fprobe: Terminate the fgraph_data list when the reservation is not filled

Overview

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

fprobe: Terminate the fgraph_data list when the reservation is not filled

fprobe_fgraph_entry() reserves shadow stack space for every fprobe with an exit handler, but only fills it for those whose entry handler returns 0. fgraph_reserve_data() does not clear the area, so fprobe_return() parses the unused tail as headers left over from an earlier call, and an exit handler can run twice or despite its entry handler asking to skip it.

Write a zero word after the last entry to terminate the walk. A zeroed slot does not decode to a NULL fprobe on the arches that encode the header into one unsigned long, since arch_decode_fprobe_header_fp() ORs in FPROBE_HEADER_MSB_PATTERN, so make read_fprobe_header() return NULL for a zeroed slot.

Affected

• Linux >= e0a384434ae1bdfb03954c46c464e3dbd3223ad6 < b7459fddcb27a4b7d01067678172afa362cf660d
• Linux >= e0a384434ae1bdfb03954c46c464e3dbd3223ad6 < 1d653a183973f5283a3db5a38cd5e195eb152244
• Linux 7.1

Remediation

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