Re: [PATCH v3] 9p/fd: set req refcount to zero to avoid uninitialized usage

[Schspa Shi <schspa@gmail.com>]

asmadeus@codewreck.org writes:

> Schspa Shi wrote on Thu, Dec 01, 2022 at 11:33:10AM +0800:
>> When the new request allocated, the refcount will be zero if it is resued
>> one. But if the request is newly allocated from slab, it is not fully
>> initialized before add it to idr.
>> 
>> 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.
>> 
>> Here is the logs from syzbot.
>> 
>> 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 can 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] == uninitialized */
>>                             m->rreq = p9_tag_lookup(m->client, m->rc.tag);
>>                               /* increments uninitalized refcount */
>> 
>>         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)
>> 
>> To fix it, we can initize the refcount to zero before add to idr.
>
> (fixed initialize typo here)
>
>> Reported-by: syzbot+8f1060e2aaf8ca55220b@syzkaller.appspotmail.com
>> Signed-off-by: Schspa Shi <schspa@gmail.com>
>> 
>> --
>> 
>> Changelog:
>> v1 -> v2:
>>         - Set refcount to fix the problem.
>> v2 -> v3:
>>         - Comment messages improve as asmadeus suggested.
>
> Just a note: when applying a patch with git am, this goes into the
> commit message -- please include the changelog below the git's three
> dashes instead (anything between the three dashes and the 'diff --git'
> below:

Thanks for the reminder, I will pay attention to this next time.

>> ---
>>  net/9p/client.c | 5 +++++
>>  1 file changed, 5 insertions(+)
>> 
>> diff --git a/net/9p/client.c b/net/9p/client.c
>
>
> Christian Schoenebeck wrote on Fri, Dec 02, 2022 at 12:48:39PM +0100:
>> > +	/* 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.
>> > +	 */
>> > +	refcount_set(&req->refcount, 0);
>> 
>> I would s/live/ready for being used/, but comment should be clear enough
>> anyway.
>
> I blame golfing to fit into three lines, sorry!
> Since it was my suggestion, I've taken the liberty to change 'live' to
> 'ready' as an half step; I think it's clearer than live and probably
> understandable enough.
>
> I've pushed this to my next branch and will submit to Linus for the
> merge window in a couple of weeks, no point in rushing this to stable
> unless it gets snatched through the net tree first...

Thanks.

-- 
BRs
Schspa Shi