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Subject[PATCH net-next 03/12] rxrpc: Provide a different lockdep key for call->user_mutex for kernel calls
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Provide a different lockdep key for rxrpc_call::user_mutex when the call is
made on a kernel socket, such as by the AFS filesystem.

The problem is that lockdep registers a false positive between userspace
calling the sendmsg syscall on a user socket where call->user_mutex is held
whilst userspace memory is accessed whereas the AFS filesystem may perform
operations with mmap_sem held by the caller.

In such a case, the following warning is produced.

======================================================
WARNING: possible circular locking dependency detected
4.14.0-fscache+ #243 Tainted: G E
------------------------------------------------------
modpost/16701 is trying to acquire lock:
(&vnode->io_lock){+.+.}, at: [<ffffffffa000fc40>] afs_begin_vnode_operation+0x33/0x77 [kafs]

but task is already holding lock:
(&mm->mmap_sem){++++}, at: [<ffffffff8104376a>] __do_page_fault+0x1ef/0x486

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #3 (&mm->mmap_sem){++++}:
__might_fault+0x61/0x89
_copy_from_iter_full+0x40/0x1fa
rxrpc_send_data+0x8dc/0xff3
rxrpc_do_sendmsg+0x62f/0x6a1
rxrpc_sendmsg+0x166/0x1b7
sock_sendmsg+0x2d/0x39
___sys_sendmsg+0x1ad/0x22b
__sys_sendmsg+0x41/0x62
do_syscall_64+0x89/0x1be
return_from_SYSCALL_64+0x0/0x75

-> #2 (&call->user_mutex){+.+.}:
__mutex_lock+0x86/0x7d2
rxrpc_new_client_call+0x378/0x80e
rxrpc_kernel_begin_call+0xf3/0x154
afs_make_call+0x195/0x454 [kafs]
afs_vl_get_capabilities+0x193/0x198 [kafs]
afs_vl_lookup_vldb+0x5f/0x151 [kafs]
afs_create_volume+0x2e/0x2f4 [kafs]
afs_mount+0x56a/0x8d7 [kafs]
mount_fs+0x6a/0x109
vfs_kern_mount+0x67/0x135
do_mount+0x90b/0xb57
SyS_mount+0x72/0x98
do_syscall_64+0x89/0x1be
return_from_SYSCALL_64+0x0/0x75

-> #1 (k-sk_lock-AF_RXRPC){+.+.}:
lock_sock_nested+0x74/0x8a
rxrpc_kernel_begin_call+0x8a/0x154
afs_make_call+0x195/0x454 [kafs]
afs_fs_get_capabilities+0x17a/0x17f [kafs]
afs_probe_fileserver+0xf7/0x2f0 [kafs]
afs_select_fileserver+0x83f/0x903 [kafs]
afs_fetch_status+0x89/0x11d [kafs]
afs_iget+0x16f/0x4f8 [kafs]
afs_mount+0x6c6/0x8d7 [kafs]
mount_fs+0x6a/0x109
vfs_kern_mount+0x67/0x135
do_mount+0x90b/0xb57
SyS_mount+0x72/0x98
do_syscall_64+0x89/0x1be
return_from_SYSCALL_64+0x0/0x75

-> #0 (&vnode->io_lock){+.+.}:
lock_acquire+0x174/0x19f
__mutex_lock+0x86/0x7d2
afs_begin_vnode_operation+0x33/0x77 [kafs]
afs_fetch_data+0x80/0x12a [kafs]
afs_readpages+0x314/0x405 [kafs]
__do_page_cache_readahead+0x203/0x2ba
filemap_fault+0x179/0x54d
__do_fault+0x17/0x60
__handle_mm_fault+0x6d7/0x95c
handle_mm_fault+0x24e/0x2a3
__do_page_fault+0x301/0x486
do_page_fault+0x236/0x259
page_fault+0x22/0x30
__clear_user+0x3d/0x60
padzero+0x1c/0x2b
load_elf_binary+0x785/0xdc7
search_binary_handler+0x81/0x1ff
do_execveat_common.isra.14+0x600/0x888
do_execve+0x1f/0x21
SyS_execve+0x28/0x2f
do_syscall_64+0x89/0x1be
return_from_SYSCALL_64+0x0/0x75

other info that might help us debug this:

