With below two cases, it will cause NULL pointer dereference when
accessing SM_I(sbi)->fcc_info in f2fs_issue_flush().

a) If kthread_run() fails in f2fs_create_flush_cmd_control(), it will
release SM_I(sbi)->fcc_info,

- mount -o noflush_merge /dev/vda /mnt/f2fs
- mount -o remount,flush_merge /dev/vda /mnt/f2fs  -- kthread_run() fails
- dd if=/dev/zero of=/mnt/f2fs/file bs=4k count=1 conv=fsync

b) we will never allocate memory for SM_I(sbi)->fcc_info w/ below
testcase,

- mount -o ro /dev/vda /mnt/f2fs
- mount -o rw,remount /dev/vda /mnt/f2fs
- dd if=/dev/zero of=/mnt/f2fs/file bs=4k count=1 conv=fsync

In order to fix this issue, let change as below:
- fix error path handling in f2fs_create_flush_cmd_control().
- allocate SM_I(sbi)->fcc_info even if readonly is on.

Signed-off-by: Chao Yu <chao@kernel.org>
---
 fs/f2fs/segment.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 2646575f43de..16f60c646cc2 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -663,8 +663,7 @@ int f2fs_create_flush_cmd_control(struct f2fs_sb_info *sbi)
 	if (IS_ERR(fcc->f2fs_issue_flush)) {
 		int err = PTR_ERR(fcc->f2fs_issue_flush);
 
-		kfree(fcc);
-		SM_I(sbi)->fcc_info = NULL;
+		fcc->f2fs_issue_flush = NULL;
 		return err;
 	}
 
@@ -5137,11 +5136,9 @@ int f2fs_build_segment_manager(struct f2fs_sb_info *sbi)
 
 	init_f2fs_rwsem(&sm_info->curseg_lock);
 
-	if (!f2fs_readonly(sbi->sb)) {
-		err = f2fs_create_flush_cmd_control(sbi);
-		if (err)
-			return err;
-	}
+	err = f2fs_create_flush_cmd_control(sbi);
+	if (err)
+		return err;
 
 	err = create_discard_cmd_control(sbi);
 	if (err)
-- 
2.36.1

Hello:

This patch was applied to jaegeuk/f2fs.git (dev)
by Jaegeuk Kim <jaegeuk@kernel.org>:

On Fri, 30 Dec 2022 23:43:32 +0800 you wrote:
> With below two cases, it will cause NULL pointer dereference when
> accessing SM_I(sbi)->fcc_info in f2fs_issue_flush().
> 
> a) If kthread_run() fails in f2fs_create_flush_cmd_control(), it will
> release SM_I(sbi)->fcc_info,
> 
> - mount -o noflush_merge /dev/vda /mnt/f2fs
> - mount -o remount,flush_merge /dev/vda /mnt/f2fs  -- kthread_run() fails
> - dd if=/dev/zero of=/mnt/f2fs/file bs=4k count=1 conv=fsync
> 
> [...]

Here is the summary with links:
  - [f2fs-dev] f2fs: fix to avoid NULL pointer dereference in f2fs_issue_flush()
    https://git.kernel.org/jaegeux/f2fs/c/b3d83066cbeb

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html

<patchwork-bot+f2fs@kernel.org> 于2023年1月4日周三 01:04写道：
>
> Hello:
>
> This patch was applied to jaegeuk/f2fs.git (dev)
> by Jaegeuk Kim <jaegeuk@kernel.org>:
>
> On Fri, 30 Dec 2022 23:43:32 +0800 you wrote:
> > With below two cases, it will cause NULL pointer dereference when
> > accessing SM_I(sbi)->fcc_info in f2fs_issue_flush().
> >
> > a) If kthread_run() fails in f2fs_create_flush_cmd_control(), it will
> > release SM_I(sbi)->fcc_info,
> >
> > - mount -o noflush_merge /dev/vda /mnt/f2fs
> > - mount -o remount,flush_merge /dev/vda /mnt/f2fs  -- kthread_run() fails
> > - dd if=/dev/zero of=/mnt/f2fs/file bs=4k count=1 conv=fsync
> >
> > [...]
>
> Here is the summary with links:
>   - [f2fs-dev] f2fs: fix to avoid NULL pointer dereference in f2fs_issue_flush()
>     https://git.kernel.org/jaegeux/f2fs/c/b3d83066cbeb
>

Hi jeageuk,
There is a character error in patchwork configuration, :).
:s/jaegeux/jeageuk/g

> You are awesome, thank you!
> --
> Deet-doot-dot, I am a bot.
> https://korg.docs.kernel.org/patchwork/pwbot.html
>
>
>
>
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