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net, scm: fix PaX detected msg_controllen overflow in scm_detach_fds
David and HacKurx reported a following/similar size overflow triggered in a grsecurity kernel, thanks to PaX's gcc size overflow plugin: (Already fixed in later grsecurity versions by Brad and PaX Team.) [ 1002.296137] PAX: size overflow detected in function scm_detach_fds net/core/scm.c:314 cicus.202_127 min, count: 4, decl: msg_controllen; num: 0; context: msghdr; [ 1002.296145] CPU: 0 PID: 3685 Comm: scm_rights_recv Not tainted 4.2.3-grsec+ #7 [ 1002.296149] Hardware name: Apple Inc. MacBookAir5,1/Mac-66F35F19FE2A0D05, [...] [ 1002.296153] ffffffff81c27366 0000000000000000 ffffffff81c27375 ffffc90007843aa8 [ 1002.296162] ffffffff818129ba 0000000000000000 ffffffff81c27366 ffffc90007843ad8 [ 1002.296169] ffffffff8121f838 fffffffffffffffc fffffffffffffffc ffffc90007843e60 [ 1002.296176] Call Trace: [ 1002.296190] [<ffffffff818129ba>] dump_stack+0x45/0x57 [ 1002.296200] [<ffffffff8121f838>] report_size_overflow+0x38/0x60 [ 1002.296209] [<ffffffff816a979e>] scm_detach_fds+0x2ce/0x300 [ 1002.296220] [<ffffffff81791899>] unix_stream_read_generic+0x609/0x930 [ 1002.296228] [<ffffffff81791c9f>] unix_stream_recvmsg+0x4f/0x60 [ 1002.296236] [<ffffffff8178dc00>] ? unix_set_peek_off+0x50/0x50 [ 1002.296243] [<ffffffff8168fac7>] sock_recvmsg+0x47/0x60 [ 1002.296248] [<ffffffff81691522>] ___sys_recvmsg+0xe2/0x1e0 [ 1002.296257] [<ffffffff81693496>] __sys_recvmsg+0x46/0x80 [ 1002.296263] [<ffffffff816934fc>] SyS_recvmsg+0x2c/0x40 [ 1002.296271] [<ffffffff8181a3ab>] entry_SYSCALL_64_fastpath+0x12/0x85 Further investigation showed that this can happen when an *odd* number of fds are being passed over AF_UNIX sockets. In these cases CMSG_LEN(i * sizeof(int)) and CMSG_SPACE(i * sizeof(int)), where i is the number of successfully passed fds, differ by 4 bytes due to the extra CMSG_ALIGN() padding in CMSG_SPACE() to an 8 byte boundary on 64 bit. The padding is used to align subsequent cmsg headers in the control buffer. When the control buffer passed in from the receiver side *lacks* these 4 bytes (e.g. due to buggy/wrong API usage), then msg->msg_controllen will overflow in scm_detach_fds(): int cmlen = CMSG_LEN(i * sizeof(int)); <--- cmlen w/o tail-padding err = put_user(SOL_SOCKET, &cm->cmsg_level); if (!err) err = put_user(SCM_RIGHTS, &cm->cmsg_type); if (!err) err = put_user(cmlen, &cm->cmsg_len); if (!err) { cmlen = CMSG_SPACE(i * sizeof(int)); <--- cmlen w/ 4 byte extra tail-padding msg->msg_control += cmlen; msg->msg_controllen -= cmlen; <--- iff no tail-padding space here ... } ... wrap-around F.e. it will wrap to a length of 18446744073709551612 bytes in case the receiver passed in msg->msg_controllen of 20 bytes, and the sender properly transferred 1 fd to the receiver, so that its CMSG_LEN results in 20 bytes and CMSG_SPACE in 24 bytes. In case of MSG_CMSG_COMPAT (scm_detach_fds_compat()), I haven't seen an issue in my tests as alignment seems always on 4 byte boundary. Same should be in case of native 32 bit, where we end up with 4 byte boundaries as well. In practice, passing msg->msg_controllen of 20 to recvmsg() while receiving a single fd would mean that on successful return, msg->msg_controllen is being set by the kernel to 24 bytes instead, thus more than the input buffer advertised. It could f.e. become an issue if such application later on zeroes or copies the control buffer based on the returned msg->msg_controllen elsewhere. Maximum number of fds we can send is a hard upper limit SCM_MAX_FD (253). Going over the code, it seems like msg->msg_controllen is not being read after scm_detach_fds() in scm_recv() anymore by the kernel, good! Relevant recvmsg() handler are unix_dgram_recvmsg() (unix_seqpacket_recvmsg()) and unix_stream_recvmsg(). Both return back to their recvmsg() caller, and ___sys_recvmsg() places the updated length, that is, new msg_control - old msg_control pointer into msg->msg_controllen (hence the 24 bytes seen in the example). Long time ago, Wei Yongjun fixed something related in commit 1ac70e7 ("[NET]: Fix function put_cmsg() which may cause usr application memory overflow"). RFC3542, section 20.2. says: The fields shown as "XX" are possible padding, between the cmsghdr structure and the data, and between the data and the next cmsghdr structure, if required by the implementation. While sending an application may or may not include padding at the end of last ancillary data in msg_controllen and implementations must accept both as valid. On receiving a portable application must provide space for padding at the end of the last ancillary data as implementations may copy out the padding at the end of the control message buffer and include it in the received msg_controllen. When recvmsg() is called if msg_controllen is too small for all the ancillary data items including any trailing padding after the last item an implementation may set MSG_CTRUNC. Since we didn't place MSG_CTRUNC for already quite a long time, just do the same as in 1ac70e7 to avoid an overflow. Btw, even man-page author got this wrong :/ See db939c9b26e9 ("cmsg.3: Fix error in SCM_RIGHTS code sample"). Some people must have copied this (?), thus it got triggered in the wild (reported several times during boot by David and HacKurx). No Fixes tag this time as pre 2002 (that is, pre history tree). Reported-by: David Sterba <[email protected]> Reported-by: HacKurx <[email protected]> Cc: PaX Team <[email protected]> Cc: Emese Revfy <[email protected]> Cc: Brad Spengler <[email protected]> Cc: Wei Yongjun <[email protected]> Cc: Eric Dumazet <[email protected]> Reviewed-by: Hannes Frederic Sowa <[email protected]> Signed-off-by: Daniel Borkmann <[email protected]> Signed-off-by: David S. Miller <[email protected]>
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