diff --git a/README.md b/README.md
index 909e5bd510..9a4dea949d 100644
--- a/README.md
+++ b/README.md
@@ -576,6 +576,21 @@ extern "Rust" {
 
     /// Miri-provided extern function to deallocate memory.
     fn miri_dealloc(ptr: *mut u8, size: usize, align: usize);
+
+    /// Convert a path from the host Miri runs on to the target Miri interprets.
+    /// Performs conversion of path separators as needed.
+    ///
+    /// Usually Miri performs this kind of conversion automatically. However, manual conversion
+    /// might be necessary when reading an environment variable that was set on the host
+    /// (such as TMPDIR) and using it as a target path.
+    ///
+    /// Only works with isolation disabled.
+    ///
+    /// `in` must point to a null-terminated string, and will be read as the input host path.
+    /// `out` must point to at least `out_size` many bytes, and the result will be stored there
+    /// with a null terminator.
+    /// Returns 0 if the `out` buffer was large enough, and the required size otherwise.
+    fn miri_host_to_target_path(path: *const i8, out: *mut i8, out_size: usize) -> usize;
 }
 ```
 
diff --git a/src/shims/foreign_items.rs b/src/shims/foreign_items.rs
index 8370e02b58..abfa73db64 100644
--- a/src/shims/foreign_items.rs
+++ b/src/shims/foreign_items.rs
@@ -1,4 +1,4 @@
-use std::{collections::hash_map::Entry, io::Write, iter};
+use std::{collections::hash_map::Entry, io::Write, iter, path::Path};
 
 use log::trace;
 
@@ -442,6 +442,21 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 }
                 this.machine.static_roots.push(alloc_id);
             }
+            "miri_host_to_target_path" => {
+                let [ptr, out, out_size] = this.check_shim(abi, Abi::Rust, link_name, args)?;
+                let ptr = this.read_pointer(ptr)?;
+                let out = this.read_pointer(out)?;
+                let out_size = this.read_scalar(out_size)?.to_machine_usize(this)?;
+
+                // The host affects program behavior here, so this requires isolation to be disabled.
+                this.check_no_isolation("`miri_host_to_target_path`")?;
+
+                // We read this as a plain OsStr and write it as a path, which will convert it to the target.
+                let path = this.read_os_str_from_c_str(ptr)?.to_owned();
+                let (success, needed_size) = this.write_path_to_c_str(Path::new(&path), out, out_size)?;
+                // Return value: 0 on success, otherwise the size it would have needed.
+                this.write_int(if success { 0 } else { needed_size }, dest)?;
+            }
 
             // Obtains the size of a Miri backtrace. See the README for details.
             "miri_backtrace_size" => {
diff --git a/src/shims/os_str.rs b/src/shims/os_str.rs
index bc7ca82997..0375a228a2 100644
--- a/src/shims/os_str.rs
+++ b/src/shims/os_str.rs
@@ -174,7 +174,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         let this = self.eval_context_ref();
         let os_str = this.read_os_str_from_c_str(ptr)?;
 
-        Ok(match this.convert_path_separator(Cow::Borrowed(os_str), PathConversion::TargetToHost) {
+        Ok(match this.convert_path(Cow::Borrowed(os_str), PathConversion::TargetToHost) {
             Cow::Borrowed(x) => Cow::Borrowed(Path::new(x)),
             Cow::Owned(y) => Cow::Owned(PathBuf::from(y)),
         })
@@ -188,10 +188,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         let this = self.eval_context_ref();
         let os_str = this.read_os_str_from_wide_str(ptr)?;
 
-        Ok(this
-            .convert_path_separator(Cow::Owned(os_str), PathConversion::TargetToHost)
-            .into_owned()
-            .into())
+        Ok(this.convert_path(Cow::Owned(os_str), PathConversion::TargetToHost).into_owned().into())
     }
 
     /// Write a Path to the machine memory (as a null-terminated sequence of bytes),
@@ -203,8 +200,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         size: u64,
     ) -> InterpResult<'tcx, (bool, u64)> {
         let this = self.eval_context_mut();
-        let os_str = this
-            .convert_path_separator(Cow::Borrowed(path.as_os_str()), PathConversion::HostToTarget);
+        let os_str =
+            this.convert_path(Cow::Borrowed(path.as_os_str()), PathConversion::HostToTarget);
