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* [PATCH] afs: Don't unlock fetched data pages until the op completes successfully
@ 2020-05-17 20:21 David Howells
  2020-05-17 21:08 ` Matthew Wilcox
  2020-05-17 22:36 ` David Howells
  0 siblings, 2 replies; 3+ messages in thread
From: David Howells @ 2020-05-17 20:21 UTC (permalink / raw)
  To: torvalds; +Cc: dhowells, linux-afs, linux-fsdevel, linux-kernel

Don't call req->page_done() on each page as we finish filling it with the
data coming from the network.  Whilst this might speed up the application a
bit, it's a problem if there's a network failure and the operation has to
be reissued.

If this happens, an oops occurs because afs_readpages_page_done() clears
the pointer to each page it unlocks and when a retry happens, the pointers
to the pages it wants to fill are now NULL (and the pages have been
unlocked anyway).

Instead, wait till the operation completes successfully and only then
release all the pages after clearing any terminal gap (the server can give
us less data than we requested as we're allowed to ask for more than is
available).

KASAN produces a bug like the following, and even without KASAN, it can
oops and panic:

BUG: KASAN: wild-memory-access in _copy_to_iter+0x323/0x5f4
Write of size 1404 at addr 0005088000000000 by task md5sum/5235

CPU: 0 PID: 5235 Comm: md5sum Not tainted 5.7.0-rc3-fscache+ #250
Hardware name: ASUS All Series/H97-PLUS, BIOS 2306 10/09/2014
Call Trace:
 dump_stack+0x97/0xd3
 ? _copy_to_iter+0x323/0x5f4
 __kasan_report+0x130/0x158
 ? _copy_to_iter+0x323/0x5f4
 kasan_report+0x42/0x51
 ? _copy_to_iter+0x323/0x5f4
 check_memory_region+0x13d/0x145
 memcpy+0x39/0x58
 _copy_to_iter+0x323/0x5f4
 ? iov_iter_get_pages_alloc+0x562/0x562
 ? match_held_lock+0x2e/0xff
 ? match_held_lock+0x2e/0xff
 ? __lock_is_held+0x2a/0x87
 ? lockdep_recursion_finish+0x1a/0x39
 __skb_datagram_iter+0x89/0x2a6
 ? copy_overflow+0x14/0x14
 ? lockdep_recursion_finish+0x1a/0x39
 skb_copy_datagram_iter+0x129/0x135
 rxrpc_recvmsg_data.isra.0+0x615/0xd42
 ? rxrpc_kernel_recv_data+0x182/0x3ae
 ? trace_rxrpc_rx_eproto.constprop.0+0x106/0x106
 ? lockdep_recursion_finish+0x1a/0x39
 ? mutex_trylock+0x96/0x96
 rxrpc_kernel_recv_data+0x1e9/0x3ae
 ? rxrpc_recvmsg_data.isra.0+0xd42/0xd42
 ? find_held_lock+0x81/0x90
 ? lockdep_recursion_finish+0x1a/0x39
 afs_extract_data+0x139/0x33a
 ? afs_send_simple_reply+0x2ef/0x2ef
 yfs_deliver_fs_fetch_data64+0x47a/0x91b
 ? afs_v2net+0x15e/0x15e
 afs_deliver_to_call+0x304/0x709
 ? afs_set_call_complete+0xcc/0xcc
 ? test_bit+0x22/0x2e
 afs_wait_for_call_to_complete+0x1cc/0x4ad
 ? afs_make_call+0x8b8/0x8b8
 ? wake_up_q+0x63/0x63
 ? __init_waitqueue_head+0x7b/0x85
 yfs_fs_fetch_data+0x279/0x288
 afs_fetch_data+0x1e1/0x38d
 ? afs_put_read+0x9a/0x9a
 ? test_bit+0x1d/0x27
 ? __kmalloc+0x16c/0x193
 ? afs_readpages+0x32f/0x72e
 afs_readpages+0x593/0x72e
 ? afs_fetch_data+0x38d/0x38d
 ? rcu_read_lock_sched_held+0x78/0xc5
 read_pages+0xf5/0x21e
 ? read_cache_pages+0x1cf/0x1cf
 ? xa_set_mark+0x34/0x34
 ? policy_nodemask+0x19/0x8b
 ? policy_node+0x3b/0x58
 __do_page_cache_readahead+0x128/0x23f
 ? blk_cgroup_congested+0x156/0x156
 ? rcu_gp_is_expedited+0x42/0x42
 ? lockdep_recursion_finish+0x1a/0x39
 ondemand_readahead+0x36e/0x37f
 generic_file_buffered_read+0x234/0x680
 ? iov_iter_init+0x8f/0x9e
 new_sync_read+0x109/0x17e
 ? generic_remap_file_range_prep+0x4a9/0x4a9
 ? __fsnotify_update_child_dentry_flags+0x18a/0x18a
 ? fsnotify_first_mark+0xa6/0xa6
 ? selinux_file_permission+0xea/0x133
 vfs_read+0xe6/0x138
 ksys_read+0xd8/0x14d
 ? kernel_write+0x74/0x74
 ? get_vtime_delta+0x9c/0xaa
 ? mark_held_locks+0x1f/0x78
 do_syscall_64+0x6e/0x8a
 entry_SYSCALL_64_after_hwframe+0x49/0xb3

