[PATCH] ntfs: set dummy blocksize to read boot_block when mounting

[PATCH] ntfs: set dummy blocksize to read boot_block when mounting

[Pedro Demarchi Gomes]

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>
---
 fs/ntfs3/super.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/ntfs3/super.c b/fs/ntfs3/super.c
index ddff94c091b8..7648663d70c9 100644
--- a/fs/ntfs3/super.c
+++ b/fs/ntfs3/super.c
@@ -933,6 +933,9 @@ 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
+	sb_min_blocksize(sb, PAGE_SIZE);
+
 read_boot:
 	bh = ntfs_bread(sb, boot_block);
 	if (!bh)
-- 
2.43.0

Re: [PATCH] ntfs: set dummy blocksize to read boot_block when mounting

[Konstantin Komarov]

On 10/3/25 17:38, Pedro Demarchi Gomes wrote:

> 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>
> ---
>   fs/ntfs3/super.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/fs/ntfs3/super.c b/fs/ntfs3/super.c
> index ddff94c091b8..7648663d70c9 100644
> --- a/fs/ntfs3/super.c
> +++ b/fs/ntfs3/super.c
> @@ -933,6 +933,9 @@ 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
> +	sb_min_blocksize(sb, PAGE_SIZE);
> +
>   read_boot:
>   	bh = ntfs_bread(sb, boot_block);
>   	if (!bh)

Queued for the next merge window, thanks.

Regards,
Konstantin