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([2804:30c:166c:ce00:17a8:586a:f651:fcd5]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-b6099f7ab4fsm4946496a12.44.2025.10.03.08.40.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 03 Oct 2025 08:40:50 -0700 (PDT) From: Pedro Demarchi Gomes To: Konstantin Komarov Cc: ntfs3@lists.linux.dev, linux-kernel@vger.kernel.org, Pedro Demarchi Gomes , syzbot+f4f84b57a01d6b8364ad@syzkaller.appspotmail.com Subject: [PATCH] ntfs: set dummy blocksize to read boot_block when mounting Date: Fri, 3 Oct 2025 12:38:50 -0300 Message-ID: <20251003153850.55643-1-pedrodemargomes@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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)=3D'./file1\x00', 0x0) r4 =3D openat$nullb(0xffffffffffffff9c, &(0x7f0000000040), 0x121403, 0x0) ioctl$FS_IOC_SETFLAGS(r4, 0x40081271, &(0x7f0000000980)=3D0x4000) mount(&(0x7f0000000140)=3D@nullb, &(0x7f0000000040)=3D'./cgroup\x00', &(0x7f0000000000)=3D'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=3Df4f84b57a01d6b8364ad Signed-off-by: Pedro Demarchi Gomes --- 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, =20 sbi->volume.blocks =3D dev_size >> PAGE_SHIFT; =20 + // Set dummy blocksize to read boot_block + sb_min_blocksize(sb, PAGE_SIZE); + read_boot: bh =3D ntfs_bread(sb, boot_block); if (!bh) --=20 2.43.0