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SubjectRe: [PATCH v2] iomap: Fix WARN_ON_ONCE() from unprivileged users
On Mon, Aug 31, 2020 at 10:48:59AM -0500, Eric Sandeen wrote:
> On 8/30/20 8:45 PM, Qian Cai wrote:
> > It is trivial to trigger a WARN_ON_ONCE(1) in iomap_dio_actor() by
> > unprivileged users which would taint the kernel, or worse - panic if
> > panic_on_warn or panic_on_taint is set. Hence, just convert it to
> > pr_warn_ratelimited() to let users know their workloads are racing.
> > Thanks Dave Chinner for the initial analysis of the racing reproducers.
> >
> > Signed-off-by: Qian Cai <cai@lca.pw>
> > ---
> >
> > v2: Record the path, pid and command as well.
> >
> > fs/iomap/direct-io.c | 17 ++++++++++++++++-
> > 1 file changed, 16 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> > index c1aafb2ab990..66a4502ef675 100644
> > --- a/fs/iomap/direct-io.c
> > +++ b/fs/iomap/direct-io.c
> > @@ -374,6 +374,7 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
> > void *data, struct iomap *iomap, struct iomap *srcmap)
> > {
> > struct iomap_dio *dio = data;
> > + char pathname[128], *path;
> >
> > switch (iomap->type) {
> > case IOMAP_HOLE:
> > @@ -389,7 +390,21 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
> > case IOMAP_INLINE:
> > return iomap_dio_inline_actor(inode, pos, length, dio, iomap);
> > default:
> > - WARN_ON_ONCE(1);
>
> It seems like we should explicitly catch IOMAP_DELALLOC for this case, and leave the
> default: as a WARN_ON that is not user-triggerable? i.e.
>
> case IOMAP_DELALLOC:
> <all the fancy warnings>
> return -EIO;
> default:
> WARN_ON_ONCE(1);
> return -EIO;
>
> > + /*
> > + * DIO is not serialised against mmap() access at all, and so
> > + * if the page_mkwrite occurs between the writeback and the
> > + * iomap_apply() call in the DIO path, then it will see the
> > + * DELALLOC block that the page-mkwrite allocated.
> > + */
> > + path = file_path(dio->iocb->ki_filp, pathname,
> > + sizeof(pathname));
> > + if (IS_ERR(path))
> > + path = "(unknown)";
> > +
> > + pr_warn_ratelimited("page_mkwrite() is racing with DIO read (iomap->type = %u).\n"
> > + "File: %s PID: %d Comm: %.20s\n",
> > + iomap->type, path, current->pid,
> > + current->comm);
>
> This is very specific ...
>
> Do we know that mmap/page_mkwrite is (and will always be) the only way to reach this
> point?

I don't know, so this could indeed be a bit misleading.

>
> It seems to me that this message won't be very useful for the admin; "pg_mkwrite" may
> mean something to us, but doubtful for the general public. And "type = 1" won't mean
> much to the reader, either.
>
> Maybe something like:
>
> "DIO encountered delayed allocation block, racing buffered+direct? File: %s Comm: %.20s\n"
>
> It just seems that a user-facing warning should be something the admin has a chance of
> acting on without needing to file a bug for analysis by the developers.
>
> (though TBH "delayed allocation" probably doesn't mean much to the admin, either)

I agree with your suggestions. I'll submit a new version.

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