[PATCH 4.7 44/45] cachefiles: Fix attempt to read i_blocks after deleting file [ver #2]

From: Greg Kroah-Hartman
Date: Fri Oct 21 2016 - 05:37:26 EST

4.7-stable review patch. If anyone has any objections, please let me know.

------------------

From: David Howells <dhowells@xxxxxxxxxx>

commit a818101d7b92e76db2f9a597e4830734767473b9 upstream.

An NULL-pointer dereference happens in cachefiles_mark_object_inactive()
when it tries to read i_blocks so that it can tell the cachefilesd daemon
how much space it's making available.

The problem is that cachefiles_drop_object() calls
cachefiles_mark_object_inactive() after calling cachefiles_delete_object()
because the object being marked active staves off attempts to (re-)use the
file at that filename until after it has been deleted. This means that
d_inode is NULL by the time we come to try to access it.

To fix the problem, have the caller of cachefiles_mark_object_inactive()
supply the number of blocks freed up.

Without this, the following oops may occur:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000098
IP: [<ffffffffa06c5cc1>] cachefiles_mark_object_inactive+0x61/0xb0 [cachefiles]
...
CPU: 11 PID: 527 Comm: kworker/u64:4 Tainted: G I ------------ 3.10.0-470.el7.x86_64 #1
Hardware name: Hewlett-Packard HP Z600 Workstation/0B54h, BIOS 786G4 v03.19 03/11/2011
Workqueue: fscache_object fscache_object_work_func [fscache]
task: ffff880035edaf10 ti: ffff8800b77c0000 task.ti: ffff8800b77c0000
RIP: 0010:[<ffffffffa06c5cc1>] cachefiles_mark_object_inactive+0x61/0xb0 [cachefiles]
RSP: 0018:ffff8800b77c3d70 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff8800bf6cc400 RCX: 0000000000000034
RDX: 0000000000000000 RSI: ffff880090ffc710 RDI: ffff8800bf761ef8
RBP: ffff8800b77c3d88 R08: 2000000000000000 R09: 0090ffc710000000
R10: ff51005d2ff1c400 R11: 0000000000000000 R12: ffff880090ffc600
R13: ffff8800bf6cc520 R14: ffff8800bf6cc400 R15: ffff8800bf6cc498
FS: 0000000000000000(0000) GS:ffff8800bb8c0000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 0000000000000098 CR3: 00000000019ba000 CR4: 00000000000007e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Stack:
ffff880090ffc600 ffff8800bf6cc400 ffff8800867df140 ffff8800b77c3db0
ffffffffa06c48cb ffff880090ffc600 ffff880090ffc180 ffff880090ffc658
ffff8800b77c3df0 ffffffffa085d846 ffff8800a96b8150 ffff880090ffc600
Call Trace:
[<ffffffffa06c48cb>] cachefiles_drop_object+0x6b/0xf0 [cachefiles]
[<ffffffffa085d846>] fscache_drop_object+0xd6/0x1e0 [fscache]
[<ffffffffa085d615>] fscache_object_work_func+0xa5/0x200 [fscache]
[<ffffffff810a605b>] process_one_work+0x17b/0x470
[<ffffffff810a6e96>] worker_thread+0x126/0x410
[<ffffffff810a6d70>] ? rescuer_thread+0x460/0x460
[<ffffffff810ae64f>] kthread+0xcf/0xe0
[<ffffffff810ae580>] ? kthread_create_on_node+0x140/0x140
[<ffffffff81695418>] ret_from_fork+0x58/0x90
[<ffffffff810ae580>] ? kthread_create_on_node+0x140/0x140

The oopsing code shows:

callq 0xffffffff810af6a0 <wake_up_bit>
mov 0xf8(%r12),%rax
mov 0x30(%rax),%rax
mov 0x98(%rax),%rax <---- oops here
lock add %rax,0x130(%rbx)

where this is:

d_backing_inode(object->dentry)->i_blocks

Fixes: a5b3a80b899bda0f456f1246c4c5a1191ea01519 (CacheFiles: Provide read-and-reset release counters for cachefilesd)
Reported-by: Jianhong Yin <jiyin@xxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
Reviewed-by: Steve Dickson <steved@xxxxxxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
fs/cachefiles/interface.c | 8 +++++++-
fs/cachefiles/internal.h | 3 ++-
fs/cachefiles/namei.c | 8 ++++----
3 files changed, 13 insertions(+), 6 deletions(-)

--- a/fs/cachefiles/interface.c
+++ b/fs/cachefiles/interface.c
@@ -253,6 +253,8 @@ static void cachefiles_drop_object(struc
struct cachefiles_object *object;
struct cachefiles_cache *cache;
const struct cred *saved_cred;
+ struct inode *inode;
+ blkcnt_t i_blocks = 0;

ASSERT(_object);

@@ -279,6 +281,10 @@ static void cachefiles_drop_object(struc
_object != cache->cache.fsdef
) {
_debug("- retire object OBJ%x", object->fscache.debug_id);
+ inode = d_backing_inode(object->dentry);
+ if (inode)
+ i_blocks = inode->i_blocks;
+
cachefiles_begin_secure(cache, &saved_cred);
cachefiles_delete_object(cache, object);
cachefiles_end_secure(cache, saved_cred);
@@ -292,7 +298,7 @@ static void cachefiles_drop_object(struc

/* note that the object is now inactive */
if (test_bit(CACHEFILES_OBJECT_ACTIVE, &object->flags))
- cachefiles_mark_object_inactive(cache, object);
+ cachefiles_mark_object_inactive(cache, object, i_blocks);

dput(object->dentry);
object->dentry = NULL;
--- a/fs/cachefiles/internal.h
+++ b/fs/cachefiles/internal.h
@@ -160,7 +160,8 @@ extern char *cachefiles_cook_key(const u
* namei.c
*/
extern void cachefiles_mark_object_inactive(struct cachefiles_cache *cache,
- struct cachefiles_object *object);
+ struct cachefiles_object *object,
+ blkcnt_t i_blocks);
extern int cachefiles_delete_object(struct cachefiles_cache *cache,
struct cachefiles_object *object);
extern int cachefiles_walk_to_object(struct cachefiles_object *parent,
--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -261,10 +261,9 @@ requeue:
* Mark an object as being inactive.
*/
void cachefiles_mark_object_inactive(struct cachefiles_cache *cache,
- struct cachefiles_object *object)
+ struct cachefiles_object *object,
+ blkcnt_t i_blocks)
{
- blkcnt_t i_blocks = d_backing_inode(object->dentry)->i_blocks;
-
write_lock(&cache->active_lock);
rb_erase(&object->active_node, &cache->active_nodes);
clear_bit(CACHEFILES_OBJECT_ACTIVE, &object->flags);
@@ -707,7 +706,8 @@ mark_active_timed_out:

check_error:
_debug("check error %d", ret);
- cachefiles_mark_object_inactive(cache, object);
+ cachefiles_mark_object_inactive(
+ cache, object, d_backing_inode(object->dentry)->i_blocks);
release_dentry:
dput(object->dentry);
object->dentry = NULL;