[syzbot] [ext4?] WARNING in __folio_mark_dirty (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 23cb64fb7625 Merge tag 'soc-fixes-6.18-3' of git://git.ker..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1287997c580000
kernel config: https://syzkaller.appspot.com/x/.config?x=38a0c4cddc846161
dashboard link: https://syzkaller.appspot.com/bug?extid=b0a0670332b6b3230a0a
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=172cd658580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=15378484580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/a14ebce6c664/disk-23cb64fb.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/7703c3032e2f/vmlinux-23cb64fb.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a16c5b94924d/bzImage-23cb64fb.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/4fa9525269b6/mount_0.gz
fsck result: failed (log: https://syzkaller.appspot.com/x/fsck.log?x=11d42a12580000)

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+b0a067...@syzkaller.appspotmail.com

EXT4-fs error (device loop0): ext4_ext_remove_space:2955: inode #15: comm syz.0.51: pblk 0 bad header/extent: invalid extent entries - magic f30a, entries 2, max 4(4), depth 0(0)
EXT4-fs error (device loop0) in ext4_setattr:6050: Corrupt filesystem
EXT4-fs error (device loop0): ext4_validate_block_bitmap:441: comm syz.0.51: bg 0: block 112: padding at end of block bitmap is not set
EXT4-fs error (device loop0): ext4_map_blocks:778: inode #15: block 3: comm syz.0.51: lblock 3 mapped to illegal pblock 3 (length 1)
EXT4-fs error (device loop0): ext4_ext_remove_space:2955: inode #15: comm syz.0.51: pblk 0 bad header/extent: invalid extent entries - magic f30a, entries 2, max 4(4), depth 0(0)
------------[ cut here ]------------
WARNING: CPU: 0 PID: 6107 at mm/page-writeback.c:2716 __folio_mark_dirty+0x1fb/0xe20 mm/page-writeback.c:2716
Modules linked in:
CPU: 0 UID: 0 PID: 6107 Comm: syz.0.51 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:__folio_mark_dirty+0x1fb/0xe20 mm/page-writeback.c:2716
Code: 3c 38 00 74 08 48 89 df e8 52 a7 26 00 4c 8b 33 4c 89 f6 48 83 e6 08 31 ff e8 d1 ed c4 ff 49 83 e6 08 75 1c e8 66 e8 c4 ff 90 <0f> 0b 90 eb 16 e8 5b e8 c4 ff e9 7e 07 00 00 e8 51 e8 c4 ff eb 05
RSP: 0018:ffffc90003a079d0 EFLAGS: 00010293
RAX: ffffffff81f9c4ea RBX: ffffea0001213400 RCX: ffff888025260000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000000
R10: dffffc0000000000 R11: fffff94000242681 R12: ffff8880416cb4f0
R13: ffff8880416cb4e8 R14: 0000000000000000 R15: dffffc0000000000
FS: 0000555587ebe500(0000) GS:ffff888126df7000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000000440 CR3: 000000002e288000 CR4: 00000000003526f0
Call Trace:
<TASK>
block_dirty_folio+0x17a/0x1d0 fs/buffer.c:754
fault_dirty_shared_page+0x103/0x570 mm/memory.c:3519
wp_page_shared mm/memory.c:3906 [inline]
do_wp_page+0x263e/0x4930 mm/memory.c:4109
handle_pte_fault mm/memory.c:6211 [inline]
__handle_mm_fault mm/memory.c:6336 [inline]
handle_mm_fault+0x97c/0x3400 mm/memory.c:6505
do_user_addr_fault+0xa7c/0x1380 arch/x86/mm/fault.c:1336
handle_page_fault arch/x86/mm/fault.c:1476 [inline]
exc_page_fault+0x82/0x100 arch/x86/mm/fault.c:1532
asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
RIP: 0033:0x7f385fe87398
Code: fc 89 37 c3 c5 fa 6f 06 c5 fa 6f 4c 16 f0 c5 fa 7f 07 c5 fa 7f 4c 17 f0 c3 66 0f 1f 84 00 00 00 00 00 48 8b 4c 16 f8 48 8b 36 <48> 89 37 48 89 4c 17 f8 c3 c5 fe 6f 54 16 e0 c5 fe 6f 5c 16 c0 c5
RSP: 002b:00007ffd3b8738f8 EFLAGS: 00010246
RAX: 0000200000000440 RBX: 0000000000000004 RCX: 0030656c69662f2e
RDX: 0000000000000008 RSI: 0030656c69662f2e RDI: 0000200000000440
RBP: 0000000000000000 R08: 0000001b2d820000 R09: 0000000000000001
R10: 0000000000000001 R11: 0000000000000009 R12: 00007f3860115fac
R13: 00007f3860115fa0 R14: fffffffffffffffe R15: 0000000000000004
</TASK>


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

[syzbot]

For archival purposes, forwarding an incoming command email to
linux-...@vger.kernel.org, syzkall...@googlegroups.com.

