From eda0273c1b36b4959e5c61e3d209029e1d8de088 Mon Sep 17 00:00:00 2001 From: Johannes Stoelp Date: Sun, 8 Aug 2021 16:50:14 +0200 Subject: add vdso proxy poc --- src/lib.rs | 184 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ src/main.rs | 144 +++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 328 insertions(+) create mode 100644 src/lib.rs create mode 100644 src/main.rs (limited to 'src') diff --git a/src/lib.rs b/src/lib.rs new file mode 100644 index 0000000..18c1b74 --- /dev/null +++ b/src/lib.rs @@ -0,0 +1,184 @@ +//! Collection of some abstractions and the error definition used by this PoC. + +/// Errors that can occur in this PoC. +#[derive(Debug)] +pub enum Error { + /// Failed to open and read the `/proc/self/maps` file. + FailedToReadMaps, + /// Failed to a parse line from the `/proc/self/maps` file. + /// Captures line that failed to parse. + ParseMapEntryError(String), + /// No `[vdso]` segment found in `/proc/self/maps`. + VdsoSegmentNotFound, + /// Failed to parse bytes as ELF file. + FailedToParseAsElf, + /// No `PT_LOAD` program header found in the ELF file. + LoadPhdrNotFound, + /// Requested symbol not found in the ELF file. + /// Captures the name of the requested symbol. + SymbolNotFound(String), +} + +/// Representation of an 64-bit virtual address. +#[derive(Debug, Copy, Clone)] +pub struct VirtAddr(pub u64); + +/// Represents an entry from the `/proc/self/maps` file with the information needed by this PoC. +/// ```text +/// man 5 proc +/// address perms offset dev inode pathname +/// 00400000-00452000 r-xp 00000000 08:02 173521 /usr/bin/dbus-daemon +/// ``` +#[derive(Debug)] +pub struct MapEntry { + /// Start address of the memory segment. + pub addr: u64, + /// Length of the memory segment. + pub len: u64, + /// Optional name of the memory segment. + pub name: Option, +} + +impl MapEntry { + /// Try to parse a [`MapEntry`] from the `line` passed as argument. + pub fn from_line<'a>(line: &'a str) -> Result { + let expect = |tok: Option<&'a str>| tok.ok_or(Error::ParseMapEntryError(line.into())); + + // Tokenize the line. + let mut toks = line.split_whitespace(); + let addr = expect(toks.next())?; + let _perms = expect(toks.next())?; + let _offset = expect(toks.next())?; + let _dev = expect(toks.next())?; + let _inode = expect(toks.next())?; + let name = toks.next().map(|name| String::from(name)); + + // Parse the address token. + let (addr, len) = { + let tou64 = |s: &'a str| { + u64::from_str_radix(s, 16) + .map_err(|e| Error::ParseMapEntryError(format!("{}\n{}", line, e))) + }; + + let mut toks = addr.split('-'); + let start = tou64(expect(toks.next())?)?; + let end = tou64(expect(toks.next())?)?; + + (start, end - start) + }; + + Ok(MapEntry { addr, len, name }) + } +} + +/// Owned [`libc::mmap`] allocation. +pub struct Mmap { + ptr: *mut libc::c_void, + len: usize, + map: MapEntry, +} + +impl Mmap { + /// Create a new allocation with `read | write | execute` permissions big enough to hold a copy + /// of `bytes` and initialize it with the `bytes` passed as argument. + pub fn new_rwx_from(bytes: &[u8]) -> Option { + use libc::{ + memcpy, mmap, sysconf, MAP_ANONYMOUS, MAP_FAILED, MAP_PRIVATE, PROT_EXEC, PROT_READ, + PROT_WRITE, _SC_PAGESIZE, + }; + + // Get the page size. + let page_size = unsafe { sysconf(_SC_PAGESIZE) } as usize; + + // Compute required size for the new allocation by rounding up to the next page size. + let len = ((bytes.len() + page_size - 1) / page_size) * page_size; + + // Allocate new `rwx` memory segment. + let ptr = unsafe { + mmap( + std::ptr::null_mut(), + len, + PROT_READ | PROT_WRITE | PROT_EXEC, + MAP_PRIVATE | MAP_ANONYMOUS, + -1, /* fd */ + 0, /* offset */ + ) + }; + + if ptr == MAP_FAILED { + return None; + } + + unsafe { + // Initialize new allocation with `bytes` passed as argument. + memcpy(ptr, bytes.as_ptr().cast(), bytes.len()); + } + + Some(Mmap { + ptr, + len, + map: MapEntry { + addr: ptr as u64, + len: len as u64, + name: Some("mmap_rwx".into()), + }, + }) + } +} + +impl Drop for Mmap { + fn drop(&mut self) { + unsafe { libc::munmap(self.ptr, self.len) }; + } +} + +impl AsRef for Mmap { + fn as_ref(&self) -> &'_ MapEntry { + &self.map + } +} + +impl AsMut<[u8]> for Mmap { + fn as_mut(&mut self) -> &mut [u8] { + unsafe { std::slice::from_raw_parts_mut(self.ptr.cast(), self.len) } + } +} + +/// An `x86_64` jump pad (trampoline) that that can be installed at a [`VirtAddr`]. +/// +/// The jump pad is implemented as: +/// ```asm +/// mov rax, imm64 ; target +/// jmp rax +/// ``` +#[allow(dead_code)] +#[cfg(target_arch = "x86_64")] +#[repr(packed)] +pub struct JmpPad { + movabs: u16, + target: u64, + jmp_rax: u16, +} + +#[cfg(target_arch = "x86_64")] +impl JmpPad { + /// Initialize a new jump pad to the destination virtual address `target`. + /// This does not install the jump pad. + pub fn to(target: VirtAddr) -> JmpPad { + JmpPad { + movabs: 0xb848, // REX.W + mov rax, imm64 + target: target.0, + jmp_rax: 0xe0ff, // jmp rax + } + } + + /// Install the jump pad at the virtual address `addr`. + /// + /// # Safety + /// The caller must guarantee the following constraints: + /// - `addr` must be a valid virtual address referring to writeable memory. + /// - There must be enough space to store [`size_of::()`](core::mem::size_of) bytes. + pub unsafe fn install_at(self, addr: VirtAddr) { + std::ptr::write(addr.0 as *mut JmpPad, self); + } +} diff --git a/src/main.rs b/src/main.rs new file mode 100644 index 0000000..400b2d7 --- /dev/null +++ b/src/main.rs @@ -0,0 +1,144 @@ +use goblin::elf::{Elf, program_header::PT_LOAD}; +use std::convert::TryFrom; +use vdso_proxy_poc::{Error, JmpPad, MapEntry, Mmap, VirtAddr}; + +#[cfg(not(target_os = "linux"))] +compile_error!("This only makes sense on Linux, as we are poking the vdso."); + +/// Find the `[vdso]` entry in `/proc/self/maps`. +fn get_vdso() -> Result { + for line in std::fs::read_to_string("/proc/self/maps") + .map_err(|_| Error::FailedToReadMaps)? + .lines() + { + let map = MapEntry::from_line(line)?; + match &map.name { + Some(n) if n == "[vdso]" => return Ok(map), + _ => {} + } + } + Err(Error::VdsoSegmentNotFound) +} + +/// Create a copy of the `vdso` memory segment. Effectively allocates memory and copies the virtual +/// address range described by `vdso`. +/// +/// # Safety: +/// The caller must guarantee that the `vdso` argument describes a valid virtual address range by +/// its `address` and `length` fields. +#[allow(unused_unsafe)] +unsafe fn copy_vdso(vdso: &MapEntry) -> Option { + let bytes = { + let ptr = vdso.addr as *const u8; + let len = usize::try_from(vdso.len) + .expect("It's required that the segment length fits into a usize!"); + // SAFETY: Validity of ptr & len must be ensured by the caller. + unsafe { std::slice::from_raw_parts(ptr, len) } + }; + + Mmap::new_rwx_from(&bytes) +} + +/// Find the `symbol_name` in the vdso described by the [`MapEntry`] memory segment. +/// +/// # Safety: +/// The caller must guarantee that the `vdso` argument describes a valid virtual address range by +/// its `address` and `length` fields. +/// +/// # Note: +/// Currently the version of the symbol is not checked, technically this is an error