[PATCH 5/5] xfs: fix a UAF when inode item push

Long Li
2024-08-23T11:09:36Z

KASAN reported a UAF bug while fault injection test:

==================================================================
BUG: KASAN: use-after-free in xfs_inode_item_push+0x2db/0x2f0
Read of size 8 at addr ffff888022f74788 by task xfsaild/sda/479

CPU: 0 PID: 479 Comm: xfsaild/sda Not tainted 6.2.0-rc7-00003-ga8a43e2eb5f6 #89
Call Trace:
<TASK>
dump_stack_lvl+0x51/0x6a
print_report+0x171/0x4a6
kasan_report+0xb7/0x130
xfs_inode_item_push+0x2db/0x2f0
xfsaild+0x729/0x1f70
kthread+0x290/0x340
ret_from_fork+0x1f/0x30
</TASK>

Allocated by task 494:
kasan_save_stack+0x22/0x40
kasan_set_track+0x25/0x30
__kasan_slab_alloc+0x58/0x70
kmem_cache_alloc+0x197/0x5d0
xfs_inode_item_init+0x62/0x170
xfs_trans_ijoin+0x15e/0x240
xfs_init_new_inode+0x573/0x1820
xfs_create+0x6a1/0x1020
xfs_generic_create+0x544/0x5d0
vfs_mkdir+0x5d0/0x980
do_mkdirat+0x14e/0x220
__x64_sys_mkdir+0x6a/0x80
do_syscall_64+0x39/0x80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

Freed by task 14:
kasan_save_stack+0x22/0x40
kasan_set_track+0x25/0x30
kasan_save_free_info+0x2e/0x40
__kasan_slab_free+0x114/0x1b0
kmem_cache_free+0xee/0x4e0
xfs_inode_free_callback+0x187/0x2a0
rcu_do_batch+0x317/0xce0
rcu_core+0x686/0xa90
__do_softirq+0x1b6/0x626

The buggy address belongs to the object at ffff888022f74758
which belongs to the cache xfs_ili of size 200
The buggy address is located 48 bytes inside of
200-byte region [ffff888022f74758, ffff888022f74820)

The buggy address belongs to the physical page:
page:ffffea00008bdd00 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x22f74
head:ffffea00008bdd00 order:1 compound_mapcount:0 subpages_mapcount:0 compound_pincount:0
flags: 0x1fffff80010200(slab|head|node=0|zone=1|lastcpupid=0x1fffff)
raw: 001fffff80010200 ffff888010ed4040 ffffea00008b2510 ffffea00008bde10
raw: 0000000000000000 00000000001a001a 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff888022f74680: 00 00 00 00 00 00 00 00 00 00 00 00 00 fc fc fc
ffff888022f74700: fc fc fc fc fc fc fc fc fc fc fc fa fb fb fb fb
>ffff888022f74780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff888022f74800: fb fb fb fb fc fc fc fc fc fc fc fc fc fc fc fc
ffff888022f74880: fc fc 00 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================

When push inode item in xfsaild, it will race with reclaim inodes task.
Consider the following call graph, both tasks deal with the same inode.
During flushing the cluster, it will enter xfs_iflush_abort() in shutdown
conditions, inode's XFS_IFLUSHING flag will be cleared and lip->li_buf set
to null. Concurrently, inode will be reclaimed in shutdown conditions,
there is no need to wait xfs buf lock because of lip->li_buf is null at
this time, inode will be freed via rcu callback if xfsaild task schedule
out during flushing the cluster. so, it is unsafe to reference lip after
flushing the cluster in xfs_inode_item_push().

