fuse: revert back to __readahead_folio() for readahead

In commit 3eab9d7bc2f4 ("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: 3eab9d7bc2f4 ("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>

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 5b5f789b37eb68811832d905ca05b59a0d5a2b2a..2b2d1b75554445bd13efd6eb96885d23cd58b4a3 100644 (file)
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -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;

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 7d92a547999858404cd7c38d423dbe2c7674fa66..d63e56fd3dd207008dfb658f1d12f787ba15fa12 100644 (file)
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -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++;