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

[Alexander Potapenko <glider@google.com>]

On Mon, Nov 7, 2022 at 6:56 PM Eric Biggers <ebiggers@kernel.org> wrote:
>
> 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@syzkaller.appspotmail.com
> >     Signed-off-by: Alexander Potapenko <glider@google.com>
> >
> > 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.

To summarize what's written here:
https://lore.kernel.org/lkml/20220701142310.2188015-44-glider@google.com/
:
 - this is UB from the C11 standpoint;
 - Linus despises UB in general, but expecting function arguments to
be initialized is reasonable;
 - if a function ends up being a no-op, tools should not warn about
uninits passed to it;
 - KMSAN only warns about functions that survive inlining.

In addition, one cannot really rely on write_end() being consistent
with write_begin(), in particular if fsdata was ignored initially and
only appeared at a later point.

>
> 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?

Will do!

> Thanks!
>
> - Eric



-- 
Alexander Potapenko
Software Engineer

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