[libunwind] [libunwind] Replace process_vm_readv with pipe (PR #74791)

[Jordan R AW via cfe-commits]

https://github.com/ajordanr-google created https://github.com/llvm/llvm-project/pull/74791

process_vm_readv is generally considered dangerous from a syscall perspective, and is frequently blanket banned in seccomp filters such as those in Chromium and ChromiumOS. We can get the same behaviour during the invalid PC address case with pipes and write/read.

Testing to ensure that process_vm_readv does not appear, I ran the output of check-unwind on an ARM64 device under strace. Previously, bad_unwind_info in particular would use process_vm_readv, but with this commit, it now uses pipe2:

```
strace test/Output/bad_unwind_info.pass.cpp.dir/t.tmp.exe \
  |& grep process_vm_readv
strace test/Output/bad_unwind_info.pass.cpp.dir/t.tmp.exe \
  |& grep pipe2
```

>From 1f4df1b82970c95684eed93c8f6bcaa6d6507b88 Mon Sep 17 00:00:00 2001
From: Jordan R Abrahams-Whitehead <ajordanr at google.com>
Date: Fri, 8 Dec 2023 00:09:59 +0000
Subject: [PATCH] [libunwind] Replace process_vm_readv with pipe

process_vm_readv is generally considered dangerous from a syscall
perspective, and is frequently blanket banned in seccomp filters such as
those in Chromium and ChromiumOS. We can get the same behaviour during
the invalid PC address case with pipes and write/read.

Testing to ensure that process_vm_readv does not appear, I ran the
output of check-unwind on an ARM64 device under strace. Previously,
bad_unwind_info in particular would use process_vm_readv, but with this
commit, it now uses pipe2:

```
strace test/Output/bad_unwind_info.pass.cpp.dir/t.tmp.exe \
  |& grep process_vm_readv
strace test/Output/bad_unwind_info.pass.cpp.dir/t.tmp.exe \
  |& grep pipe2
```
---
 libunwind/src/UnwindCursor.hpp | 50 ++++++++++++++++++++--------------
 1 file changed, 29 insertions(+), 21 deletions(-)

diff --git a/libunwind/src/UnwindCursor.hpp b/libunwind/src/UnwindCursor.hpp
index 647a5a9c9d92d..5e4e376220daa 100644
--- a/libunwind/src/UnwindCursor.hpp
+++ b/libunwind/src/UnwindCursor.hpp
@@ -33,6 +33,7 @@
 #if defined(_LIBUNWIND_TARGET_LINUX) &&                                        \
     (defined(_LIBUNWIND_TARGET_AARCH64) || defined(_LIBUNWIND_TARGET_RISCV) || \
      defined(_LIBUNWIND_TARGET_S390X))
+#include <fcntl.h>
 #include <sys/syscall.h>
 #include <sys/uio.h>
 #include <unistd.h>
@@ -2700,19 +2701,18 @@ bool UnwindCursor<A, R>::setInfoForSigReturn(Registers_arm64 &) {
   // [1] https://github.com/torvalds/linux/blob/master/arch/arm64/kernel/vdso/sigreturn.S
   const pint_t pc = static_cast<pint_t>(this->getReg(UNW_REG_IP));
   // The PC might contain an invalid address if the unwind info is bad, so
-  // directly accessing it could cause a segfault. Use process_vm_readv to read
-  // the memory safely instead. process_vm_readv was added in Linux 3.2, and
-  // AArch64 supported was added in Linux 3.7, so the syscall is guaranteed to
-  // be present. Unfortunately, there are Linux AArch64 environments where the
-  // libc wrapper for the syscall might not be present (e.g. Android 5), so call
-  // the syscall directly instead.
+  // directly accessing it could cause a segfault. Use pipe/write/read to read
+  // the memory safely instead.
+  int pipefd[2];
+  if (pipe2(pipefd, O_CLOEXEC | O_NONBLOCK) == -1)
+    return false;
   uint32_t instructions[2];
-  struct iovec local_iov = {&instructions, sizeof instructions};
-  struct iovec remote_iov = {reinterpret_cast<void *>(pc), sizeof instructions};
-  long bytesRead =
-      syscall(SYS_process_vm_readv, getpid(), &local_iov, 1, &remote_iov, 1, 0);
+  const auto bufferSize = sizeof instructions;
+  if (write(pipefd[1], reinterpret_cast<void *>(pc), bufferSize) != bufferSize)
+    return false;
+  const auto bytesRead = read(pipefd[0], instructions, bufferSize);
   // Look for instructions: mov x8, #0x8b; svc #0x0
-  if (bytesRead != sizeof instructions || instructions[0] != 0xd2801168 ||
+  if (bytesRead != bufferSize || instructions[0] != 0xd2801168 ||
       instructions[1] != 0xd4000001)
     return false;
 
