[PATCH] security/keyring: avoid pagefaults in keyring_read_iterator
From: Chris von Recklinghausen
Date: Fri Oct 18 2019 - 14:40:41 EST
under a debug kernel, the following circular locking dependency was observed:
[ 5896.294840] ======================================================
[ 5896.294846] [ INFO: possible circular locking dependency detected ]
[ 5896.294852] 3.10.0-957.31.1.el7.ppc64le.debug #1 Tainted: G OE ------------ T
[ 5896.294857] -------------------------------------------------------
[ 5896.294863] keyctl/21719 is trying to acquire lock:
[ 5896.294867] (&mm->mmap_sem){++++++}, at: [<c000000000331db8>] might_fault+0x88/0xf0
[ 5896.294881]
[ 5896.294881] but task is already holding lock:
[ 5896.294886] (&type->lock_class){+++++.}, at: [<c0000000004ff504>] keyctl_read_key+0xb4/0x170
[ 5896.294899]
[ 5896.294899] which lock already depends on the new lock.
[ 5896.294899]
[ 5896.294905]
[ 5896.294905] the existing dependency chain (in reverse order) is:
[ 5896.294911]
-> #1 (&type->lock_class){+++++.}:
[ 5896.294920] [<c0000000001caaf4>] check_prevs_add+0x144/0x1d0
[ 5896.294929] [<c0000000001ce338>] lock_acquire+0xe38/0x16c0
[ 5896.294936] [<c000000000b8e5e4>] down_write+0x84/0x130
[ 5896.294943] [<c0000000004fd330>] key_link+0x90/0x2e0
[ 5896.294949] [<c000000000503f44>] call_sbin_request_key+0x154/0x640
[ 5896.294956] [<c000000000bb1424>] construct_key_and_link+0x38c/0x464
[ 5896.294964] [<c000000000504bb4>] request_key+0x214/0x230
[ 5896.294970] [<d0000000047e2490>] nfs_idmap_get_key+0x110/0x460 [nfsv4]
[ 5896.294986] [<d0000000047e3464>] nfs_map_name_to_uid+0x84/0x2f0 [nfsv4]
[ 5896.294999] [<d0000000047c3180>] decode_attr_owner+0x1d0/0x2c0 [nfsv4]
[ 5896.295010] [<d0000000047c6f18>] decode_getfattr_attrs+0x5a8/0xb80 [nfsv4]
[ 5896.295022] [<d0000000047c75cc>] decode_getfattr_generic.constprop.100+0xdc/0x200 [nfsv4]
[ 5896.295033] [<d0000000047c8048>] nfs4_xdr_dec_getattr+0xa8/0xb0 [nfsv4]
[ 5896.295044] [<d0000000035eff58>] rpcauth_unwrap_resp+0xf8/0x150 [sunrpc]
[ 5896.295060] [<d0000000035d357c>] call_decode+0x29c/0x910 [sunrpc]
[ 5896.295071] [<d0000000035eb940>] __rpc_execute+0xf0/0x870 [sunrpc]
[ 5896.295083] [<d0000000035d233c>] rpc_run_task+0x14c/0x1c0 [sunrpc]
[ 5896.295094] [<d0000000047a12f0>] nfs4_call_sync_sequence+0x70/0xb0 [nfsv4]
[ 5896.295105] [<d0000000047a2254>] _nfs4_proc_getattr+0xc4/0xf0 [nfsv4]
[ 5896.295115] [<d0000000047b9ee4>] nfs4_proc_getattr+0x84/0x220 [nfsv4]
[ 5896.295126] [<d00000000454519c>] __nfs_revalidate_inode+0x1cc/0x7a0 [nfs]
[ 5896.295138] [<d000000004546284>] nfs_revalidate_mapping+0x1f4/0x520 [nfs]
[ 5896.295150] [<d00000000453df98>] nfs_file_mmap+0x78/0xb0 [nfs]
[ 5896.295160] [<c000000000343df8>] mmap_region+0x518/0x780
[ 5896.295167] [<c000000000344488>] do_mmap+0x428/0x510
[ 5896.295173] [<c000000000317508>] vm_mmap_pgoff+0x108/0x150
[ 5896.295179] [<c000000000340f1c>] SyS_mmap_pgoff+0xec/0x2c0
[ 5896.295186] [<c0000000000173b8>] sys_mmap+0x78/0x90
[ 5896.295192] [<c00000000000a294>] system_call+0x3c/0x100
[ 5896.295199]
-> #0 (&mm->mmap_sem){++++++}:
[ 5896.295207] [<c0000000001ca990>] check_prev_add+0xa50/0xa70
[ 5896.295214] [<c0000000001caaf4>] check_prevs_add+0x144/0x1d0
[ 5896.295221] [<c0000000001ce338>] lock_acquire+0xe38/0x16c0
[ 5896.295228] [<c000000000331de4>] might_fault+0xb4/0xf0
[ 5896.295235] [<c0000000004fc644>] keyring_read_iterator+0x54/0xd0
[ 5896.295242] [<c00000000060fe98>] assoc_array_subtree_iterate+0x4d8/0x790
[ 5896.295249] [<c0000000004fbc00>] keyring_read+0x80/0xa0
[ 5896.295255] [<c0000000004ff5a4>] keyctl_read_key+0x154/0x170
[ 5896.295262] [<c00000000000a294>] system_call+0x3c/0x100
[ 5896.295269]
[ 5896.295269] other info that might help us debug this:
[ 5896.295275] Possible unsafe locking scenario:
[ 5896.295275]
[ 5896.295281] CPU0 CPU1
[ 5896.295285] ---- ----
[ 5896.295289] lock(&type->lock_class);
