On Mon, Nov 7, 2022 at 6:56 PM Eric Biggers <ebiggers@xxxxxxxxxx> 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@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.
To summarize what's written here:
https://lore.kernel.org/lkml/20220701142310.2188015-44-glider@xxxxxxxxxx/
:
- 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
Google Germany GmbH
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Geschäftsführer: Paul Manicle, Liana Sebastian
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