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

[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+7be88937363ac7ab7bb0@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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Re: [syzbot] [ntfs3?] KMSAN: uninit-value in ntfs_iomap_begin

[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;

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

[syzbot]

Hello,

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

Reported-by: syzbot+7be88937363ac7ab7bb0@syzkaller.appspotmail.com
Tested-by: syzbot+7be88937363ac7ab7bb0@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.

[PATCH] fs/ntfs3: prevent uninitialized lcn caused by zero len

[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+7be88937363ac7ab7bb0@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7be88937363ac7ab7bb0
Tested-by: syzbot+7be88937363ac7ab7bb0@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
 fs/ntfs3/inode.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

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;
-- 
2.43.0

Re: [PATCH] fs/ntfs3: prevent uninitialized lcn caused by zero len

[Konstantin Komarov]

On 2/23/26 09:01, Edward Adam Davis wrote:

> 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+7be88937363ac7ab7bb0@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=7be88937363ac7ab7bb0
> Tested-by: syzbot+7be88937363ac7ab7bb0@syzkaller.appspotmail.com
> Signed-off-by: Edward Adam Davis <eadavis@qq.com>
> ---
>   fs/ntfs3/inode.c | 10 +++++-----
>   1 file changed, 5 insertions(+), 5 deletions(-)
>
> 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;

Hello,

Your patch is applied. Thanks.

Regards,
Konstantin