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From:   Liu Shixin <liushixin2@huawei.com>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        William Kucharski <william.kucharski@oracle.com>,
        "Christoph Hellwig" <hch@lst.de>
CC:     <linux-kernel@vger.kernel.org>, <stable@vger.kernel.org>,
        Liu Shixin <liushixin2@huawei.com>
Subject: [PATCH 5.15 v3] mm/filemap: fix UAF in find_lock_entries
Date:   Wed, 6 Jul 2022 15:45:27 +0800
Message-ID: <20220706074527.1406924-1-liushixin2@huawei.com>
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Release refcount after xas_set to fix UAF which may cause panic like this:

 page:ffffea000491fa40 refcount:1 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x1247e9
 head:ffffea000491fa00 order:3 compound_mapcount:0 compound_pincount:0
 memcg:ffff888104f91091
 flags: 0x2fffff80010200(slab|head|node=0|zone=2|lastcpupid=0x1fffff)
...
page dumped because: VM_BUG_ON_PAGE(PageTail(page))
 ------------[ cut here ]------------
 kernel BUG at include/linux/page-flags.h:632!
 invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN
 CPU: 1 PID: 7642 Comm: sh Not tainted 5.15.51-dirty #26
...
 Call Trace:
  <TASK>
  __invalidate_mapping_pages+0xe7/0x540
  drop_pagecache_sb+0x159/0x320
  iterate_supers+0x120/0x240
  drop_caches_sysctl_handler+0xaa/0xe0
  proc_sys_call_handler+0x2b4/0x480
  new_sync_write+0x3d6/0x5c0
  vfs_write+0x446/0x7a0
  ksys_write+0x105/0x210
  do_syscall_64+0x35/0x80
  entry_SYSCALL_64_after_hwframe+0x44/0xae
 RIP: 0033:0x7f52b5733130
...

This problem has been fixed on mainline by patch 6b24ca4a1a8d ("mm: Use
multi-index entries in the page cache") since it deletes the related code.

Fixes: 5c211ba29deb ("mm: add and use find_lock_entries")
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
---
 mm/filemap.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 00e391e75880..c3e1fc0520dc 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2090,7 +2090,11 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
 
 	rcu_read_lock();
 	while ((page = find_get_entry(&xas, end, XA_PRESENT))) {
+		unsigned long next_idx = xas.xa_index;
+
 		if (!xa_is_value(page)) {
+			if (PageTransHuge(page))
+				next_idx = page->index + thp_nr_pages(page);
 			if (page->index < start)
 				goto put;
 			if (page->index + thp_nr_pages(page) - 1 > end)
@@ -2111,13 +2115,11 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
 put:
 		put_page(page);
 next:
-		if (!xa_is_value(page) && PageTransHuge(page)) {
-			unsigned int nr_pages = thp_nr_pages(page);
-
+		if (next_idx != xas.xa_index) {
 			/* Final THP may cross MAX_LFS_FILESIZE on 32-bit */
-			xas_set(&xas, page->index + nr_pages);
-			if (xas.xa_index < nr_pages)
+			if (next_idx < xas.xa_index)
 				break;
+			xas_set(&xas, next_idx);
 		}
 	}
 	rcu_read_unlock();
-- 
2.25.1