2022-07-06 07:37:59

by Liu Shixin

Subject: [PATCH 5.15 v3] mm/filemap: fix UAF in find_lock_entries

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 <[email protected]>
---
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

2022-07-06 14:19:34

by Matthew Wilcox

Subject: Re: [PATCH 5.15 v3] mm/filemap: fix UAF in find_lock_entries

On Wed, Jul 06, 2022 at 03:45:27PM +0800, Liu Shixin wrote:
> rcu_read_lock();
> while ((page = find_get_entry(&xas, end, XA_PRESENT))) {
> + unsigned long next_idx = xas.xa_index;

It's confusing to have next_idx not be the actual next index.
That was why I made it 'xas.xa_index + 1'. I know it's somewhat
used as an indicator that we don't need to call xas_set(), and so
it doesn't really matter, but let's say what we mean.

2022-07-07 01:31:52

by Liu Shixin

Subject: Re: [PATCH 5.15 v3] mm/filemap: fix UAF in find_lock_entries

On 2022/7/6 22:08, Matthew Wilcox wrote:
> On Wed, Jul 06, 2022 at 03:45:27PM +0800, Liu Shixin wrote:
>> rcu_read_lock();
>> while ((page = find_get_entry(&xas, end, XA_PRESENT))) {
>> + unsigned long next_idx = xas.xa_index;
> It's confusing to have next_idx not be the actual next index.
> That was why I made it 'xas.xa_index + 1'. I know it's somewhat
> used as an indicator that we don't need to call xas_set(), and so
> it doesn't really matter, but let's say what we mean.
I'll modify it and resend again, thanks.
>
>
> .
>