Re: [syzbot] KMSAN: uninit-value in pagecache_write

[Eric Biggers]

On Mon, Nov 07, 2022 at 10:46:13AM +0100, 'Alexander Potapenko' via syzkaller-bugs wrote:
>    ext4: initialize fsdata in pagecache_write()
> 
>     When aops->write_begin() does not initialize fsdata, KMSAN reports
>     an error passing the latter to aops->write_end().
> 
>     Fix this by unconditionally initializing fsdata.
> 
>     Fixes: c93d8f885809 ("ext4: add basic fs-verity support")
>     Reported-by: syzbot+9767be679ef5016b6082@xxxxxxxxxxxxxxxxxxxxxxxxx
>     Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> 
> diff --git a/fs/ext4/verity.c b/fs/ext4/verity.c
> index 3c640bd7ecaeb..30e3b65798b50 100644
> --- a/fs/ext4/verity.c
> +++ b/fs/ext4/verity.c
> @@ -79,7 +79,7 @@ static int pagecache_write(struct inode *inode,
> const void *buf, size_t count,
>                 size_t n = min_t(size_t, count,
>                                  PAGE_SIZE - offset_in_page(pos));
>                 struct page *page;
> -               void *fsdata;
> +               void *fsdata = NULL;
>                 int res;
> 
>                 res = aops->write_begin(NULL, mapping, pos, n, &page, &fsdata);

Are you sure that KMSAN should be reporting this?  The uninitialized value is
passed as a function parameter, but it's never actually used.

Anyway, this patch doesn't hurt, I suppose.  Can please you send it out as a
formal patch to linux-ext4?  It would be easy for people to miss this patch
buried in this thread.  Also, can you please send a patch to linux-f2fs-devel
for the same code in fs/f2fs/verity.c?

Thanks!

- Eric