         this.write_os_str_to_c_str(&os_str, ptr, size)
     }
 
@@ -217,8 +214,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         size: u64,
     ) -> InterpResult<'tcx, (bool, u64)> {
         let this = self.eval_context_mut();
-        let os_str = this
-            .convert_path_separator(Cow::Borrowed(path.as_os_str()), PathConversion::HostToTarget);
+        let os_str =
+            this.convert_path(Cow::Borrowed(path.as_os_str()), PathConversion::HostToTarget);
         this.write_os_str_to_wide_str(&os_str, ptr, size)
     }
 
@@ -230,18 +227,20 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         memkind: MemoryKind<MiriMemoryKind>,
     ) -> InterpResult<'tcx, Pointer<Option<Provenance>>> {
         let this = self.eval_context_mut();
-        let os_str = this
-            .convert_path_separator(Cow::Borrowed(path.as_os_str()), PathConversion::HostToTarget);
+        let os_str =
+            this.convert_path(Cow::Borrowed(path.as_os_str()), PathConversion::HostToTarget);
         this.alloc_os_str_as_c_str(&os_str, memkind)
     }
 
-    fn convert_path_separator<'a>(
+    #[allow(clippy::get_first)]
+    fn convert_path<'a>(
         &self,
         os_str: Cow<'a, OsStr>,
         direction: PathConversion,
     ) -> Cow<'a, OsStr> {
         let this = self.eval_context_ref();
         let target_os = &this.tcx.sess.target.os;
+
         #[cfg(windows)]
         return if target_os == "windows" {
             // Windows-on-Windows, all fine.
@@ -252,24 +251,71 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 PathConversion::HostToTarget => ('\\', '/'),
                 PathConversion::TargetToHost => ('/', '\\'),
             };
-            let converted = os_str
+            let mut converted = os_str
                 .encode_wide()
                 .map(|wchar| if wchar == from as u16 { to as u16 } else { wchar })
                 .collect::<Vec<_>>();
+            // We also have to ensure that absolute paths remain absolute.
+            match direction {
+                PathConversion::HostToTarget => {
+                    // If this is an absolute Windows path that starts with a drive letter (`C:/...`
+                    // after separator conversion), it would not be considered absolute by Unix
+                    // target code.
+                    if converted.get(1).copied() == Some(b':' as u16)
+                        && converted.get(2).copied() == Some(b'/' as u16)
+                    {
+                        // We add a `/` at the beginning, to store the absolute Windows
+                        // path in something that looks like an absolute Unix path.
+                        converted.insert(0, b'/' as u16);
+                    }
+                }
+                PathConversion::TargetToHost => {
+                    // If the path is `\C:\`, the leading backslash was probably added by the above code
+                    // and we should get rid of it again.
+                    if converted.get(0).copied() == Some(b'\\' as u16)
+                        && converted.get(2).copied() == Some(b':' as u16)
+                        && converted.get(3).copied() == Some(b'\\' as u16)
+                    {
+                        converted.remove(0);
+                    }
+                }
+            }
             Cow::Owned(OsString::from_wide(&converted))
         };
         #[cfg(unix)]
         return if target_os == "windows" {
             // Windows target, Unix host.
             let (from, to) = match direction {
-                PathConversion::HostToTarget => ('/', '\\'),
-                PathConversion::TargetToHost => ('\\', '/'),
+                PathConversion::HostToTarget => (b'/', b'\\'),
+                PathConversion::TargetToHost => (b'\\', b'/'),
             };
-            let converted = os_str
+            let mut converted = os_str
                 .as_bytes()
                 .iter()
-                .map(|&wchar| if wchar == from as u8 { to as u8 } else { wchar })
+                .map(|&wchar| if wchar == from { to } else { wchar })
                 .collect::<Vec<_>>();
+            // We also have to ensure that absolute paths remain absolute.
+            match direction {
+                PathConversion::HostToTarget => {
+                    // If this start withs a `\`, we add `\\?` so it starts with `\\?\` which is
+                    // some magic path on Windos that *is* considered absolute.
+                    if converted.get(0).copied() == Some(b'\\') {
+                        converted.splice(0..0, b"\\\\?".iter().copied());
+                    }
+                }
+                PathConversion::TargetToHost => {
+                    // If this starts with `//?/`, it was probably produced by the above code and we
+                    // remove the `//?` that got added to get the Unix path back out.
+                    if converted.get(0).copied() == Some(b'/')
+                        && converted.get(1).copied() == Some(b'/')
+                        && converted.get(2).copied() == Some(b'?')
+                        && converted.get(3).copied() == Some(b'/')
+                    {
+                        // Remove first 3 characters
+                        converted.splice(0..3, std::iter::empty());
+                    }
+                }
+            }
             Cow::Owned(OsString::from_vec(converted))
         } else {
             // Unix-on-Unix, all is fine.
diff --git a/src/shims/unix/fs.rs b/src/shims/unix/fs.rs
index bf99412af6..8b869a6525 100644
--- a/src/shims/unix/fs.rs
+++ b/src/shims/unix/fs.rs
@@ -1667,7 +1667,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 // 'readlink' truncates the resolved path if the provided buffer is not large
                 // enough, and does *not* add a null terminator. That means we cannot use the usual
                 // `write_path_to_c_str` and have to re-implement parts of it ourselves.
-                let resolved = this.convert_path_separator(
+                let resolved = this.convert_path(
                     Cow::Borrowed(resolved.as_ref()),
                     crate::shims::os_str::PathConversion::HostToTarget,
                 );
diff --git a/test-cargo-miri/src/main.rs b/test-cargo-miri/src/main.rs
index 41c52b7017..048dbbbaa0 100644
--- a/test-cargo-miri/src/main.rs
+++ b/test-cargo-miri/src/main.rs
@@ -2,6 +2,7 @@ use byteorder::{BigEndian, ByteOrder};
 use std::env;
 #[cfg(unix)]
 use std::io::{self, BufRead};
+use std::path::PathBuf;
 
 fn main() {
     // Check env var set by `build.rs`.
@@ -21,12 +22,30 @@ fn main() {
     // If there were no arguments, access stdin and test working dir.
     // (We rely on the test runner to always disable isolation when passing no arguments.)
     if std::env::args().len() <= 1 {
+        fn host_to_target_path(path: String) -> PathBuf {
+            use std::ffi::{CStr, CString};
+
+            let path = CString::new(path).unwrap();
+            let mut out = Vec::with_capacity(1024);
+
+            unsafe {
+                extern "Rust" {
+                    fn miri_host_to_target_path(
+                        path: *const i8,
+                        out: *mut i8,
+                        out_size: usize,
+                    ) -> usize;
+                }
+                let ret = miri_host_to_target_path(path.as_ptr(), out.as_mut_ptr(), out.capacity());
+                assert_eq!(ret, 0);
+                let out = CStr::from_ptr(out.as_ptr()).to_str().unwrap();
+                PathBuf::from(out)
+            }
+        }
+
         // CWD should be crate root.
