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charset="utf-8"; x-default="true" syzbot reported the following uninit splat: =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D BUG: KMSAN: uninit-value in mptcp_write_data_fin net/mptcp/options.c:542 [i= nline] BUG: KMSAN: uninit-value in mptcp_established_options_dss net/mptcp/options= .c:590 [inline] BUG: KMSAN: uninit-value in mptcp_established_options+0x112f/0x3530 net/mpt= cp/options.c:874 mptcp_write_data_fin net/mptcp/options.c:542 [inline] mptcp_established_options_dss net/mptcp/options.c:590 [inline] mptcp_established_options+0x112f/0x3530 net/mptcp/options.c:874 tcp_established_options+0x312/0xcc0 net/ipv4/tcp_output.c:1192 __tcp_transmit_skb+0x5dc/0x5fe0 net/ipv4/tcp_output.c:1575 __tcp_send_ack+0x967/0xad0 net/ipv4/tcp_output.c:4499 tcp_send_ack+0x3d/0x60 net/ipv4/tcp_output.c:4505 mptcp_subflow_shutdown+0x164/0x690 net/mptcp/protocol.c:3137 mptcp_check_send_data_fin+0x31b/0x3d0 net/mptcp/protocol.c:3218 __mptcp_wr_shutdown net/mptcp/protocol.c:3234 [inline] __mptcp_close+0x860/0x1360 net/mptcp/protocol.c:3313 mptcp_close+0x42/0x260 net/mptcp/protocol.c:3367 inet_release+0x1ee/0x2a0 net/ipv4/af_inet.c:442 __sock_release net/socket.c:722 [inline] sock_close+0xd6/0x2f0 net/socket.c:1514 __fput+0x60e/0x1010 fs/file_table.c:510 ____fput+0x25/0x30 fs/file_table.c:538 task_work_run+0x208/0x2b0 kernel/task_work.c:233 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline] __exit_to_user_mode_loop kernel/entry/common.c:67 [inline] exit_to_user_mode_loop+0x306/0x1b60 kernel/entry/common.c:98 __exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline] syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:238 [in= line] syscall_exit_to_user_mode include/linux/entry-common.h:318 [inline] __do_fast_syscall_32+0x2c7/0x460 arch/x86/entry/syscall_32.c:310 do_fast_syscall_32+0x37/0x80 arch/x86/entry/syscall_32.c:332 do_SYSENTER_32+0x1f/0x30 arch/x86/entry/syscall_32.c:370 entry_SYSENTER_compat_after_hwframe+0x84/0x8e Local variable opts created at: __tcp_transmit_skb+0x4d/0x5fe0 net/ipv4/tcp_output.c:1536 __tcp_send_ack+0x967/0xad0 net/ipv4/tcp_output.c:4499 =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D The output path currently omit initializing the mptcp extension `use_map` flag in a few corner cases. Address the issue always zeroing all the extensions flags before eventually initializing the individual bits. To that extent, introduce and use a struct_group to avoid multiple bitwise operations. Reported-by: syzbot+ff020673c5e3d94d9478@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=3Dff020673c5e3d94d9478 Fixes: 6d0060f600ad ("mptcp: Write MPTCP DSS headers to outgoing data packe= ts") Signed-off-by: Paolo Abeni Reviewed-by: Matthieu Baerts (NGI0) --- Note that this patch intentionally omit removing the now unneeded opts->ext_copy.ack64 =3D 0; assignment later on in mptcp_established_options_dss(), to avoid conflict with pending next patches. --- include/net/mptcp.h | 7 +++++-- net/mptcp/options.c | 6 +++++- 2 files changed, 10 insertions(+), 3 deletions(-) diff --git a/include/net/mptcp.h b/include/net/mptcp.h index aef2dbeb847b..8e666347b56d 100644 --- a/include/net/mptcp.h +++ b/include/net/mptcp.h @@ -23,7 +23,9 @@ struct mptcp_ext { u32 subflow_seq; u16 data_len; __sum16 csum; - u8 use_map:1, + + struct_group(flags, + u8 use_map:1, dsn64:1, data_fin:1, use_ack:1, @@ -31,9 +33,10 @@ struct mptcp_ext { mpc_map:1, frozen:1, reset_transient:1; - u8 reset_reason:4, + u8 reset_reason:4, csum_reqd:1, infinite_map:1; + ); /* end of flags group */ }; =20 #define MPTCPOPT_HMAC_LEN 20 diff --git a/net/mptcp/options.c b/net/mptcp/options.c index 2b35bdc113a5..7301225bac98 100644 --- a/net/mptcp/options.c +++ b/net/mptcp/options.c @@ -572,6 +572,11 @@ static bool mptcp_established_options_dss(struct sock = *sk, struct sk_buff *skb, unsigned int ack_size; bool ret =3D false; =20 + /* Zero `use_ack` and `use_map` flags with one shot. */ + BUILD_BUG_ON(sizeof_field(struct mptcp_ext, flags) !=3D sizeof(u16)); + BUILD_BUG_ON(!IS_ALIGNED(offsetof(struct mptcp_ext, flags), + sizeof(u16))); + *(u16 *)&opts->ext_copy.flags =3D 0; opts->csum_reqd =3D READ_ONCE(msk->csum_enabled); mpext =3D skb ? mptcp_get_ext(skb) : NULL; =20 @@ -595,7 +600,6 @@ static bool mptcp_established_options_dss(struct sock *= sk, struct sk_buff *skb, /* passive sockets msk will set the 'can_ack' after accept(), even * if the first subflow may have the already the remote key handy */ - opts->ext_copy.use_ack =3D 0; if (!READ_ONCE(msk->can_ack)) { *size =3D ALIGN(dss_size, 4); return ret; --=20 2.54.0