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

ring-buffer: Fix event length with forced 8-byte alignment

When RB_FORCE_8BYTE_ALIGNMENT is true, rb_calculate_event_length() reserves the space of event->array[0] for placing the data length and rb_update_event() stores the data length in event->array[0] accordingly. As a result the whole event length will add extra 4 bytes for sizeof(event.array[0]) unconditionally.

But ring_buffer_event_length() only subtracts the sizeof(event->array[0]) for events larger than RB_MAX_SMALL_DATA + sizeof(event->array[0]). As a result, small events on architectures with RB_FORCE_8BYTE_ALIGNMENT=true report a data length that is 4 bytes larger than expected.

To fix it, add the RB_FORCE_8BYTE_ALIGNMENT as a condition to subtract the size of that length field whenever RB_FORCE_8BYTE_ALIGNMENT is true.

This issue is observed in a riscv64 kernel with CONFIG_HAVE_64BIT_ALIGNED_ACCESS set to y, when we run ftrace selftest trace_marker_raw.tc, we get the weird log: for cases where the id is 1..100, the number of data field is 8N, but once id exceeds 100, the number of data field becomes 8N+4:

1 buf: 58 00 00 00 80 5e d1 63 (number of data field is 8*1)

…

a buf: 58 … (number of data field is 8*2)

…

64 buf: 58 … (number of data field is 8*13)

65 buf: 58 … (number of data field is 8*13+4)

After applying this change, the number of data field keeps being 8*N+4 consistently.

Affected Software

References

• https://git.kernel.org/stable/c/14057268e79654c3e8ea2c9b5204cb9644b2964d
• https://git.kernel.org/stable/c/24c3fa71f9947b0e1f3b954db1b769b44140192e
• https://git.kernel.org/stable/c/3a63a11897c7ba32d1be7a3fdbc48a8b01cf4992
• https://git.kernel.org/stable/c/7c9f0ccf9f04142458d2ac3d39414f4acae242f0
• https://git.kernel.org/stable/c/c37e0a4b79a6bbb96ce5ffe279d7c001e20529e0
• https://git.kernel.org/stable/c/cfada73fabe2ccc06ec77fe2ceaa088689213326
• https://git.kernel.org/stable/c/dbcb8635b1eb7603818591cf745c7b1d714f7ac6
• https://git.kernel.org/stable/c/ec5e96aee75d27779b9a860307679f13f33adb0c