-        // We have to normalize slashes, as the env var might be set for a different target's conventions.
         let env_dir = env::current_dir().unwrap();
-        let env_dir = env_dir.to_string_lossy().replace("\\", "/");
-        let crate_dir = env::var_os("CARGO_MANIFEST_DIR").unwrap();
-        let crate_dir = crate_dir.to_string_lossy().replace("\\", "/");
+        let crate_dir = host_to_target_path(env::var("CARGO_MANIFEST_DIR").unwrap());
         assert_eq!(env_dir, crate_dir);
 
         #[cfg(unix)]
diff --git a/test-cargo-miri/subcrate/main.rs b/test-cargo-miri/subcrate/main.rs
index 4ce80b3707..1cb8091f87 100644
--- a/test-cargo-miri/subcrate/main.rs
+++ b/test-cargo-miri/subcrate/main.rs
@@ -4,13 +4,30 @@ use std::path::PathBuf;
 fn main() {
     println!("subcrate running");
 
+    fn host_to_target_path(path: String) -> PathBuf {
+        use std::ffi::{CStr, CString};
+
+        let path = CString::new(path).unwrap();
+        let mut out = Vec::with_capacity(1024);
+
+        unsafe {
+            extern "Rust" {
+                fn miri_host_to_target_path(
+                    path: *const i8,
+                    out: *mut i8,
+                    out_size: usize,
+                ) -> usize;
+            }
+            let ret = miri_host_to_target_path(path.as_ptr(), out.as_mut_ptr(), out.capacity());
+            assert_eq!(ret, 0);
+            let out = CStr::from_ptr(out.as_ptr()).to_str().unwrap();
+            PathBuf::from(out)
+        }
+    }
+
     // CWD should be workspace root, i.e., one level up from crate root.
-    // We have to normalize slashes, as the env var might be set for a different target's conventions.
     let env_dir = env::current_dir().unwrap();
-    let env_dir = env_dir.to_string_lossy().replace("\\", "/");
-    let crate_dir = env::var_os("CARGO_MANIFEST_DIR").unwrap();
-    let crate_dir = crate_dir.to_string_lossy().replace("\\", "/");
-    let crate_dir = PathBuf::from(crate_dir);
-    let crate_dir = crate_dir.parent().unwrap().to_string_lossy();
+    let crate_dir = host_to_target_path(env::var("CARGO_MANIFEST_DIR").unwrap());
+    let crate_dir = crate_dir.parent().unwrap();
     assert_eq!(env_dir, crate_dir);
 }
diff --git a/test-cargo-miri/subcrate/test.rs b/test-cargo-miri/subcrate/test.rs
index 77e3c2878c..619d8c72fd 100644
--- a/test-cargo-miri/subcrate/test.rs
+++ b/test-cargo-miri/subcrate/test.rs
@@ -1,16 +1,37 @@
 use std::env;
 
+use std::path::PathBuf;
+
 use byteorder::{ByteOrder, LittleEndian};
 
 fn main() {
     println!("subcrate testing");
 
+    fn host_to_target_path(path: String) -> PathBuf {
+        use std::ffi::{CStr, CString};
+
+        let path = CString::new(path).unwrap();
+        let mut out = Vec::with_capacity(1024);
+
+        unsafe {
+            extern "Rust" {
+                fn miri_host_to_target_path(
+                    path: *const i8,
+                    out: *mut i8,
+                    out_size: usize,
+                ) -> usize;
+            }
+            let ret = miri_host_to_target_path(path.as_ptr(), out.as_mut_ptr(), out.capacity());
+            assert_eq!(ret, 0);
+            let out = CStr::from_ptr(out.as_ptr()).to_str().unwrap();
+            PathBuf::from(out)
+        }
+    }
+
     // CWD should be crate root.
