7.3-stable review patch. If anyone has any objections, please let me know. ------------------ From: Fang Xieyan commit f5263bee9eb84d2388fd7555a18d518e17226894 upstream. write_comp_data() reads the comparison record count from area[0] and uses it to index the coverage buffer: area = (u64 *)t->kcov_area; max_pos = t->kcov_size * sizeof(unsigned long); count = READ_ONCE(area[0]); /* Every record is KCOV_WORDS_PER_CMP 64-bit words. */ start_index = 1 + count * KCOV_WORDS_PER_CMP; end_pos = (start_index + KCOV_WORDS_PER_CMP) * sizeof(u64); if (likely(end_pos <= max_pos)) { The buffer is mmap'd writable into the collecting process, so count is under its control and end_pos <= max_pos is its only bound. A count that wraps the u64 multiply leaves end_pos below max_pos, so the check passes while the record store lands 24 bytes before the buffer, in the unmapped vmalloc guard page, and faults: BUG: unable to handle page fault for address: ffa0000000b60fe8 #PF: supervisor write access in kernel mode #PF: error_code(0x0002) - not-present page Oops: 0002 [#1] SMP KASAN NOPTI RIP: 0010:write_comp_data+0x7e/0xa0 ... Kernel panic - not syncing: Fatal exception Bound count first: only max_pos / (sizeof(u64) * KCOV_WORDS_PER_CMP) records fit, so a larger count is not a valid index and is dropped. kcov_move_area() bounds the same untrusted count this way, and no count the end_pos <= max_pos check accepts reaches that limit, so no valid record is lost. kcov is a root-only debugfs file (debugfs_create_file_unsafe("kcov", 0600, ...)) and write_comp_data() exists only under CONFIG_KCOV_ENABLE_COMPARISONS, so this is a local, debug-kernel robustness fix: the process corrupts its own buffer and the kernel oopses. It crosses no privilege boundary. Additional details at [1] Link: https://lore.kernel.org/20260917104306.22145-1-fangxy@xiaopeng.com [1] Fixes: ded97d2c2b2c ("kcov: support comparison operands collection") Signed-off-by: Fang Xieyan Signed-off-by: Andrew Morton Reviewed-by: Alexander Potapenko Assisted-by: Hawkeye:GLM-5.3-flash Assisted-by: Qoder:Qwen3.8-Max Cc: Andrey Konovalov Cc: Dmitry Vyukov Cc: Marco Elver Cc: Victor Chibotaru Cc: Signed-off-by: Greg Kroah-Hartman --- kernel/kcov.c | 8 ++++++++ 1 file changed, 8 insertions(+) --- a/kernel/kcov.c +++ b/kernel/kcov.c @@ -253,6 +253,14 @@ static void notrace write_comp_data(u64 count = READ_ONCE(area[0]); + /* + * area[0] is writable by the collecting process, so count cannot be + * trusted. Bound it to the records that fit, as kcov_move_area() + * does, so the end_pos multiply below cannot wrap past its check. + */ + if (count >= max_pos / (sizeof(u64) * KCOV_WORDS_PER_CMP)) + return; + /* Every record is KCOV_WORDS_PER_CMP 64-bit words. */ start_index = 1 + count * KCOV_WORDS_PER_CMP; end_pos = (start_index + KCOV_WORDS_PER_CMP) * sizeof(u64);