linux-kernel - [PATCH] ntfs: set dummy blocksize to read boot

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Message-ID: <20251003153850.55643-1-pedrodemargomes@gmail.com>
Date: Fri,  3 Oct 2025 12:38:50 -0300
From: Pedro Demarchi Gomes <pedrodemargomes@...il.com>
To: Konstantin Komarov <almaz.alexandrovich@...agon-software.com>
Cc: ntfs3@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	Pedro Demarchi Gomes <pedrodemargomes@...il.com>,
	syzbot+f4f84b57a01d6b8364ad@...kaller.appspotmail.com
Subject: [PATCH] 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)=@...lb, &(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@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f4f84b57a01d6b8364ad
Signed-off-by: Pedro Demarchi Gomes <pedrodemargomes@...il.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