CVE-2021-46999

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

sctp: do asoc update earlier in sctp_sf_do_dupcook_a

Theres a panic that occurs in a few of envs, the call trace is as below:

[] general protection fault, … 0x29acd70f1000a: 0000 [#1] SMP PTI [] RIP: 0010:sctp_ulpevent_notify_peer_addr_change+0x4b/0x1fa [sctp] [] sctp_assoc_control_transport+0x1b9/0x210 [sctp] [] sctp_do_8_2_transport_strike.isra.16+0x15c/0x220 [sctp] [] sctp_cmd_interpreter.isra.21+0x1231/0x1a10 [sctp] [] sctp_do_sm+0xc3/0x2a0 [sctp] [] sctp_generate_timeout_event+0x81/0xf0 [sctp]

This is caused by a transport use-after-free issue. When processing a duplicate COOKIE-ECHO chunk in sctp_sf_do_dupcook_a(), both COOKIE-ACK and SHUTDOWN chunks are allocated with the transort from the new asoc. However, later in the sideeffect machine, the old asoc is used to send them out and old asocs shutdown_last_sent_to is set to the transport that SHUTDOWN chunk attached to in sctp_cmd_setup_t2(), which actually belongs to the new asoc. After the new_asoc is freed and the old asoc T2 timeout, the old asocs shutdown_last_sent_to that is already freed would be accessed in sctp_sf_t2_timer_expire().

Thanks Alexander and Jere for helping dig into this issue.

To fix it, this patch is to do the asoc update first, then allocate the COOKIE-ACK and SHUTDOWN chunks with the updated old asoc. This would make more sense, as a chunk from an asoc shouldnt be sent out with another asoc. We had fixed quite a few issues caused by this.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/0bfd913c2121b3d553bfd52810fe6061d542d625
• https://git.kernel.org/stable/c/35b4f24415c854cd718ccdf38dbea6297f010aae
• https://git.kernel.org/stable/c/61b877bad9bb0d82b7d8841be50872557090a704
• https://git.kernel.org/stable/c/b1b31948c0af44628e43353828453461bb74098f
• https://git.kernel.org/stable/c/d624f2991b977821375fbd56c91b0c91d456a697
• https://git.kernel.org/stable/c/f01988ecf3654f805282dce2d3bb9afe68d2691e
• https://git.kernel.org/stable/c/0bfd913c2121b3d553bfd52810fe6061d542d625
• https://git.kernel.org/stable/c/35b4f24415c854cd718ccdf38dbea6297f010aae
• https://git.kernel.org/stable/c/61b877bad9bb0d82b7d8841be50872557090a704
• https://git.kernel.org/stable/c/b1b31948c0af44628e43353828453461bb74098f
• https://git.kernel.org/stable/c/d624f2991b977821375fbd56c91b0c91d456a697
• https://git.kernel.org/stable/c/f01988ecf3654f805282dce2d3bb9afe68d2691e