Fixes: 196ee9cd2d04 ("afs: Make afs_fs_fetch_data() take a list of pages")
Fixes: 30062bd13e36 ("afs: Implement YFS support in the fs client")
Signed-off-by: David Howells <dhowells@redhat.com>
---

 fs/afs/fsclient.c  |    8 ++++----
 fs/afs/yfsclient.c |    8 ++++----
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/fs/afs/fsclient.c b/fs/afs/fsclient.c
index a3e3a16b32d6..401de063996c 100644
--- a/fs/afs/fsclient.c
+++ b/fs/afs/fsclient.c
@@ -385,8 +385,6 @@ static int afs_deliver_fs_fetch_data(struct afs_call *call)
 		ASSERTCMP(req->offset, <=, PAGE_SIZE);
 		if (req->offset == PAGE_SIZE) {
 			req->offset = 0;
-			if (req->page_done)
-				req->page_done(req);
 			req->index++;
 			if (req->remain > 0)
 				goto begin_page;
@@ -440,11 +438,13 @@ static int afs_deliver_fs_fetch_data(struct afs_call *call)
 		if (req->offset < PAGE_SIZE)
 			zero_user_segment(req->pages[req->index],
 					  req->offset, PAGE_SIZE);
-		if (req->page_done)
-			req->page_done(req);
 		req->offset = 0;
 	}
 
+	if (req->page_done)
+		for (req->index = 0; req->index < req->nr_pages; req->index++)
+			req->page_done(req);
+
 	_leave(" = 0 [done]");
 	return 0;
 }
diff --git a/fs/afs/yfsclient.c b/fs/afs/yfsclient.c
index b5b45c57e1b1..fe413e7a5cf4 100644
--- a/fs/afs/yfsclient.c
+++ b/fs/afs/yfsclient.c
@@ -497,8 +497,6 @@ static int yfs_deliver_fs_fetch_data64(struct afs_call *call)
 		ASSERTCMP(req->offset, <=, PAGE_SIZE);
 		if (req->offset == PAGE_SIZE) {
 			req->offset = 0;
-			if (req->page_done)
-				req->page_done(req);
 			req->index++;
 			if (req->remain > 0)
 				goto begin_page;
@@ -556,11 +554,13 @@ static int yfs_deliver_fs_fetch_data64(struct afs_call *call)
 		if (req->offset < PAGE_SIZE)
 			zero_user_segment(req->pages[req->index],
 					  req->offset, PAGE_SIZE);
-		if (req->page_done)
-			req->page_done(req);
 		req->offset = 0;
 	}
 
+	if (req->page_done)
+		for (req->index = 0; req->index < req->nr_pages; req->index++)
+			req->page_done(req);
+
 	_leave(" = 0 [done]");
 	return 0;
 }



^ permalink raw reply	[flat|nested] 3+ messages in thread

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2020-05-17 20:21 [PATCH] afs: Don't unlock fetched data pages until the op completes successfully David Howells
2020-05-17 21:08 ` Matthew Wilcox
2020-05-17 22:36 ` David Howells

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