ntfs: set dummy blocksize to read boot_block when mounting

In the Linux kernel, the following vulnerability has been resolved:

ntfs: set dummy blocksize to read boot_block when mounting

When mounting, sb->s_blocksize is used to read the boot_block without

being defined or validated. Set a dummy blocksize before attempting to

read the boot_block.

The issue can be triggered with the following syz reproducer:

mkdirat(0xffffffffffffff9c, &(0x7f0000000080)='./file1\x00', 0x0)

r4 = openat$nullb(0xffffffffffffff9c, &(0x7f0000000040), 0x121403, 0x0)

ioctl$FS_IOC_SETFLAGS(r4, 0x40081271, &(0x7f0000000980)=0x4000)

mount(&(0x7f0000000140)=@nullb, &(0x7f0000000040)='./cgroup\x00',

&(0x7f0000000000)='ntfs3\x00', 0x2208004, 0x0)

syz_clone(0x88200200, 0x0, 0x0, 0x0, 0x0, 0x0)

Here, the ioctl sets the bdev block size to 16384. During mount,

get_tree_bdev_flags() calls sb_set_blocksize(sb, block_size(bdev)),

but since block_size(bdev) > PAGE_SIZE, sb_set_blocksize() leaves

sb->s_blocksize at zero.

Later, ntfs_init_from_boot() attempts to read the boot_block while

sb->s_blocksize is still zero, which triggers the bug.

[[email protected]: changed comment style, added

return value handling]