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SubjectRe: [PATCH v2] iomap: Fix WARN_ON_ONCE() from unprivileged users
Date
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?

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)

-Eric

> return -EIO;
> }
> }
>

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