Messages in this thread | | | From | Schspa Shi <> | Subject | Re: [PATCH] 9p: fix crash when transaction killed | Date | Wed, 30 Nov 2022 10:22:44 +0800 |
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asmadeus@codewreck.org writes:
> Schspa Shi wrote on Wed, Nov 30, 2022 at 12:22:51AM +0800: >> The transport layer of fs does not fully support the cancel request. >> When the request is in the REQ_STATUS_SENT state, p9_fd_cancelled >> will forcibly delete the request, and at this time p9_[read/write]_work >> may continue to use the request. Therefore, it causes UAF . >> >> There 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 the root cause seems to be the same. >> >> T21124 p9_write_work p9 read_work >> ======================== first trans ================================= >> p9_client_walk >> p9_client_rpc >> p9_client_prepare_req >> /* req->refcount == 2 */ >> c->trans_mod->request(c, req); >> p9_fd_request >> req move to unsent_req_list >> req->status = REQ_STATUS_SENT; >> req move to req_list >> << send to server >> >> wait_event_killable >> << get kill signal >> >> if (c->trans_mod->cancel(c, req)) >> p9_client_flush(c, req); >> /* send flush request */ >> req = p9_client_rpc(c, P9_TFLUSH, "w", oldtag); >> if (c->trans_mod->cancelled) >> c->trans_mod->cancelled(c, oldreq); >> /* old req was deleted from req_list */ >> /* req->refcount == 1 */ >> p9_req_put >> /* req->refcount == 0 */ >> << preempted >> >> << get response, UAF here >> >> m->rreq = p9_tag_lookup(m->client, m->rc.tag); >> /* req->refcount == 1 */ >> << do response >> >> p9_client_cb(m->client, m->rreq, REQ_STATUS_RCVD); >> /* req->refcount == 0 */ >> p9_fcall_fini >> /* request have been freed */ >> p9_fcall_fini >> /* double free */ >> p9_req_put(m->client, m->rreq); >> /* req->refcount == 1 */ >> >> To fix it, we can wait the request with status REQ_STATUS_SENT returned. > > Christian replied on this (we cannot wait) but I agree with him -- the
Yes, this is where I worry about too, this wait maybe cause a deadlock.
> scenario you describe is proteced by p9_tag_lookup checking for refcount > with refcount_inc_not_zero (p9_req_try_get).
Thanks for pointing out the zero value check here, the scene in the commit message does not hold.
> > The normal scenarii for flush are as follow: > - cancel before request is sent: no flush, just free > - flush is ignored and reply comes first: we get reply from original > request then reply from flush > - flush is handled and reply never comes: we only get reply from flush > > Protocol-wise, we can safely reuse the tag after the flush reply got > received; and as far as I can follow the code we only ever free the tag > (last p9_call_fini) after flush has returned so the entry should be > protected. > > If we receive a response on the given tag between cancelled and the main > thread going out the request has been marked as FLSHD and should be > ignored. . . here is one p9_req_put in p9_read_work() in this case but > it corresponds to the ref obtained by p9_tag_lookup() so it should be > valid. > > > I'm happy to believe we have a race somewhere (even if no sane server > would produce it), but right now I don't see it looking at the code.. :/
And I think there is a race too. because the syzbot report about 9p fs memory corruption multi times.
As for the problem, the p9_tag_lookup only takes the rcu_read_lock when accessing the IDR, why it doesn't take the p9_client->lock? Maybe the root cause is that a lock is missing here.
-- BRs Schspa Shi
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