     // We have to normalize slashes, as the env var might be set for a different target's conventions.
     let env_dir = env::current_dir().unwrap();
-    let env_dir = env_dir.to_string_lossy().replace("\\", "/");
-    let crate_dir = env::var_os("CARGO_MANIFEST_DIR").unwrap();
-    let crate_dir = crate_dir.to_string_lossy().replace("\\", "/");
+    let crate_dir = host_to_target_path(env::var("CARGO_MANIFEST_DIR").unwrap());
     assert_eq!(env_dir, crate_dir);
 
     // Make sure we can call dev-dependencies.
diff --git a/tests/pass-dep/shims/libc-fs.rs b/tests/pass-dep/shims/libc-fs.rs
index 93c0fad9c1..ba5b269f65 100644
--- a/tests/pass-dep/shims/libc-fs.rs
+++ b/tests/pass-dep/shims/libc-fs.rs
@@ -5,7 +5,7 @@
 #![feature(io_error_uncategorized)]
 
 use std::convert::TryInto;
-use std::ffi::CString;
+use std::ffi::{CStr, CString};
 use std::fs::{canonicalize, remove_dir_all, remove_file, File};
 use std::io::{Error, ErrorKind, Write};
 use std::os::unix::ffi::OsStrExt;
@@ -23,20 +23,21 @@ fn main() {
 }
 
 fn tmp() -> PathBuf {
-    std::env::var("MIRI_TEMP")
-        .map(|tmp| {
-            // MIRI_TEMP is set outside of our emulated
-            // program, so it may have path separators that don't
-            // correspond to our target platform. We normalize them here
-            // before constructing a `PathBuf`
-
-            #[cfg(windows)]
-            return PathBuf::from(tmp.replace("/", "\\"));
-
-            #[cfg(not(windows))]
-            return PathBuf::from(tmp.replace("\\", "/"));
-        })
-        .unwrap_or_else(|_| std::env::temp_dir())
+    let path = std::env::var("MIRI_TEMP")
+        .unwrap_or_else(|_| std::env::temp_dir().into_os_string().into_string().unwrap());
+    // These are host paths. We need to convert them to the target.
+    let path = CString::new(path).unwrap();
+    let mut out = Vec::with_capacity(1024);
+
+    unsafe {
+        extern "Rust" {
+            fn miri_host_to_target_path(path: *const i8, out: *mut i8, out_size: usize) -> usize;
+        }
+        let ret = miri_host_to_target_path(path.as_ptr(), out.as_mut_ptr(), out.capacity());
+        assert_eq!(ret, 0);
+        let out = CStr::from_ptr(out.as_ptr()).to_str().unwrap();
+        PathBuf::from(out)
+    }
 }
 
 /// Prepare: compute filename and make sure the file does not exist.
diff --git a/tests/pass-dep/shims/libc-misc.rs b/tests/pass-dep/shims/libc-misc.rs
index 2a4300fcd0..20e96a92c7 100644
--- a/tests/pass-dep/shims/libc-misc.rs
+++ b/tests/pass-dep/shims/libc-misc.rs
@@ -7,15 +7,23 @@ use std::os::unix::io::AsRawFd;
 use std::path::PathBuf;
 
 fn tmp() -> PathBuf {
-    std::env::var("MIRI_TEMP")
-        .map(|tmp| {
-            // MIRI_TEMP is set outside of our emulated
-            // program, so it may have path separators that don't
-            // correspond to our target platform. We normalize them here
-            // before constructing a `PathBuf`
-            return PathBuf::from(tmp.replace("\\", "/"));
-        })
-        .unwrap_or_else(|_| std::env::temp_dir())
+    use std::ffi::{CStr, CString};
+
+    let path = std::env::var("MIRI_TEMP")
+        .unwrap_or_else(|_| std::env::temp_dir().into_os_string().into_string().unwrap());
+    // These are host paths. We need to convert them to the target.
