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authorKiran Kumar Modukuri <kiran.modukuri@gmail.com>2018-06-21 22:31:44 +0200
committerDavid Howells <dhowells@redhat.com>2018-07-25 15:49:00 +0200
commitf29507ce66701084c39aeb1b0ae71690cbff3554 (patch)
tree32c6a56a27fc9a00b9e5f956a618193210603f00 /fs/fscache/cookie.c
parentcachefiles: Fix refcounting bug in backing-file read monitoring (diff)
downloadlinux-f29507ce66701084c39aeb1b0ae71690cbff3554.tar.xz
linux-f29507ce66701084c39aeb1b0ae71690cbff3554.zip
fscache: Fix reference overput in fscache_attach_object() error handling
When a cookie is allocated that causes fscache_object structs to be allocated, those objects are initialised with the cookie pointer, but aren't blessed with a ref on that cookie unless the attachment is successfully completed in fscache_attach_object(). If attachment fails because the parent object was dying or there was a collision, fscache_attach_object() returns without incrementing the cookie counter - but upon failure of this function, the object is released which then puts the cookie, whether or not a ref was taken on the cookie. Fix this by taking a ref on the cookie when it is assigned in fscache_object_init(), even when we're creating a root object. Analysis from Kiran Kumar: This bug has been seen in 4.4.0-124-generic #148-Ubuntu kernel BugLink: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1776277 fscache cookie ref count updated incorrectly during fscache object allocation resulting in following Oops. kernel BUG at /build/linux-Y09MKI/linux-4.4.0/fs/fscache/internal.h:321! kernel BUG at /build/linux-Y09MKI/linux-4.4.0/fs/fscache/cookie.c:639! [Cause] Two threads are trying to do operate on a cookie and two objects. (1) One thread tries to unmount the filesystem and in process goes over a huge list of objects marking them dead and deleting the objects. cookie->usage is also decremented in following path: nfs_fscache_release_super_cookie -> __fscache_relinquish_cookie ->__fscache_cookie_put ->BUG_ON(atomic_read(&cookie->usage) <= 0); (2) A second thread tries to lookup an object for reading data in following path: fscache_alloc_object 1) cachefiles_alloc_object -> fscache_object_init -> assign cookie, but usage not bumped. 2) fscache_attach_object -> fails in cant_attach_object because the cookie's backing object or cookie's->parent object are going away 3) fscache_put_object -> cachefiles_put_object ->fscache_object_destroy ->fscache_cookie_put ->BUG_ON(atomic_read(&cookie->usage) <= 0); [NOTE from dhowells] It's unclear as to the circumstances in which (2) can take place, given that thread (1) is in nfs_kill_super(), however a conflicting NFS mount with slightly different parameters that creates a different superblock would do it. A backtrace from Kiran seems to show that this is a possibility: kernel BUG at/build/linux-Y09MKI/linux-4.4.0/fs/fscache/cookie.c:639! ... RIP: __fscache_cookie_put+0x3a/0x40 [fscache] Call Trace: __fscache_relinquish_cookie+0x87/0x120 [fscache] nfs_fscache_release_super_cookie+0x2d/0xb0 [nfs] nfs_kill_super+0x29/0x40 [nfs] deactivate_locked_super+0x48/0x80 deactivate_super+0x5c/0x60 cleanup_mnt+0x3f/0x90 __cleanup_mnt+0x12/0x20 task_work_run+0x86/0xb0 exit_to_usermode_loop+0xc2/0xd0 syscall_return_slowpath+0x4e/0x60 int_ret_from_sys_call+0x25/0x9f [Fix] Bump up the cookie usage in fscache_object_init, when it is first being assigned a cookie atomically such that the cookie is added and bumped up if its refcount is not zero. Remove the assignment in fscache_attach_object(). [Testcase] I have run ~100 hours of NFS stress tests and not seen this bug recur. [Regression Potential] - Limited to fscache/cachefiles. Fixes: ccc4fc3d11e9 ("FS-Cache: Implement the cookie management part of the netfs API") Signed-off-by: Kiran Kumar Modukuri <kiran.modukuri@gmail.com> Signed-off-by: David Howells <dhowells@redhat.com>
Diffstat (limited to 'fs/fscache/cookie.c')
-rw-r--r--fs/fscache/cookie.c7
1 files changed, 4 insertions, 3 deletions
diff --git a/fs/fscache/cookie.c b/fs/fscache/cookie.c
index 97137d7ec5ee..83bfe04456b6 100644
--- a/fs/fscache/cookie.c
+++ b/fs/fscache/cookie.c
@@ -516,6 +516,7 @@ static int fscache_alloc_object(struct fscache_cache *cache,
goto error;
}
+ ASSERTCMP(object->cookie, ==, cookie);
fscache_stat(&fscache_n_object_alloc);
object->debug_id = atomic_inc_return(&fscache_object_debug_id);
@@ -571,6 +572,8 @@ static int fscache_attach_object(struct fscache_cookie *cookie,
_enter("{%s},{OBJ%x}", cookie->def->name, object->debug_id);
+ ASSERTCMP(object->cookie, ==, cookie);
+
spin_lock(&cookie->lock);
/* there may be multiple initial creations of this object, but we only
@@ -610,9 +613,7 @@ static int fscache_attach_object(struct fscache_cookie *cookie,
spin_unlock(&cache->object_list_lock);
}
- /* attach to the cookie */
- object->cookie = cookie;
- fscache_cookie_get(cookie, fscache_cookie_get_attach_object);
+ /* Attach to the cookie. The object already has a ref on it. */
hlist_add_head(&object->cookie_link, &cookie->backing_objects);
fscache_objlist_add(object);