From d6f3385b1a3f2fba8e14d6794bece1dcdd9e479e Mon Sep 17 00:00:00 2001 From: Aleksandr Nogikh Date: Wed, 12 Jan 2022 14:40:51 +0000 Subject: all: add syz_clone() and syz_clone3() pseudo calls As was pointed out in #2921, the current approach of limiting the number of pids per process does not work on all Linux-based kernels. We could just treat fork, clone and clone3 in a special way (e.g. exit on a zero return). However, in that case we also need to sanitize the arguments for clone and clone3 - if CLONE_VM is passed and stack is 0, the forked child processes (threads) will become nearly unindentifiable and will corrupt syz-executor's memory. While we could sanitize clone's arguments, we cannot do so for clone3 - nothing can guarantee that they will not be changed concurrently. Instead of calling those syscalls directly, introduce a special pseudo syscall syz_clone3. It copies and sanitizes the arguments and then executes clone3 (or fork, if we're on an older kernel) in such a way so as to prevent fork bombs from happening. Also introduce syz_clone() to still be able to fuzz it on older systems. --- executor/common_linux.h | 59 +++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 59 insertions(+) (limited to 'executor') diff --git a/executor/common_linux.h b/executor/common_linux.h index 479c229dc..520c13506 100644 --- a/executor/common_linux.h +++ b/executor/common_linux.h @@ -5215,3 +5215,62 @@ static long syz_80211_join_ibss(volatile long a0, volatile long a1, volatile lon } #endif + +#if SYZ_EXECUTOR || __NR_syz_clone || __NR_syz_clone3 +#if SYZ_EXECUTOR +// The slowdown multiplier is already taken into account. +#define USLEEP_FORKED_CHILD (3 * syscall_timeout_ms * 1000) +#else +#define USLEEP_FORKED_CHILD (3 * /*{{{BASE_CALL_TIMEOUT_MS}}}*/ *1000) +#endif + +static long handle_clone_ret(long ret) +{ + if (ret != 0) + return ret; + // Exit if we're in the child process - not all kernels provide the proper means + // to prevent fork-bombs. + // But first sleep for some time. This will hopefully foster IPC fuzzing. + usleep(USLEEP_FORKED_CHILD); + // Note that exit_group is a bad choice here because if we created just a thread, then + // the whole process will be killed. A plain exit will work fine in any case. + syscall(__NR_exit, 0); + while (1) { + } +} +#endif + +#if SYZ_EXECUTOR || __NR_syz_clone +// syz_clone is mostly needed on kernels which do not suport clone3. +static long syz_clone(volatile long flags, volatile long stack, volatile long stack_len, + volatile long ptid, volatile long ctid, volatile long tls) +{ + // ABI requires 16-byte stack alignment. + long sp = (stack + stack_len) & ~15; + // Clear the CLONE_VM flag. Otherwise it'll very likely corrupt syz-executor. + long ret = (long)syscall(__NR_clone, flags & ~CLONE_VM, sp, ptid, ctid, tls); + return handle_clone_ret(ret); +} +#endif + +#if SYZ_EXECUTOR || __NR_syz_clone3 +#include +#include + +#define MAX_CLONE_ARGS_BYTES 256 +static long syz_clone3(volatile long a0, volatile long a1) +{ + unsigned long copy_size = a1; + if (copy_size < sizeof(uint64) || copy_size > MAX_CLONE_ARGS_BYTES) + return -1; + // The structure may have different sizes on different kernel versions, so copy it as raw bytes. + char clone_args[MAX_CLONE_ARGS_BYTES]; + memcpy(&clone_args, (void*)a0, copy_size); + + // As in syz_clone, clear the CLONE_VM flag. Flags are in the first 8-byte integer field. + uint64* flags = (uint64*)&clone_args; + *flags &= ~CLONE_VM; + return handle_clone_ret((long)syscall(__NR_clone3, &clone_args, copy_size)); +} + +#endif -- cgit mrf-deployment