Messages in this thread | | | Date | Wed, 30 Nov 2022 07:38:06 +0900 | From | asmadeus@codewrec ... | Subject | Re: [PATCH] 9p: fix crash when transaction killed |
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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 scenario you describe is proteced by p9_tag_lookup checking for refcount with refcount_inc_not_zero (p9_req_try_get).
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.. :/
-- Dominique
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