift to equal
ilog2(s_fsize) keeps the shift that produces blks correct, and requiring
s_cssize to equal s_ncg times sizeof(struct ufs_csum) rounded up to a
fragment ties the buffer to the real cylinder group count and makes it a
fragment multiple, so blks times s_fsize equals s_cssize and the copy stays
in bounds. struct ufs_csum is four 32 bit fields for both ufs1 and ufs2, and
this expected size matches the fragroundup(ncg times sizeof(csum)) sizing
that newfs writes.

s_ncg, s_fshift and s_fsize are all assigned earlier in ufs_fill_super(),
before this check runs, so the validation uses initialised values.

Reviewed-by: Ali Ahmet Memis <[email protected]>