MIPS: ftrace: Fix memory corruption when kernel is located beyond 32 bits

Overview

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

MIPS: ftrace: Fix memory corruption when kernel is located beyond 32 bits

Since commit e424054000878 ("MIPS: Tracing: Reduce the overhead of dynamic Function Tracer"), the macro UASM_i_LA_mostly has been used, and this macro can generate more than 2 instructions. At the same time, the code in ftrace assumes that no more than 2 instructions can be generated, which is why it stores them in an int[2] array. However, as previously noted, the macro UASM_i_LA_mostly (and now UASM_i_LA) causes a buffer overflow when _mcount is beyond 32 bits. This leads to corruption of the variables located in the __read_mostly section.

This corruption was observed because the variable __cpu_primary_thread_mask was corrupted, causing a hang very early during boot.

This fix prevents the corruption by avoiding the generation of instructions if they could exceed 2 instructions in length. Fortunately, insn_la_mcount is only used if the instrumented code is located outside the kernel code section, so dynamic ftrace can still be used, albeit in a more limited scope. This is still preferable to corrupting memory and/or crashing the kernel.

Affected

• Linux >= e424054000878d7eb11e44289242886d6e219d22 < 9cfda98cf02eacfa6b09076e98bd479fd73407c2
• Linux >= e424054000878d7eb11e44289242886d6e219d22 < e3e33ac2eb69d595079a1a1e444c2fb98efdd42d
• Linux >= e424054000878d7eb11e44289242886d6e219d22 < 7f39b9d0e86ed6236b9a5fb67616ab1f76c4f150
• Linux >= e424054000878d7eb11e44289242886d6e219d22 < 36dac9a3dda1f2bae343191bc16b910c603cac25
• Linux 2.6.35

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.