which can be +/// fatal in case of a binary incompatibility, but that's accepted for this PoC. +#[allow(unused_unsafe)] +unsafe fn get_vdso_sym(vdso: &MapEntry, symbol_name: &str) -> Result { + // Turn `vdso` maps entry into slice of bytes. + let bytes = { + let ptr = vdso.addr as *const u8; + let len = usize::try_from(vdso.len) + .expect("It's required that the segment length fits into a usize!"); + // SAFETY: Validity of ptr & len must be ensured by the caller. + unsafe { std::slice::from_raw_parts(ptr, len) } + }; + + // Parse vdso bytes as ELF. + let elf = Elf::parse(bytes).map_err(|_| Error::FailedToParseAsElf)?; + + // Compute the dynamic shared object (dso) base address. Symbol offsets are relative to this + // dso base address. + let dso_base = { + let phdr_load = elf + .program_headers + .iter() + .find(|p| p.p_type == PT_LOAD) + .ok_or(Error::LoadPhdrNotFound)?; + vdso.addr - phdr_load.p_offset - phdr_load.p_vaddr + }; + assert_ne!(dso_base, 0, "If the dso base address is 0 that means the symbols contain absolute addresses, we don't want to support that!"); + + // Try to find the requested symbol. + let sym = elf + .dynsyms + .iter() + .filter(|sym| sym.is_function()) + .find(|sym| matches!(elf.dynstrtab.get_at(sym.st_name), Some(sym) if sym == symbol_name)) + .ok_or(Error::SymbolNotFound(symbol_name.into()))?; + + // Compute the absolute virtual address of the requested symbol. + Ok(VirtAddr(dso_base + sym.st_value)) +} + +/// Represent the `struct timeval` C structure (see `man 2 gettimeofday`). +#[repr(C)] +struct Timeval { + tv_sec: i64, + tv_usec: i64, +} + +fn main() -> Result<(), Error> { + // This represents the _new_ vdso pages that the kernel mapped into the restoring process. + let orig_vdso = get_vdso()?; + + // This represents the _old_ vdso pages that were captured in the memory dump of the process + // checkpoint. + // + // SAFETY: orig_vdso describes a valid memory region as we got it from /proc/self/maps. + let copy_vdso = unsafe { copy_vdso(&orig_vdso).expect("Copy of vdso must succeed!") }; + + let (orig_sym_addr, copy_sym_addr) = unsafe { + // SAFETY: orig_vdso describes a valid memory region as we got it from /proc/self/maps. + let orig = get_vdso_sym(&orig_vdso, "__vdso_gettimeofday")?; + // SAFETY: copy_vdso describes a valid and owned memory allocation. + let copy = get_vdso_sym(©_vdso.as_ref(), "__vdso_gettimeofday")?; + + (orig, copy) + }; + + // As an example, install a trampoline for the `__vdso_gettimeofday` symbol. The trampoline is + // installed in the _old_ vdso pages, where the user code from the checkpoint image links to, + // and forwards the calls into the _new_ vdso pages. + let pad = JmpPad::to(orig_sym_addr); + // SAFETY: copy_sym_addr is a valid virtual address as we got it from the symbol lookup. + unsafe { pad.install_at(copy_sym_addr) }; + + let mut tv: Timeval = Timeval { + tv_sec: 0, + tv_usec: 0, + }; + + unsafe { + // Mimic a call to `__vdso_gettimeofday` from user code which is still linked to the _old_ + // vdso. + + // SAFETY: copy_sym_addr is a valid virtual address pointing to the `__vdso_gettimeofday` + // function. + let gettimeofday: extern "C" fn(*mut Timeval, *mut libc::c_void) -> i32 = + std::mem::transmute(copy_sym_addr.0 as *const ()); + + // Invoke the `__vdso_gettimeofday` function in the copied memory region (_old_ vdso). This + // should forward to the function in the original memory region. + gettimeofday(&mut tv as *mut Timeval, std::ptr::null_mut()); + } + + println!("Timeval tv_sec : {} tv_usec : {}", tv.tv_sec, tv.tv_usec); + + Ok(()) +} -- cgit v1.2.3