Chain exists of:
&vnode->io_lock --> &call->user_mutex --> &mm->mmap_sem

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&mm->mmap_sem);
lock(&call->user_mutex);
lock(&mm->mmap_sem);
lock(&vnode->io_lock);

*** DEADLOCK ***

1 lock held by modpost/16701:
#0: (&mm->mmap_sem){++++}, at: [<ffffffff8104376a>] __do_page_fault+0x1ef/0x486

stack backtrace:
CPU: 0 PID: 16701 Comm: modpost Tainted: G E 4.14.0-fscache+ #243
Hardware name: ASUS All Series/H97-PLUS, BIOS 2306 10/09/2014
Call Trace:
dump_stack+0x67/0x8e
print_circular_bug+0x341/0x34f
check_prev_add+0x11f/0x5d4
? add_lock_to_list.isra.12+0x8b/0x8b
? add_lock_to_list.isra.12+0x8b/0x8b
? __lock_acquire+0xf77/0x10b4
__lock_acquire+0xf77/0x10b4
lock_acquire+0x174/0x19f
? afs_begin_vnode_operation+0x33/0x77 [kafs]
__mutex_lock+0x86/0x7d2
? afs_begin_vnode_operation+0x33/0x77 [kafs]
? afs_begin_vnode_operation+0x33/0x77 [kafs]
? afs_begin_vnode_operation+0x33/0x77 [kafs]
afs_begin_vnode_operation+0x33/0x77 [kafs]
afs_fetch_data+0x80/0x12a [kafs]
afs_readpages+0x314/0x405 [kafs]
__do_page_cache_readahead+0x203/0x2ba
? filemap_fault+0x179/0x54d
filemap_fault+0x179/0x54d
__do_fault+0x17/0x60
__handle_mm_fault+0x6d7/0x95c
handle_mm_fault+0x24e/0x2a3
__do_page_fault+0x301/0x486
do_page_fault+0x236/0x259
page_fault+0x22/0x30
RIP: 0010:__clear_user+0x3d/0x60
RSP: 0018:ffff880071e93da0 EFLAGS: 00010202
RAX: 0000000000000000 RBX: 000000000000011c RCX: 000000000000011c
RDX: 0000000000000000 RSI: 0000000000000008 RDI: 000000000060f720
RBP: 000000000060f720 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: ffff8800b5459b68 R12: ffff8800ce150e00
R13: 000000000060f720 R14: 00000000006127a8 R15: 0000000000000000
padzero+0x1c/0x2b
load_elf_binary+0x785/0xdc7
search_binary_handler+0x81/0x1ff
do_execveat_common.isra.14+0x600/0x888
do_execve+0x1f/0x21
SyS_execve+0x28/0x2f
do_syscall_64+0x89/0x1be
entry_SYSCALL64_slow_path+0x25/0x25
RIP: 0033:0x7fdb6009ee07
RSP: 002b:00007fff566d9728 EFLAGS: 00000246 ORIG_RAX: 000000000000003b
RAX: ffffffffffffffda RBX: 000055ba57280900 RCX: 00007fdb6009ee07
RDX: 000055ba5727f270 RSI: 000055ba5727cac0 RDI: 000055ba57280900
RBP: 000055ba57280900 R08: 00007fff566d9700 R09: 0000000000000000
R10: 000055ba5727cac0 R11: 0000000000000246 R12: 0000000000000000
R13: 000055ba5727cac0 R14: 000055ba5727f270 R15: 0000000000000000

Signed-off-by: David Howells <dhowells@redhat.com>
---

net/rxrpc/ar-internal.h | 2 +-
net/rxrpc/call_accept.c | 2 +-
net/rxrpc/call_object.c | 19 +++++++++++++++----
3 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/net/rxrpc/ar-internal.h b/net/rxrpc/ar-internal.h
index b2151993d384..a972887b3f5d 100644
--- a/net/rxrpc/ar-internal.h
+++ b/net/rxrpc/ar-internal.h
@@ -672,7 +672,7 @@ extern unsigned int rxrpc_max_call_lifetime;
extern struct kmem_cache *rxrpc_call_jar;