***

Subject: [PATCH] ext4: check folio uptodate state in ext4_page_mkwrite()
Author: karti...@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master


When a write fault occurs on a memory-mapped ext4 file, ext4_page_mkwrite()
is called to prepare the folio for writing. However, if the folio could
not be read successfully due to filesystem corruption or I/O errors, it
will not be marked uptodate.

Attempting to write to a non-uptodate folio is problematic because:
1. We don't have valid data from the backing store to preserve
2. A subsequent writeback could write uninitialized data to disk
3. It triggers a warning in __folio_mark_dirty():
WARN_ON_ONCE(warn && !folio_test_uptodate(folio))

This issue can be reproduced by:
1. Creating a corrupted ext4 filesystem with invalid extent entries
2. Memory-mapping a file on that filesystem
3. Attempting to write to the mapped region

The sequence of events is:
- User reads/writes to mmap region -> page fault
- ext4_filemap_fault() -> ext4_map_blocks() detects corruption
- Returns error, folio allocated but NOT marked uptodate
- User writes to same region -> ext4_page_mkwrite() called
- No uptodate check -> folio marked dirty -> WARNING

Fix this by checking folio_test_uptodate() after locking the folio in
ext4_page_mkwrite(). If the folio is not uptodate, unlock it and return
VM_FAULT_SIGBUS to signal the error to userspace. This is consistent
with how we handle other error conditions like truncated folios.

Reported-by: syzbot+b0a067...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b0a0670332b6b3230a0a
Signed-off-by: Deepanshu Kartikey <karti...@gmail.com>
---
fs/buffer.c | 5 +++--
fs/ext4/inode.c | 11 ++++++++++-
2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 6a8752f7bbed..805cc0a2ecab 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2621,7 +2621,8 @@ int block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
unsigned long end;
loff_t size;
int ret;
-
+ pr_info("BUFFER_DEBUG: block_page_mkwrite called for inode %lu, folio index %lu, uptodate=%d\n",
+ inode->i_ino, folio->index, folio_test_uptodate(folio));
folio_lock(folio);
size = i_size_read(inode);
if ((folio->mapping != inode->i_mapping) ||
@@ -2630,7 +2631,7 @@ int block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
ret = -EFAULT;
goto out_unlock;
}
-
+ pr_info("BUFFER_DEBUG: Folio checks passed, calling __block_write_begin\n");
end = folio_size(folio);
/* folio is wholly or partially inside EOF */
if (folio_pos(folio) + end > size)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index e99306a8f47c..535da8fa9233 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -6675,7 +6675,8 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf)
struct address_space *mapping = inode->i_mapping;
get_block_t *get_block = ext4_get_block;
int retries = 0;
-
+ pr_info("EXT4_DEBUG: ext4_page_mkwrite called for inode %lu, folio index %lu, uptodate=%d\n",
+ inode->i_ino, folio->index, folio_test_uptodate(folio));
if (unlikely(IS_IMMUTABLE(inode)))
return VM_FAULT_SIGBUS;

@@ -6700,19 +6701,27 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf)
/* Delalloc case is easy... */
if (test_opt(inode->i_sb, DELALLOC) &&
!ext4_nonda_switch(inode->i_sb)) {
+ pr_info("EXT4_DEBUG: Taking delalloc path, calling block_page_mkwrite\n");
do {
err = block_page_mkwrite(vma, vmf,
ext4_da_get_block_prep);
+ pr_info("EXT4_DEBUG: block_page_mkwrite returned err=%d\n", err);
} while (err == -ENOSPC &&
ext4_should_retry_alloc(inode->i_sb, &retries));
goto out_ret;
}

folio_lock(folio);
+ if (!folio_test_uptodate(folio)) {
+ folio_unlock(folio);
+ ret = VM_FAULT_SIGBUS;
+ goto out;
+ }
size = i_size_read(inode);
/* Page got truncated from under us? */
if (folio->mapping != mapping || folio_pos(folio) > size) {
folio_unlock(folio);
+ pr_info("EXT4_DEBUG: Folio truncated, returning VM_FAULT_NOPAGE\n");
ret = VM_FAULT_NOPAGE;
goto out;
}
--
2.43.0

[syzbot]

Hello,

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

Reported-by: syzbot+b0a067...@syzkaller.appspotmail.com
Tested-by: syzbot+b0a067...@syzkaller.appspotmail.com

Tested on:

commit: fd95357f Merge tag 'sched_ext-for-6.18-rc6-fixes-2' of..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16457a12580000

kernel config: https://syzkaller.appspot.com/x/.config?x=38a0c4cddc846161
dashboard link: https://syzkaller.appspot.com/bug?extid=b0a0670332b6b3230a0a
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

patch: https://syzkaller.appspot.com/x/patch.diff?x=14e7e8b4580000

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