[ 5896.295294] lock(&mm->mmap_sem);
[ 5896.295301] lock(&type->lock_class);
[ 5896.295308] lock(&mm->mmap_sem);
[ 5896.295313]
[ 5896.295313] *** DEADLOCK ***
[ 5896.295313]
[ 5896.295320] 1 lock held by keyctl/21719:
[ 5896.295323] #0: (&type->lock_class){+++++.}, at: [<c0000000004ff504>] keyctl_read_key+0xb4/0x170
[ 5896.295337]
[ 5896.295337] stack backtrace:
[ 5896.295343] CPU: 1 PID: 21719 Comm: keyctl Kdump: loaded Tainted: G OE ------------ T 3.10.0-957.31.1.el7.ppc64le.debug #1
[ 5896.295351] Call Trace:
[ 5896.295355] [c00000016100f8e0] [c0000000000205d0] show_stack+0x90/0x390 (unreliable)
[ 5896.295363] [c00000016100f9a0] [c000000000bb37d0] dump_stack+0x30/0x44
[ 5896.295371] [c00000016100f9c0] [c000000000ba7f3c] print_circular_bug+0x36c/0x3a0
[ 5896.295379] [c00000016100fa60] [c0000000001ca990] check_prev_add+0xa50/0xa70
[ 5896.295386] [c00000016100fb60] [c0000000001caaf4] check_prevs_add+0x144/0x1d0
[ 5896.295393] [c00000016100fbb0] [c0000000001ce338] lock_acquire+0xe38/0x16c0
[ 5896.295400] [c00000016100fce0] [c000000000331de4] might_fault+0xb4/0xf0
[ 5896.295407] [c00000016100fd00] [c0000000004fc644] keyring_read_iterator+0x54/0xd0
[ 5896.295415] [c00000016100fd40] [c00000000060fe98] assoc_array_subtree_iterate+0x4d8/0x790
[ 5896.295423] [c00000016100fd90] [c0000000004fbc00] keyring_read+0x80/0xa0
[ 5896.295430] [c00000016100fde0] [c0000000004ff5a4] keyctl_read_key+0x154/0x170
[ 5896.295437] [c00000016100fe30] [c00000000000a294] system_call+0x3c/0x100
The put_user call from keyring_read_iterator caused a page fault which attempts
to lock mm->mmap_sem and type->lock_class (key->sem) in the reverse order that
keyring_read_iterator did, thus causing the circular locking dependency.
Remedy this by using access_ok and __put_user instead of put_user so we'll
return an error instead of faulting in the page.
Also to prevent potential changes in behavior to applications, pre-fault the
page(s) with the key in keyctl_read_key before taking the read semaphore to
ensure that the page is present by the time keyring_read_iterator is called.
Suggested-by: Waiman Long <longman@xxxxxxxxxx>
Signed-off-by: Chris von Recklinghausen <crecklin@xxxxxxxxxx>
---
security/keys/keyctl.c | 10 ++++++++--
security/keys/keyring.c | 7 +++----
2 files changed, 11 insertions(+), 6 deletions(-)
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 9b898c9..f8a2553 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -846,9 +846,15 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
can_read_key:
ret = -EOPNOTSUPP;
if (key->type->read) {
- /* Read the data with the semaphore held (since we might sleep)
- * to protect against the key being updated or revoked.
+ /*
+ * Read the data with the semaphore held (since we might sleep)
+ * to protect against the key being updated or revoked. The
+ * user buffer, if not mapped yet, will be faulted in to
+ * prevent read failure.
*/
+ key_serial_t tmp;
+
+ get_user(tmp, buffer); /* Prefault */
down_read(&key->sem);
ret = key_validate(key);
if (ret == 0)
diff --git a/security/keys/keyring.c b/security/keys/keyring.c
index febf36c..7cac3c7 100644
--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -459,7 +459,6 @@ static int keyring_read_iterator(const void *object, void *data)
{
struct keyring_read_iterator_context *ctx = data;
const struct key *key = keyring_ptr_to_key(object);
- int ret;
kenter("{%s,%d},,{%zu/%zu}",
key->type->name, key->serial, ctx->count, ctx->buflen);
@@ -467,9 +466,9 @@ static int keyring_read_iterator(const void *object, void *data)
if (ctx->count >= ctx->buflen)
return 1;
- ret = put_user(key->serial, ctx->buffer);
- if (ret < 0)
- return ret;
+ if (!access_ok(ctx->buffer, sizeof(key->serial)) ||
+ __put_user(key->serial, ctx->buffer) < 0)
+ return -EFAULT;
ctx->buffer++;
ctx->count += sizeof(key->serial);
return 0;
--
1.8.3.1