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.
Fixes: 1d8f430c15 ("[PATCH] Input: add modalias support")
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Hasan Basbunar <basbunarhasan@gmail.com>
Link: https://patch.msgid.link/20260505161102.44087-1-basbunarhasan@gmail.com
Signed-off-by: Nicolas Schier <nsc@kernel.org>
49f8fcde68