@@ -2762,17 +2762,20 @@ int UnwindCursor<A, R>::stepThroughSigReturn(Registers_arm64 &) {
 template <typename A, typename R>
 bool UnwindCursor<A, R>::setInfoForSigReturn(Registers_riscv &) {
   const pint_t pc = static_cast<pint_t>(getReg(UNW_REG_IP));
+  int pipefd[2];
+  if (pipe2(pipefd, O_CLOEXEC | O_NONBLOCK) == -1)
+    return false;
   uint32_t instructions[2];
-  struct iovec local_iov = {&instructions, sizeof instructions};
-  struct iovec remote_iov = {reinterpret_cast<void *>(pc), sizeof instructions};
-  long bytesRead =
-      syscall(SYS_process_vm_readv, getpid(), &local_iov, 1, &remote_iov, 1, 0);
+  const auto bufferSize = sizeof instructions;
+  if (write(pipefd[1], reinterpret_cast<void *>(pc), bufferSize) != bufferSize)
+    return false;
+  const auto bytesRead = read(pipefd[0], instructions, bufferSize);
   // Look for the two instructions used in the sigreturn trampoline
   // __vdso_rt_sigreturn:
   //
   // 0x08b00893 li a7,0x8b
   // 0x00000073 ecall
-  if (bytesRead != sizeof instructions || instructions[0] != 0x08b00893 ||
+  if (bytesRead != bufferSize || instructions[0] != 0x08b00893 ||
       instructions[1] != 0x00000073)
     return false;
 
@@ -2822,13 +2825,18 @@ bool UnwindCursor<A, R>::setInfoForSigReturn(Registers_s390x &) {
   // onto the stack.
   const pint_t pc = static_cast<pint_t>(this->getReg(UNW_REG_IP));
   // The PC might contain an invalid address if the unwind info is bad, so
-  // directly accessing it could cause a segfault. Use process_vm_readv to
+  // directly accessing it could cause a segfault. Use pipe/write/read to
   // read the memory safely instead.
   uint16_t inst;
-  struct iovec local_iov = {&inst, sizeof inst};
-  struct iovec remote_iov = {reinterpret_cast<void *>(pc), sizeof inst};
-  long bytesRead = process_vm_readv(getpid(), &local_iov, 1, &remote_iov, 1, 0);
-  if (bytesRead == sizeof inst && (inst == 0x0a77 || inst == 0x0aad)) {
+  int pipefd[2];
+  if (pipe2(pipefd, O_CLOEXEC | O_NONBLOCK) == -1)
+    return false;
+  uint16_t inst;
+  const auto bufferSize = sizeof inst;
+  if (write(pipefd[1], reinterpret_cast<void *>(pc), bufferSize) != bufferSize)
+    return false;
+  const auto bytesRead = read(pipefd[0], &inst, bufferSize);
+  if (bytesRead == bufferSize && (inst == 0x0a77 || inst == 0x0aad)) {
     _info = {};
     _info.start_ip = pc;
     _info.end_ip = pc + 2;