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Subject[PATCH AUTOSEL 5.4 32/35] xenbus: req->body should be updated before req->state
Date
From: Dongli Zhang <dongli.zhang@oracle.com>

[ Upstream commit 1b6a51e86cce38cf4d48ce9c242120283ae2f603 ]

The req->body should be updated before req->state is updated and the
order should be guaranteed by a barrier.

Otherwise, read_reply() might return req->body = NULL.

Below is sample callstack when the issue is reproduced on purpose by
reordering the updates of req->body and req->state and adding delay in
code between updates of req->state and req->body.

[ 22.356105] general protection fault: 0000 [#1] SMP PTI
[ 22.361185] CPU: 2 PID: 52 Comm: xenwatch Not tainted 5.5.0xen+ #6
[ 22.366727] Hardware name: Xen HVM domU, BIOS ...
[ 22.372245] RIP: 0010:_parse_integer_fixup_radix+0x6/0x60
... ...
[ 22.392163] RSP: 0018:ffffb2d64023fdf0 EFLAGS: 00010246
[ 22.395933] RAX: 0000000000000000 RBX: 75746e7562755f6d RCX: 0000000000000000
[ 22.400871] RDX: 0000000000000000 RSI: ffffb2d64023fdfc RDI: 75746e7562755f6d
[ 22.405874] RBP: 0000000000000000 R08: 00000000000001e8 R09: 0000000000cdcdcd
[ 22.410945] R10: ffffb2d6402ffe00 R11: ffff9d95395eaeb0 R12: ffff9d9535935000
[ 22.417613] R13: ffff9d9526d4a000 R14: ffff9d9526f4f340 R15: ffff9d9537654000
[ 22.423726] FS: 0000000000000000(0000) GS:ffff9d953bc80000(0000) knlGS:0000000000000000
[ 22.429898] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 22.434342] CR2: 000000c4206a9000 CR3: 00000001ea3fc002 CR4: 00000000001606e0
[ 22.439645] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 22.444941] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 22.450342] Call Trace:
[ 22.452509] simple_strtoull+0x27/0x70
[ 22.455572] xenbus_transaction_start+0x31/0x50
[ 22.459104] netback_changed+0x76c/0xcc1 [xen_netfront]
[ 22.463279] ? find_watch+0x40/0x40
[ 22.466156] xenwatch_thread+0xb4/0x150
[ 22.469309] ? wait_woken+0x80/0x80
[ 22.472198] kthread+0x10e/0x130
[ 22.474925] ? kthread_park+0x80/0x80
[ 22.477946] ret_from_fork+0x35/0x40
[ 22.480968] Modules linked in: xen_kbdfront xen_fbfront(+) xen_netfront xen_blkfront
[ 22.486783] ---[ end trace a9222030a747c3f7 ]---
[ 22.490424] RIP: 0010:_parse_integer_fixup_radix+0x6/0x60

The virt_rmb() is added in the 'true' path of test_reply(). The "while"
is changed to "do while" so that test_reply() is used as a read memory
barrier.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Link: https://lore.kernel.org/r/20200303221423.21962-1-dongli.zhang@oracle.com
Reviewed-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/xen/xenbus/xenbus_comms.c | 2 ++
drivers/xen/xenbus/xenbus_xs.c | 9 ++++++---
2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c
index d239fc3c5e3de..852ed161fc2a7 100644
--- a/drivers/xen/xenbus/xenbus_comms.c
+++ b/drivers/xen/xenbus/xenbus_comms.c
@@ -313,6 +313,8 @@ static int process_msg(void)
req->msg.type = state.msg.type;
req->msg.len = state.msg.len;
req->body = state.body;
+ /* write body, then update state */
+ virt_wmb();
req->state = xb_req_state_got_reply;
req->cb(req);
} else
diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c
index ddc18da61834e..3a06eb699f333 100644
--- a/drivers/xen/xenbus/xenbus_xs.c
+++ b/drivers/xen/xenbus/xenbus_xs.c
@@ -191,8 +191,11 @@ static bool xenbus_ok(void)

static bool test_reply(struct xb_req_data *req)
{
- if (req->state == xb_req_state_got_reply || !xenbus_ok())
+ if (req->state == xb_req_state_got_reply || !xenbus_ok()) {
+ /* read req->state before all other fields */
+ virt_rmb();
return true;
+ }

/* Make sure to reread req->state each time. */
barrier();
@@ -202,7 +205,7 @@ static bool test_reply(struct xb_req_data *req)

static void *read_reply(struct xb_req_data *req)
{
- while (req->state != xb_req_state_got_reply) {
+ do {
wait_event(req->wq, test_reply(req));

if (!xenbus_ok())
@@ -216,7 +219,7 @@ static void *read_reply(struct xb_req_data *req)
if (req->err)
return ERR_PTR(req->err);

- }
+ } while (req->state != xb_req_state_got_reply);

return req->body;
}
--
2.20.1
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