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.

Fixes: 8c97a6ddc956 ("minix: Add required sanity checking to minix_check_superblock()")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Link: https://patch.msgid.link/20260618143922.3066874-1-michael.bommarito@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

diff --git a/fs/minix/minix.h b/fs/minix/minix.h
index f2025c9b58252f1027d78dbc42caecacece0161a..9e52d4302f0d2ea62225a4dcfe08ef1bef686687 100644 (file)
--- a/fs/minix/minix.h
+++ b/fs/minix/minix.h
@@ -97,7 +97,7 @@ static inline struct minix_inode_info *minix_i(struct inode *inode)
 
 static inline unsigned minix_blocks_needed(unsigned bits, unsigned blocksize)
 {
-       return DIV_ROUND_UP(bits, blocksize * 8);
+       return DIV_ROUND_UP_POW2(bits, blocksize * 8);
 }
 
 #if defined(CONFIG_MINIX_FS_NATIVE_ENDIAN) && \