[syzbot] [ntfs3?] KMSAN: uninit-value in ntfs_iomap_begin

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: d79526b89571 Merge tag 'spi-fix-v7.0-merge-window' of git:..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17c90006580000
kernel config: https://syzkaller.appspot.com/x/.config?x=e9e76fdaa4072fa5
dashboard link: https://syzkaller.appspot.com/bug?extid=7be88937363ac7ab7bb0
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1016255a580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16197c02580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/9632d1edcebe/disk-d79526b8.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/c9f69df31226/vmlinux-d79526b8.xz
kernel image: https://storage.googleapis.com/syzbot-assets/54f77eae730c/bzImage-d79526b8.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/62ad4352d8b0/mount_0.gz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+7be889...@syzkaller.appspotmail.com

loop0: detected capacity change from 0 to 4096
ntfs3(loop0): Different NTFS sector size (2048) and media sector size (512).
=====================================================
BUG: KMSAN: uninit-value in ntfs_iomap_begin+0x8c0/0x1460 fs/ntfs3/inode.c:825
ntfs_iomap_begin+0x8c0/0x1460 fs/ntfs3/inode.c:825
iomap_iter+0x9b7/0x1540 fs/iomap/iter.c:110
iomap_read_folio+0x262/0x750 fs/iomap/buffered-io.c:587
ntfs_read_folio+0x74a/0x770 fs/ntfs3/inode.c:697
filemap_read_folio mm/filemap.c:2496 [inline]
do_read_cache_folio+0x993/0x1470 mm/filemap.c:4096
do_read_cache_page mm/filemap.c:4162 [inline]
read_cache_page+0x63/0x1e0 mm/filemap.c:4171
read_mapping_page include/linux/pagemap.h:1022 [inline]
inode_read_data+0xb3/0x6b0 fs/ntfs3/inode.c:1116
ntfs_fill_super+0x6a24/0x7e50 fs/ntfs3/super.c:1549
get_tree_bdev_flags+0x6e6/0x920 fs/super.c:1694
get_tree_bdev+0x38/0x50 fs/super.c:1717
ntfs_fs_get_tree+0x35/0x40 fs/ntfs3/super.c:1788
vfs_get_tree+0xb3/0x5d0 fs/super.c:1754
fc_mount fs/namespace.c:1193 [inline]
do_new_mount_fc fs/namespace.c:3760 [inline]
do_new_mount+0x885/0x1dd0 fs/namespace.c:3836
path_mount+0x7a2/0x20b0 fs/namespace.c:4146
do_mount fs/namespace.c:4159 [inline]
__do_sys_mount fs/namespace.c:4348 [inline]
__se_sys_mount+0x704/0x7f0 fs/namespace.c:4325
__x64_sys_mount+0xe4/0x150 fs/namespace.c:4325
x64_sys_call+0x39f0/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:166
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x134/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Local variable lcn created at:
ntfs_iomap_begin+0x15d/0x1460 fs/ntfs3/inode.c:786
iomap_iter+0x9b7/0x1540 fs/iomap/iter.c:110

CPU: 1 UID: 0 PID: 6051 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
=====================================================


---
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If the report is already addressed, let syzbot know by replying with:
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If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
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#syz undup

[Edward Adam Davis]

#syz test

diff --git a/fs/ntfs3/inode.c b/fs/ntfs3/inode.c
index 6e65066ebcc1..eac421cf98a8 100644
--- a/fs/ntfs3/inode.c
+++ b/fs/ntfs3/inode.c
@@ -822,6 +822,11 @@ static int ntfs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
return err;
}

+ if (!clen) {
+ /* broken file? */
+ return -EINVAL;
+ }
+
if (lcn == EOF_LCN) {
/* request out of file. */
if (flags & IOMAP_REPORT) {
@@ -855,11 +860,6 @@ static int ntfs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
return 0;
}

- if (!clen) {
- /* broken file? */
- return -EINVAL;
- }
-
iomap->bdev = inode->i_sb->s_bdev;
iomap->offset = offset;
iomap->length = ((loff_t)clen << cluster_bits) - off;

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+7be889...@syzkaller.appspotmail.com
Tested-by: syzbot+7be889...@syzkaller.appspotmail.com

Tested on:

commit: 6de23f81 Linux 7.0-rc1
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1716655a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=52cbe69fade9ccad

dashboard link: https://syzkaller.appspot.com/bug?extid=7be88937363ac7ab7bb0
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

patch: https://syzkaller.appspot.com/x/patch.diff?x=1026655a580000

Note: testing is done by a robot and is best-effort only.

[Edward Adam Davis]

syzbot reported a uninit-value in ntfs_iomap_begin [1].

Since runs was not touched yet, run_lookup_entry() immediately fails
and returns false, which makes the value of "*len" 0.
Simultaneously, the new value and err value are also 0, causing the
logic in attr_data_get_block_locked() to jump directly to ok, ultimately
resulting in *lcn being triggered before it is set [1].

In ntfs_iomap_begin(), the check for a 0 value in clen is moved forward
to before updating lcn to avoid this [1].

[1]

BUG: KMSAN: uninit-value in ntfs_iomap_begin+0x8c0/0x1460 fs/ntfs3/inode.c:825
ntfs_iomap_begin+0x8c0/0x1460 fs/ntfs3/inode.c:825
iomap_iter+0x9b7/0x1540 fs/iomap/iter.c:110

Local variable lcn created at:
ntfs_iomap_begin+0x15d/0x1460 fs/ntfs3/inode.c:786

Fixes: 10d7c95af043 ("fs/ntfs3: add delayed-allocation (delalloc) support")
Reported-by: syzbot+7be889...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7be88937363ac7ab7bb0
Tested-by: syzbot+7be889...@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <ead...@qq.com>
---
fs/ntfs3/inode.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)

--
2.43.0