Commit 0c67c37e authored Feb 11, 2025 by Joanne Koong Committed by Miklos Szeredi Feb 14, 2025

fuse: revert back to __readahead_folio() for readahead

In commit 3eab9d7b ("fuse: convert readahead to use folios"), the
logic was converted to using the new folio readahead code, which drops
the reference on the folio once it is locked, using an inferred
reference on the folio. Previously we held a reference on the folio for
the entire duration of the readpages call.

This is fine, however for the case for splice pipe responses where we
will remove the old folio and splice in the new folio (see
fuse_try_move_page()), we assume that there is a reference held on the
folio for ap->folios, which is no longer the case.

To fix this, revert back to __readahead_folio() which allows us to hold
the reference on the folio for the duration of readpages until either we
drop the reference ourselves in fuse_readpages_end() or the reference is
dropped after it's replaced in the page cache in the splice case.
This will fix the UAF bug that was reported.

Link: https://lore.kernel.org/linux-fsdevel/2f681f48-00f5-4e09-8431-2b3dbfaa881e@heusel.eu/

Fixes: 3eab9d7b ("fuse: convert readahead to use folios")
Reported-by: Christian Heusel <christian@heusel.eu>
Closes: https://lore.kernel.org/all/2f681f48-00f5-4e09-8431-2b3dbfaa881e@heusel.eu/
Closes: https://gitlab.archlinux.org/archlinux/packaging/packages/linux/-/issues/110

Reported-by: Mantas Mikulėnas <grawity@gmail.com>
Closes: https://lore.kernel.org/all/34feb867-09e2-46e4-aa31-d9660a806d1a@gmail.com/
Closes: https://bugzilla.opensuse.org/show_bug.cgi?id=1236660

Cc: <stable@vger.kernel.org> # v6.13
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

parent a64dcfb4

fs/fuse/dev.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -838,6 +838,12 @@ static int fuse_check_folio(struct folio *folio)
		return 0;
		}

		/*
		* Attempt to steal a page from the splice() pipe and move it into the
		* pagecache. If successful, the pointer in @pagep will be updated. The
		* folio that was originally in @pagep will lose a reference and the new
		* folio returned in @pagep will carry a reference.
		*/
		static int fuse_try_move_page(struct fuse_copy_state cs, struct page *pagep)
		{
		int err;

+11 −2

Original line number	Diff line number	Diff line
		@@ -955,8 +955,10 @@ static void fuse_readpages_end(struct fuse_mount fm, struct fuse_args args,
		fuse_invalidate_atime(inode);
		}

		for (i = 0; i < ap->num_folios; i++)
		for (i = 0; i < ap->num_folios; i++) {
		folio_end_read(ap->folios[i], !err);
		folio_put(ap->folios[i]);
		}
		if (ia->ff)
		fuse_file_put(ia->ff, false);

		@@ -1048,7 +1050,14 @@ static void fuse_readahead(struct readahead_control *rac)
		ap = &ia->ap;

		while (ap->num_folios < cur_pages) {
		folio = readahead_folio(rac);
		/*
		* This returns a folio with a ref held on it.
		* The ref needs to be held until the request is
		* completed, since the splice case (see
		* fuse_try_move_page()) drops the ref after it's
		* replaced in the page cache.
		*/
		folio = __readahead_folio(rac);
		ap->folios[ap->num_folios] = folio;
		ap->descs[ap->num_folios].length = folio_size(folio);
		ap->num_folios++;