In the Linux kernel, the following vulnerability has been resolved:

tcp: Fix shift-out-of-bounds in dctcp_update_alpha().

In dctcp_update_alpha(), we use a module parameter dctcp_shift_g as follows:

alpha -= min_not_zero(alpha, alpha » dctcp_shift_g); … delivered_ce «= (10 - dctcp_shift_g);

It seems syzkaller started fuzzing module parameters and triggered shift-out-of-bounds [0] by setting 100 to dctcp_shift_g:

memcpy((void*)0x20000080, /sys/module/tcp_dctcp/parameters/dctcp_shift_g000, 47); res = syscall(__NR_openat, /fd=/0xffffffffffffff9cul, /file=/0x20000080ul, /flags=/2ul, /mode=/0ul); memcpy((void*)0x20000000, 100000, 4); syscall(__NR_write, /fd=/r[0], /val=/0x20000000ul, /len=/4ul);

Lets limit the max value of dctcp_shift_g by param_set_uint_minmax().

With this patch:

echo 10 > /sys/module/tcp_dctcp/parameters/dctcp_shift_g

cat /sys/module/tcp_dctcp/parameters/dctcp_shift_g

10

echo 11 > /sys/module/tcp_dctcp/parameters/dctcp_shift_g

-bash: echo: write error: Invalid argument

[0]: UBSAN: shift-out-of-bounds in net/ipv4/tcp_dctcp.c:143:12 shift exponent 100 is too large for 32-bit type u32 (aka unsigned int) CPU: 0 PID: 8083 Comm: syz-executor345 Not tainted 6.9.0-05151-g1b294a1f3561 #2 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014 Call Trace: __dump_stack lib/dump_stack.c:88 [inline] dump_stack_lvl+0x201/0x300 lib/dump_stack.c:114 ubsan_epilogue lib/ubsan.c:231 [inline] __ubsan_handle_shift_out_of_bounds+0x346/0x3a0 lib/ubsan.c:468 dctcp_update_alpha+0x540/0x570 net/ipv4/tcp_dctcp.c:143 tcp_in_ack_event net/ipv4/tcp_input.c:3802 [inline] tcp_ack+0x17b1/0x3bc0 net/ipv4/tcp_input.c:3948 tcp_rcv_state_process+0x57a/0x2290 net/ipv4/tcp_input.c:6711 tcp_v4_do_rcv+0x764/0xc40 net/ipv4/tcp_ipv4.c:1937 sk_backlog_rcv include/net/sock.h:1106 [inline] __release_sock+0x20f/0x350 net/core/sock.c:2983 release_sock+0x61/0x1f0 net/core/sock.c:3549 mptcp_subflow_shutdown+0x3d0/0x620 net/mptcp/protocol.c:2907 mptcp_check_send_data_fin+0x225/0x410 net/mptcp/protocol.c:2976 __mptcp_close+0x238/0xad0 net/mptcp/protocol.c:3072 mptcp_close+0x2a/0x1a0 net/mptcp/protocol.c:3127 inet_release+0x190/0x1f0 net/ipv4/af_inet.c:437 __sock_release net/socket.c:659 [inline] sock_close+0xc0/0x240 net/socket.c:1421 __fput+0x41b/0x890 fs/file_table.c:422 task_work_run+0x23b/0x300 kernel/task_work.c:180 exit_task_work include/linux/task_work.h:38 [inline] do_exit+0x9c8/0x2540 kernel/exit.c:878 do_group_exit+0x201/0x2b0 kernel/exit.c:1027 __do_sys_exit_group kernel/exit.c:1038 [inline] __se_sys_exit_group kernel/exit.c:1036 [inline] __x64_sys_exit_group+0x3f/0x40 kernel/exit.c:1036 do_syscall_x64 arch/x86/entry/common.c:52 [inline] do_syscall_64+0xe4/0x240 arch/x86/entry/common.c:83 entry_SYSCALL_64_after_hwframe+0x67/0x6f RIP: 0033:0x7f6c2b5005b6 Code: Unable to access opcode bytes at 0x7f6c2b50058c. RSP: 002b:00007ffe883eb948 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7 RAX: ffffffffffffffda RBX: 00007f6c2b5862f0 RCX: 00007f6c2b5005b6 RDX: 0000000000000001 RSI: 000000000000003c RDI: 0000000000000001 RBP: 0000000000000001 R08: 00000000000000e7 R09: ffffffffffffffc0 R10: 0000000000000006 R11: 0000000000000246 R12: 00007f6c2b5862f0 R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000001

Weakness

The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.

Affected Software

Potential Mitigations

• Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
• If possible, choose a language or compiler that performs automatic bounds checking.
• Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid [REF-1482].
• Use libraries or frameworks that make it easier to handle numbers without unexpected consequences.
• Examples include safe integer handling packages such as SafeInt (C++) or IntegerLib (C or C++). [REF-106]
• Perform input validation on any numeric input by ensuring that it is within the expected range. Enforce that the input meets both the minimum and maximum requirements for the expected range.
• Use unsigned integers where possible. This makes it easier to perform validation for integer overflows. When signed integers are required, ensure that the range check includes minimum values as well as maximum values.
• Understand the programming language’s underlying representation and how it interacts with numeric calculation (CWE-681). Pay close attention to byte size discrepancies, precision, signed/unsigned distinctions, truncation, conversion and casting between types, “not-a-number” calculations, and how the language handles numbers that are too large or too small for its underlying representation. [REF-7]
• Also be careful to account for 32-bit, 64-bit, and other potential differences that may affect the numeric representation.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/92.html

References

• https://git.kernel.org/stable/c/02261d3f9dc7d1d7be7d778f839e3404ab99034c
• https://git.kernel.org/stable/c/06d0fe049b51b0a92a70df8333fd85c4ba3eb2c6
• https://git.kernel.org/stable/c/237340dee373b97833a491d2e99fcf1d4a9adafd
• https://git.kernel.org/stable/c/3ebc46ca8675de6378e3f8f40768e180bb8afa66
• https://git.kernel.org/stable/c/6aacaa80d962f4916ccf90e2080306cec6c90fcf
• https://git.kernel.org/stable/c/8602150286a2a860a1dc55cbd04f99316f19b40a
• https://git.kernel.org/stable/c/e65d13ec00a738fa7661925fd5929ab3c765d4be
• https://git.kernel.org/stable/c/e9b2f60636d18dfd0dd4965b3316f88dfd6a2b31
• https://git.kernel.org/stable/c/02261d3f9dc7d1d7be7d778f839e3404ab99034c
• https://git.kernel.org/stable/c/06d0fe049b51b0a92a70df8333fd85c4ba3eb2c6
• https://git.kernel.org/stable/c/237340dee373b97833a491d2e99fcf1d4a9adafd
• https://git.kernel.org/stable/c/3ebc46ca8675de6378e3f8f40768e180bb8afa66
• https://git.kernel.org/stable/c/6aacaa80d962f4916ccf90e2080306cec6c90fcf
• https://git.kernel.org/stable/c/8602150286a2a860a1dc55cbd04f99316f19b40a
• https://git.kernel.org/stable/c/e65d13ec00a738fa7661925fd5929ab3c765d4be
• https://git.kernel.org/stable/c/e9b2f60636d18dfd0dd4965b3316f88dfd6a2b31