[PATCH 6.12 022/567] ntfs: set dummy blocksize to read boot_block when mounting

Greg Kroah-Hartman
2026-01-06T17:15:30Z

From: Pedro Demarchi Gomes <pedrodemargomes@gmail.com>

[ Upstream commit d1693a7d5a38acf6424235a6070bcf5b186a360d ]

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.

Reported-by: syzbot+f4f84b57a01d6b8364ad@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f4f84b57a01d6b8364ad
Signed-off-by: Pedro Demarchi Gomes <pedrodemargomes@gmail.com>
[almaz.alexandrovich@paragon-software.com: changed comment style, added
return value handling]
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/ntfs3/super.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/fs/ntfs3/super.c b/fs/ntfs3/super.c
index 6a0f6b0a3ab2a..89d126c155c7d 100644
--- a/fs/ntfs3/super.c
+++ b/fs/ntfs3/super.c
@@ -892,6 +892,11 @@ static int ntfs_init_from_boot(struct super_block *sb, u32 sector_size,

sbi->volume.blocks = dev_size >> PAGE_SHIFT;

+ /* Set dummy blocksize to read boot_block. */
+ if (!sb_min_blocksize(sb, PAGE_SIZE)) {
+ return -EINVAL;
+ }
+
read_boot:
bh = ntfs_bread(sb, boot_block);
if (!bh)
--
2.51.0