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Message-ID: <m2r0xj6f2x.fsf@gmail.com>
Date: Thu, 01 Dec 2022 10:14:35 +0800
From: Schspa Shi <schspa@...il.com>
To: asmadeus@...ewreck.org
Cc: ericvh@...il.com, lucho@...kov.net, linux_oss@...debyte.com,
davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
pabeni@...hat.com, v9fs-developer@...ts.sourceforge.net,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
syzbot+8f1060e2aaf8ca55220b@...kaller.appspotmail.com
Subject: Re: [PATCH v2] 9p/fd: set req refcount to zero to avoid
uninitialized usage
asmadeus@...ewreck.org writes:
> Schspa Shi wrote on Wed, Nov 30, 2022 at 09:08:31PM +0800:
>> When the transport layer of fs cancels the request, it is deleted from the
>> client side. But the server can send a response with the freed tag.
>>
>> When the new request allocated, we add it to idr, and use the id form idr
>> as tag, which will have the same tag with high probability. Then initialize
>> the refcount after adding it to idr.
>
> ultimately this bug has nothing to do with tag reuse -- we don't
> actually need flush at all to trigger it.
>
> - thread1 starts new request; idr initialized with tag X
> - thread2 receives something for tag X, increments refcount before
> refcount init
> - thread1 resets refcount to two incorrectly
>
> This could happen on any new message where the server voluntarily sends
> a reply with tag X before the request has been sent; in a cyclic model
> as suggested in the other thread it would be easy to guess just last+1
> for an hypothetical attacker.
>
> This scenario with flush is just how syzbot happened to trigger it, but
> I think it's just superfluous to this commit message.
>
> A few more nitpicks on wording below; happy to adjust things myself as I
> apply patches but might as well comment first...
>
>> If the p9_read_work got a response before the refcount initiated. It will
>> use a uninitialized req, which will result in a bad request data struct.
>>
>> There is the logs from syzbot.
>
> English: Here is ...
>
>> Corrupted memory at 0xffff88807eade00b [ 0xff 0x07 0x00 0x00 0x00 0x00
>> 0x00 0x00 . . . . . . . . ] (in kfence-#110):
>> p9_fcall_fini net/9p/client.c:248 [inline]
>> p9_req_put net/9p/client.c:396 [inline]
>> p9_req_put+0x208/0x250 net/9p/client.c:390
>> p9_client_walk+0x247/0x540 net/9p/client.c:1165
>> clone_fid fs/9p/fid.h:21 [inline]
>> v9fs_fid_xattr_set+0xe4/0x2b0 fs/9p/xattr.c:118
>> v9fs_xattr_set fs/9p/xattr.c:100 [inline]
>> v9fs_xattr_handler_set+0x6f/0x120 fs/9p/xattr.c:159
>> __vfs_setxattr+0x119/0x180 fs/xattr.c:182
>> __vfs_setxattr_noperm+0x129/0x5f0 fs/xattr.c:216
>> __vfs_setxattr_locked+0x1d3/0x260 fs/xattr.c:277
>> vfs_setxattr+0x143/0x340 fs/xattr.c:309
>> setxattr+0x146/0x160 fs/xattr.c:617
>> path_setxattr+0x197/0x1c0 fs/xattr.c:636
>> __do_sys_setxattr fs/xattr.c:652 [inline]
>> __se_sys_setxattr fs/xattr.c:648 [inline]
>> __ia32_sys_setxattr+0xc0/0x160 fs/xattr.c:648
>> do_syscall_32_irqs_on arch/x86/entry/common.c:112 [inline]
>> __do_fast_syscall_32+0x65/0xf0 arch/x86/entry/common.c:178
>> do_fast_syscall_32+0x33/0x70 arch/x86/entry/common.c:203
>> entry_SYSENTER_compat_after_hwframe+0x70/0x82
>>
>> Below is a similar scenario, the scenario in the syzbot log looks more
>> complicated than this one, but this patch seems can fix it.
>
> English: seems to fix it?
> (thanks for checking!)
>
Sorry for my bad english, this patch will fix it.
>>
>> T21124 p9_read_work
>> ======================== second trans =================================
>> p9_client_walk
>> p9_client_rpc
>> p9_client_prepare_req
>> p9_tag_alloc
>> req = kmem_cache_alloc(p9_req_cache, GFP_NOFS);
>> tag = idr_alloc
>> << preempted >>
>> req->tc.tag = tag;
>> /* req->[refcount/tag] == uninitilzed */
>
> typo: uninitialized
thanks for check.
>
>> m->rreq = p9_tag_lookup(m->client, m->rc.tag);
>
> it's not obvious for someone reading this not familiar with 9p that
> lookup will increment refcount
>
>>
>> refcount_set(&req->refcount, 2);
>> << do response/error >>
>> p9_req_put(m->client, m->rreq);
>> /* req->refcount == 1 */
>>
>> /* req->refcount == 1 */
>> << got a bad refcount >>
>
> it's not obvious from this going back to thread 1 with a refcount of 1
> would be a bad refcount, either.
> One possible scenario would be:
>
> /* increments uninitalized refcount */
> req = p9_tag_lookup(tag)
> refcount_set(req->refcount, 2)
> /* cb drops one ref */
> p9_client_cb(req)
> /* reader thread drops its ref:
> request is incorrectly freed */
> p9_req_put(req)
> /* use after free and ref underflow */
> p9_req_put(req)
>
OK, this is more clear.
>
>
>> To fix it, we can initize the refcount to zero before add to idr.
>>
>> Reported-by: syzbot+8f1060e2aaf8ca55220b@...kaller.appspotmail.com
>>
>
> There should be no empty line between the tags; tags are part of the
> "trailer" and some tools handle it as such (like git interpret-trailers),
> which would ignore that Reported-by as it is not part of the last block
> of text.
>
Thanks for reminding the format issue here.
>> Signed-off-by: Schspa Shi <schspa@...il.com>
>> ---
>> net/9p/client.c | 4 ++++
>> 1 file changed, 4 insertions(+)
>>
>> diff --git a/net/9p/client.c b/net/9p/client.c
>> index aaa37b07e30a..a72cb597a8ab 100644
>> --- a/net/9p/client.c
>> +++ b/net/9p/client.c
>> @@ -297,6 +297,10 @@ p9_tag_alloc(struct p9_client *c, int8_t type, uint t_size, uint r_size,
>> p9pdu_reset(&req->rc);
>> req->t_err = 0;
>> req->status = REQ_STATUS_ALLOC;
>> + /* p9_tag_lookup relies on this refcount to be zero to avoid
>> + * getting a freed request.
>
> A freed request would have 0 by definition, if it isn't zero then this
> is a newly allocated uninit request, so this comment is incorrect.
>
> How about:
> /* refcount needs to be set to 0 before inserting into the idr
> * so p9_tag_lookup does not accept a request that is not fully
> * initialized. refcount_set to 2 below will mark request live.
> */
>
Your comment is more clear, I will change to this one.
>> + */
>> + refcount_set(&req->refcount, 0);
>> init_waitqueue_head(&req->wq);
>> INIT_LIST_HEAD(&req->req_list);
>>
--
BRs
Schspa Shi
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