<log item is in AIL>
<filesystem shutdown>
spin_lock(&ailp->ail_lock)
xfs_inode_item_push(lip)
xfs_buf_trylock(bp)
spin_unlock(&lip->li_ailp->ail_lock)
xfs_iflush_cluster(bp)
if (xfs_is_shutdown())
xfs_iflush_abort(ip)
xfs_trans_ail_delete(ip)
spin_lock(&ailp->ail_lock)
spin_unlock(&ailp->ail_lock)
xfs_iflush_abort_clean(ip)
error = -EIO
<log item removed from AIL>
<log item li_buf set to null>
if (error)
xfs_force_shutdown()
xlog_shutdown_wait(mp->m_log)
might_sleep()
xfs_reclaim_inode(ip)
if (shutdown)
xfs_iflush_shutdown_abort(ip)
if (!bp)
xfs_iflush_abort(ip)
return
__xfs_inode_free(ip)
call_rcu(ip, xfs_inode_free_callback)
......
<rcu grace period expires>
<rcu free callbacks run somewhere>
xfs_inode_free_callback(ip)
kmem_cache_free(ip->i_itemp)
......
<starts running again>
xfs_buf_ioend_fail(bp);
xfs_buf_ioend(bp)
xfs_buf_relse(bp);
return error
spin_lock(&lip->li_ailp->ail_lock)
<UAF on log item>

Additionally, after xfsaild_push_item(), the tracepoints can still access
the log item, potentially causing a UAF. I've previously submitted two
versions [1][2] attempting to solve this issue, but the solutions had
flaws.

Fix it by returning XFS_ITEM_UNSAFE in xfs_inode_item_push() when the log
item might be freed, ensuring xfsaild does not access the log item after
it is pushed.

[1] https://patchwork.kernel.org/project/xfs/patch/20230211022941.GA1515023@ceph-admin/
[2] https://patchwork.kernel.org/project/xfs/patch/20230722025721.312909-1-leo.lilong@huawei.com/
Fixes: 90c60e164012 ("xfs: xfs_iflush() is no longer necessary")
Signed-off-by: Long Li <leo.lilong@huawei.com>
---
fs/xfs/xfs_inode_item.c | 21 ++++++++++++++-------
1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/fs/xfs/xfs_inode_item.c b/fs/xfs/xfs_inode_item.c
index b509cbd191f4..c855cd2c81a5 100644
--- a/fs/xfs/xfs_inode_item.c
+++ b/fs/xfs/xfs_inode_item.c
@@ -720,10 +720,11 @@ STATIC uint
xfs_inode_item_push(
struct xfs_log_item *lip,
struct list_head *buffer_list)
- __releases(&lip->li_ailp->ail_lock)
- __acquires(&lip->li_ailp->ail_lock)
+ __releases(&ailp->ail_lock)
+ __acquires(&ailp->ail_lock)
{
struct xfs_inode_log_item *iip = INODE_ITEM(lip);
+ struct xfs_ail *ailp = lip->li_ailp;
struct xfs_inode *ip = iip->ili_inode;
struct xfs_buf *bp = lip->li_buf;
uint rval = XFS_ITEM_SUCCESS;
@@ -748,7 +749,7 @@ xfs_inode_item_push(
if (!xfs_buf_trylock(bp))
return XFS_ITEM_LOCKED;

- spin_unlock(&lip->li_ailp->ail_lock);
+ spin_unlock(&ailp->ail_lock);

/*
* We need to hold a reference for flushing the cluster buffer as it may
@@ -762,17 +763,23 @@ xfs_inode_item_push(
if (!xfs_buf_delwri_queue(bp, buffer_list))
rval = XFS_ITEM_FLUSHING;
xfs_buf_relse(bp);
- } else {
+ } else if (error == -EAGAIN) {
/*
* Release the buffer if we were unable to flush anything. On
* any other error, the buffer has already been released.
*/
- if (error == -EAGAIN)
- xfs_buf_relse(bp);
+ xfs_buf_relse(bp);
rval = XFS_ITEM_LOCKED;
+ } else {
+ /*
+ * The filesystem has already been shut down. If there's a race
+ * between inode flush and inode reclaim, the inode might be
+ * freed. Accessing the item after this point would be unsafe.
+ */
+ rval = XFS_ITEM_UNSAFE;
}

- spin_lock(&lip->li_ailp->ail_lock);
+ spin_lock(&ailp->ail_lock);
return rval;
}

--
2.39.2