@@ -58,6 +58,7 @@ static inline void netfs_proc_del_rreq(struct netfs_io_request *rreq) {}
/*
* misc.c
*/
+struct folio_queue *netfs_buffer_make_space(struct netfs_io_request *rreq);
int netfs_buffer_append_folio(struct netfs_io_request *rreq, struct folio *folio,
bool needs_put);
struct folio_queue *netfs_delete_buffer_head(struct netfs_io_request *wreq);
@@ -9,34 +9,66 @@
#include "internal.h"
/*
- * Append a folio to the rolling queue.
+ * Make sure there's space in the rolling queue.
*/
-int netfs_buffer_append_folio(struct netfs_io_request *rreq, struct folio *folio,
- bool needs_put)
+struct folio_queue *netfs_buffer_make_space(struct netfs_io_request *rreq)
{
- struct folio_queue *tail = rreq->buffer_tail;
- unsigned int slot, order = folio_order(folio);
+ struct folio_queue *tail = rreq->buffer_tail, *prev;
+ unsigned int prev_nr_slots = 0;
if (WARN_ON_ONCE(!rreq->buffer && tail) ||
WARN_ON_ONCE(rreq->buffer && !tail))
- return -EIO;
-
- if (!tail || folioq_full(tail)) {
- tail = kmalloc(sizeof(*tail), GFP_NOFS);
- if (!tail)
- return -ENOMEM;
- netfs_stat(&netfs_n_folioq);
- folioq_init(tail);
- tail->prev = rreq->buffer_tail;
- if (tail->prev)
- tail->prev->next = tail;
- rreq->buffer_tail = tail;
- if (!rreq->buffer) {
- rreq->buffer = tail;
- iov_iter_folio_queue(&rreq->io_iter, ITER_SOURCE, tail, 0, 0, 0);
+ return ERR_PTR(-EIO);
+
+ prev = tail;
+ if (prev) {
+ if (!folioq_full(tail))
+ return tail;
+ prev_nr_slots = folioq_nr_slots(tail);
+ }
+
+ tail = kmalloc(sizeof(*tail), GFP_NOFS);
+ if (!tail)
+ return ERR_PTR(-ENOMEM);
+ netfs_stat(&netfs_n_folioq);
+ folioq_init(tail);
+ tail->prev = prev;
+ if (prev)
+ /* [!] NOTE: After we set prev->next, the consumer is entirely
+ * at liberty to delete prev.
+ */
+ WRITE_ONCE(prev->next, tail);
+
+ rreq->buffer_tail = tail;
+ if (!rreq->buffer) {
+ rreq->buffer = tail;
+ iov_iter_folio_queue(&rreq->io_iter, ITER_SOURCE, tail, 0, 0, 0);
+ } else {
+ /* Make sure we don't leave the master iterator pointing to a
+ * block that might get immediately consumed.
+ */
+ if (rreq->io_iter.folioq == prev &&
+ rreq->io_iter.folioq_slot == prev_nr_slots) {
+ rreq->io_iter.folioq = tail;
+ rreq->io_iter.folioq_slot = 0;
}
- rreq->buffer_tail_slot = 0;
}
+ rreq->buffer_tail_slot = 0;
+ return tail;
+}
+
+/*
+ * Append a folio to the rolling queue.
+ */
+int netfs_buffer_append_folio(struct netfs_io_request *rreq, struct folio *folio,
+ bool needs_put)
+{
+ struct folio_queue *tail;
+ unsigned int slot, order = folio_order(folio);
+
+ tail = netfs_buffer_make_space(rreq);
+ if (IS_ERR(tail))
+ return PTR_ERR(tail);
rreq->io_iter.count += PAGE_SIZE << order;
@@ -153,12 +153,22 @@ static void netfs_prepare_write(struct netfs_io_request *wreq,
loff_t start)
{
struct netfs_io_subrequest *subreq;
+ struct iov_iter *wreq_iter = &wreq->io_iter;
+
+ /* Make sure we don't point the iterator at a used-up folio_queue
+ * struct being used as a placeholder to prevent the queue from
+ * collapsing. In such a case, extend the queue.