+    let path = CString::new(path).unwrap();
+    let mut out = Vec::with_capacity(1024);
+
+    unsafe {
+        extern "Rust" {
+            fn miri_host_to_target_path(path: *const i8, out: *mut i8, out_size: usize) -> usize;
+        }
+        let ret = miri_host_to_target_path(path.as_ptr(), out.as_mut_ptr(), out.capacity());
+        assert_eq!(ret, 0);
+        let out = CStr::from_ptr(out.as_ptr()).to_str().unwrap();
+        PathBuf::from(out)
+    }
 }
 
 /// Test allocating variant of `realpath`.
diff --git a/tests/pass/shims/env/current_dir.rs b/tests/pass/shims/env/current_dir.rs
index 069b462ab3..ca90912eab 100644
--- a/tests/pass/shims/env/current_dir.rs
+++ b/tests/pass/shims/env/current_dir.rs
@@ -3,8 +3,9 @@ use std::env;
 use std::io::ErrorKind;
 
 fn main() {
-    // Test that `getcwd` is available
+    // Test that `getcwd` is available and an absolute path
     let cwd = env::current_dir().unwrap();
+    assert!(cwd.is_absolute(), "cwd {:?} is not absolute", cwd);
     // Test that changing dir to `..` actually sets the current directory to the parent of `cwd`.
     // The only exception here is if `cwd` is the root directory, then changing directory must
     // keep the current directory equal to `cwd`.
diff --git a/tests/pass/shims/fs.rs b/tests/pass/shims/fs.rs
index 65cf6fe66b..a7d4800fae 100644
--- a/tests/pass/shims/fs.rs
+++ b/tests/pass/shims/fs.rs
@@ -15,6 +15,7 @@ use std::io::{Error, ErrorKind, IsTerminal, Read, Result, Seek, SeekFrom, Write}
 use std::path::{Path, PathBuf};
 
 fn main() {
+    test_path_conversion();
     test_file();
     test_file_clone();
     test_file_create_new();
@@ -30,21 +31,28 @@ fn main() {
     test_from_raw_os_error();
 }
 
+fn host_to_target_path(path: String) -> PathBuf {
+    use std::ffi::{CStr, CString};
+
+    let path = CString::new(path).unwrap();
+    let mut out = Vec::with_capacity(1024);
+
+    unsafe {
+        extern "Rust" {
+            fn miri_host_to_target_path(path: *const i8, out: *mut i8, out_size: usize) -> usize;
+        }
+        let ret = miri_host_to_target_path(path.as_ptr(), out.as_mut_ptr(), out.capacity());
+        assert_eq!(ret, 0);
+        let out = CStr::from_ptr(out.as_ptr()).to_str().unwrap();
+        PathBuf::from(out)
+    }
+}
+
 fn tmp() -> PathBuf {
-    std::env::var("MIRI_TEMP")
-        .map(|tmp| {
-            // MIRI_TEMP is set outside of our emulated
-            // program, so it may have path separators that don't
-            // correspond to our target platform. We normalize them here
-            // before constructing a `PathBuf`
-
-            #[cfg(windows)]
-            return PathBuf::from(tmp.replace("/", "\\"));
-
-            #[cfg(not(windows))]
-            return PathBuf::from(tmp.replace("\\", "/"));
-        })
-        .unwrap_or_else(|_| std::env::temp_dir())
+    let path = std::env::var("MIRI_TEMP")
+        .unwrap_or_else(|_| std::env::temp_dir().into_os_string().into_string().unwrap());
+    // These are host paths. We need to convert them to the target.
+    host_to_target_path(path)
 }
 
 /// Prepare: compute filename and make sure the file does not exist.
@@ -71,6 +79,12 @@ fn prepare_with_content(filename: &str, content: &[u8]) -> PathBuf {
     path
 }
 
+fn test_path_conversion() {
+    let tmp = tmp();
+    assert!(tmp.is_absolute(), "{:?} is not absolute", tmp);
+    assert!(tmp.is_dir(), "{:?} is not a directory", tmp);
+}
+
 fn test_file() {
     let bytes = b"Hello, World!\n";
     let path = prepare("miri_test_fs_file.txt");