minix: avoid overflow in bitmap block count calculation

In the Linux kernel, the following vulnerability has been resolved: minix: avoid overflow in bitmap block count calculation minix_check_superblock() uses minix_blocks_needed() to verify that the on-disk imap and zmap block counts are large enough for the advertised inode and zone counts. The helper currently performs DIV_ROUND_UP() in unsigned int arithmetic. A Minix v3 image can set s_ninodes or s_zones near UINT_MAX so the addition inside DIV_ROUND_UP() wraps to zero. That makes a zero imap/zmap block count look valid, after which minix_fill_super() can dereference s_imap[0] or s_zmap[0] even though no bitmap buffers were allocated. Impact: mounting a crafted Minix v3 image whose s_ninodes or s_zones is near UINT_MAX makes minix_check_superblock() accept a zero bitmap-block count and minix_fill_super() dereference s_imap[0]/s_zmap[0], panicking the kernel. The divisor is the bitmap capacity in bits, blocksize * 8, which is always a power of two: minix_fill_super() obtains the block size through sb_set_blocksize(), and blk_validate_block_size() rejects any size that is not a power of two. Use DIV_ROUND_UP_POW2(), which divides before adding the round-up term and so cannot overflow for a power-of-two divisor.

The current structured CVE record identifies a security weakness, but the root cause requires confirmation in the linked vendor material.

An attacker operating through local access may attempt exploitation with low privileges. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.