struct rxrpc_call *rxrpc_find_call_by_user_ID(struct rxrpc_sock *, unsigned long);
-struct rxrpc_call *rxrpc_alloc_call(gfp_t);
+struct rxrpc_call *rxrpc_alloc_call(struct rxrpc_sock *, gfp_t);
struct rxrpc_call *rxrpc_new_client_call(struct rxrpc_sock *,
struct rxrpc_conn_parameters *,
struct sockaddr_rxrpc *,
diff --git a/net/rxrpc/call_accept.c b/net/rxrpc/call_accept.c
index cbd1701e813a..3028298ca561 100644
--- a/net/rxrpc/call_accept.c
+++ b/net/rxrpc/call_accept.c
@@ -94,7 +94,7 @@ static int rxrpc_service_prealloc_one(struct rxrpc_sock *rx,
/* Now it gets complicated, because calls get registered with the
* socket here, particularly if a user ID is preassigned by the user.
*/
- call = rxrpc_alloc_call(gfp);
+ call = rxrpc_alloc_call(rx, gfp);
if (!call)
return -ENOMEM;
call->flags |= (1 << RXRPC_CALL_IS_SERVICE);
diff --git a/net/rxrpc/call_object.c b/net/rxrpc/call_object.c
index 4c7fbc6dcce7..1f141dc08ad2 100644
--- a/net/rxrpc/call_object.c
+++ b/net/rxrpc/call_object.c
@@ -55,6 +55,8 @@ static void rxrpc_call_timer_expired(unsigned long _call)
rxrpc_set_timer(call, rxrpc_timer_expired, ktime_get_real());
}

+static struct lock_class_key rxrpc_call_user_mutex_lock_class_key;
+
/*
* find an extant server call
* - called in process context with IRQs enabled
@@ -95,7 +97,7 @@ struct rxrpc_call *rxrpc_find_call_by_user_ID(struct rxrpc_sock *rx,
/*
* allocate a new call
*/
-struct rxrpc_call *rxrpc_alloc_call(gfp_t gfp)
+struct rxrpc_call *rxrpc_alloc_call(struct rxrpc_sock *rx, gfp_t gfp)
{
struct rxrpc_call *call;

@@ -114,6 +116,14 @@ struct rxrpc_call *rxrpc_alloc_call(gfp_t gfp)
goto nomem_2;

mutex_init(&call->user_mutex);
+
+ /* Prevent lockdep reporting a deadlock false positive between the afs
+ * filesystem and sys_sendmsg() via the mmap sem.
+ */
+ if (rx->sk.sk_kern_sock)
+ lockdep_set_class(&call->user_mutex,
+ &rxrpc_call_user_mutex_lock_class_key);
+
setup_timer(&call->timer, rxrpc_call_timer_expired,
(unsigned long)call);
INIT_WORK(&call->processor, &rxrpc_process_call);
@@ -151,7 +161,8 @@ struct rxrpc_call *rxrpc_alloc_call(gfp_t gfp)
/*
* Allocate a new client call.
*/
-static struct rxrpc_call *rxrpc_alloc_client_call(struct sockaddr_rxrpc *srx,
+static struct rxrpc_call *rxrpc_alloc_client_call(struct rxrpc_sock *rx,
+ struct sockaddr_rxrpc *srx,
gfp_t gfp)
{
struct rxrpc_call *call;
@@ -159,7 +170,7 @@ static struct rxrpc_call *rxrpc_alloc_client_call(struct sockaddr_rxrpc *srx,

_enter("");

- call = rxrpc_alloc_call(gfp);
+ call = rxrpc_alloc_call(rx, gfp);
if (!call)
return ERR_PTR(-ENOMEM);
call->state = RXRPC_CALL_CLIENT_AWAIT_CONN;
@@ -210,7 +221,7 @@ struct rxrpc_call *rxrpc_new_client_call(struct rxrpc_sock *rx,

_enter("%p,%lx", rx, user_call_ID);

- call = rxrpc_alloc_client_call(srx, gfp);
+ call = rxrpc_alloc_client_call(rx, srx, gfp);
if (IS_ERR(call)) {
release_sock(&rx->sk);
_leave(" = %ld", PTR_ERR(call));
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