+ */
+ if (iov_iter_is_folioq(wreq_iter) &&
+ wreq_iter->folioq_slot >= folioq_nr_slots(wreq_iter->folioq)) {
+ netfs_buffer_make_space(wreq);
+ }
subreq = netfs_alloc_subrequest(wreq);
subreq->source = stream->source;
subreq->start = start;
subreq->stream_nr = stream->stream_nr;
- subreq->io_iter = wreq->io_iter;
+ subreq->io_iter = *wreq_iter;
_enter("R=%x[%x]", wreq->debug_id, subreq->debug_index);
In netfslib, a buffered writeback operation has a 'write queue' of folios that are being written, held in a linear sequence of folio_queue structs. The 'issuer' adds new folio_queues on the leading edge of the queue and populates each one progressively; the 'collector' pops them off the trailing edge and discards them and the folios they point to as they are consumed. The queue is required to always retain at least one folio_queue structure. This allows the queue to be accessed without locking and with just a bit of barriering. When a new subrequest is prepared, its ->io_iter iterator is pointed at the current end of the write queue and then the iterator is extended as more data is added to the queue until the subrequest is committed. Now, the problem is that the folio_queue at the leading edge of the write queue when a subrequest is prepared might have been entirely consumed - but not yet removed from the queue as it is the only remaining one and is preventing the queue from collapsing. So, what happens is that subreq->io_iter is pointed at the spent folio_queue, then a new folio_queue is added, and, at that point, the collector is at entirely at liberty to immediately delete the spent folio_queue. This leaves the subreq->io_iter pointing at a freed object. If the system is lucky, iterate_folioq() sees ->io_iter, sees the as-yet uncorrupted freed object and advances to the next folio_queue in the queue. In the case seen, however, the freed object gets recycled and put back onto the queue at the tail and filled to the end. This confuses iterate_folioq() and it tries to step ->next, which may be NULL - resulting in an oops. Fix this by the following means: (1) When preparing a write subrequest, make sure there's a folio_queue struct with space in it at the leading edge of the queue. A function to make space is split out of the function to append a folio so that it can be called for this purpose. (2) If the request struct iterator is pointing to a completely spent folio_queue when we make space, then advance the iterator to the newly allocated folio_queue. The subrequest's iterator will then be set from this. The oops could be triggered using the generic/346 xfstest with a filesystem on9P over TCP with cache=loose. The oops looked something like: BUG: kernel NULL pointer dereference, address: 0000000000000008 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page ... RIP: 0010:_copy_from_iter+0x2db/0x530 ... Call Trace: <TASK> ... p9pdu_vwritef+0x3d8/0x5d0 p9_client_prepare_req+0xa8/0x140 p9_client_rpc+0x81/0x280 p9_client_write+0xcf/0x1c0 v9fs_issue_write+0x87/0xc0 netfs_advance_write+0xa0/0xb0 netfs_write_folio.isra.0+0x42d/0x500 netfs_writepages+0x15a/0x1f0 do_writepages+0xd1/0x220 filemap_fdatawrite_wbc+0x5c/0x80 v9fs_mmap_vm_close+0x7d/0xb0 remove_vma+0x35/0x70 vms_complete_munmap_vmas+0x11a/0x170 do_vmi_align_munmap+0x17d/0x1c0 do_vmi_munmap+0x13e/0x150 __vm_munmap+0x92/0xd0 __x64_sys_munmap+0x17/0x20 do_syscall_64+0x80/0xe0 entry_SYSCALL_64_after_hwframe+0x71/0x79 This also fixed a similar-looking issue with cifs and generic/074. Reported-by: kernel test robot <oliver.sang@intel.com> Closes: https://lore.kernel.org/oe-lkp/202409180928.f20b5a08-oliver.sang@intel.com Closes: https://lore.kernel.org/oe-lkp/202409131438.3f225fbf-oliver.sang@intel.com Signed-off-by: David Howells <dhowells@redhat.com> Tested-by: kernel test robot <oliver.sang@intel.com> cc: Eric Van Hensbergen <ericvh@kernel.org> cc: Latchesar Ionkov <lucho@ionkov.net> cc: Dominique Martinet <asmadeus@codewreck.org> cc: Christian Schoenebeck <linux_oss@crudebyte.com> cc: Steve French <sfrench@samba.org> cc: Paulo Alcantara <pc@manguebit.com> cc: Jeff Layton <jlayton@kernel.org> cc: v9fs@lists.linux.dev cc: linux-cifs@vger.kernel.org cc: netfs@lists.linux.dev cc: linux-fsdevel@vger.kernel.org --- Changes: v2) - Drop netfs_folioq_alloc() and code what it did directly in the caller. The cleanup can be done in a follow-up patch. fs/netfs/internal.h | 1 fs/netfs/misc.c | 74 +++++++++++++++++++++++++++++++++++-------------- fs/netfs/write_issue.c | 12 +++++++ 3 files changed, 65 insertions(+), 22 deletions(-)