modpost: prevent stack buffer overflow in do_input_entry() and do_dmi_entry()

Several functions in scripts/mod/file2alias.c build the module alias string by repeatedly appending into a fixed-size on-stack buffer: char alias[256] = {}; ... sprintf(alias + strlen(alias), "%X,*", i); This pattern is unbounded and silently corrupts the stack when the formatted output exceeds the destination size. Two functions in this file are realistically reachable with input that overflows their buffer: 1. do_input_entry() appends across nine bitmap classes (evbit/keybit/relbit/absbit/mscbit/ledbit/sndbit/ffbit/swbit). The keybit case alone scans bits from INPUT_DEVICE_ID_KEY_MIN_INTERESTING (0x71) to INPUT_DEVICE_ID_KEY_MAX (0x2ff), 655 iterations; if a MODULE_DEVICE_TABLE(input, ...) populates keybit[] densely, the emission reaches ~3132 bytes — overflowing the 256-byte buffer by about 12x. include/linux/mod_devicetable.h declares storage for the full bit range ("keybit[INPUT_DEVICE_ID_KEY_MAX / BITS_PER_LONG + 1]"), so the worst case is reachable per the ABI. 2. do_dmi_entry() emits one ":<prefix>*<filtered_substr>*" segment per matched DMI field, up to 4 matches per dmi_system_id. Each substr is sized as char[79] in struct dmi_strmatch (mod_devicetable.h:584), and dmi_ascii_filter() copies it verbatim into the alias buffer without bounds. Worst case: 4 × (1 + 3 + 1 + 79 + 1) = 336 bytes into alias[256], an 80-byte overflow. No driver in the current tree triggers either case — every in-tree INPUT_DEVICE_ID_MATCH_KEYBIT user populates keybit[] very sparsely (1-3 bits), and no in-tree dmi_system_id has four maximally-long matches. The concern is defense-in-depth: both unbounded sprintf chains are silent stack-corruption primitives in a host build tool, and the buffer sizes have not been revisited since the corresponding code was first introduced. The other do_*_entry() handlers in this file (do_usb_entry, do_cpu_entry, do_typec_entry, ...) were audited and are bounded by their input field sizes (uint16 IDs, fixed-length keys); their alias buffers do not need this treatment. Reproduced under AddressSanitizer with a stand-alone harness mirroring do_input on a fully-populated keybit: ==18319==ERROR: AddressSanitizer: stack-buffer-overflow WRITE of size 2 at offset 288 in frame [32, 288) 'alias' #6 do_input poc.c:44 Stack-canary build: Abort trap: 6 (strlen(alias)=3134, cap was 256-1) Add a small alias_append() helper around vsnprintf with a remaining- space check and call fatal() on overflow, matching the modpost style for unrecoverable build conditions. do_input() takes the buffer size as a new parameter; do_input_entry() and do_dmi_entry() pass sizeof(alias) at every call site. dmi_ascii_filter() takes the remaining buffer size as well and aborts on truncation. This bounds every write into the on-stack buffers and turns the latent overflow into a clean build error if it is ever reached.

HBHasan Basbunar committed 6 months ago