samiandClaude Sonnet 5
7514f5a4c4
The residual section claimed the leaf/component TOCTOU is "closed in practice by CORE's O_NOFOLLOW open of the final file". Verified: it is not — core/src/io/sparse_file.cpp:77 opens O_WRONLY|O_CREAT|O_CLOEXEC, no O_NOFOLLOW, no O_EXCL. Requested the flags from CORE via PKG/QA. Doc now states the residual is currently OPEN, names the file:line and flags checked and the date, says what actually limits exposure today (0700 parent dirs), and flags this as the boundary where a reader stops checking. Step 5 reworded the same way. Re-verify the flags when the CORE change lands. Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01Upd9WhG9oppieig5nRDLig
96 lines
8.4 KiB
Markdown
96 lines
8.4 KiB
Markdown
# `saveDir` / `filename` → filesystem destination: the adversarial table
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`veloxd` is the only process that turns an untrusted string into a place bytes get
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written. `capture.offer` means that string can originate from a web page, and
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`download.add` over the Unix socket is reachable by any same-UID process. CLAUDE.md §4
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("paths are canonicalized and checked against allowed roots before any write") and the M1
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DoD ("no path traversal in `saveDir` … → `-32011`") make this a security boundary, not a
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formatting nicety.
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This table is written **before** `fs/safepath.cpp`, the way EXT did for `shouldCapture`.
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Every row is a test in `daemon/tests/safepath_test.cpp`.
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Roots for the examples: `allowedRoots = ["/home/u/Downloads", "/data/dl"]`, already
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`realpath`-resolved and stored canonical at load time. `$HOME = /home/u`.
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| # | Input (`saveDir`, `filename`) | Attack | Required outcome |
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|---|---|---|---|
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| A1 | `/home/u/Downloads/../.ssh`, `authorized_keys` | `..` climbs out of the root | `-32011`, `data.path` = the input `saveDir`. No dir created. |
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| A2 | `/home/u/Downloads/a/b/../../../etc`, `x` | `..` chain escaping after descending | `-32011`. |
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| A3 | `/etc`, `cron.d-payload` | absolute path, simply outside every root | `-32011`. |
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| A4 | `/home/u/Downloads-evil`, `x` | prefix-match confusion with `/home/u/Downloads` | `-32011` — containment is component-wise, not `starts_with`. |
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| A5 | `/home/u/Downloads`, `../.bashrc` | `..` in the **leaf**, not the dir | leaf rejected → `-32011` (or `InvalidParams`); a leaf is one component, never a path. |
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| A6 | `/home/u/Downloads`, `sub/dir/file` | `/` in the leaf | leaf rejected — `filename` names a file, not a subpath. |
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| A7 | `/home/u/Downloads/link-out` where `link-out` → `/etc` (pre-existing symlink) | symlink component points outside a root | `realpath` resolves it to `/etc`; `-32011`. |
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| A8 | `/home/u/Downloads/goodsub`, `iso.img` — but between our check and CORE's `open`, `goodsub` is swapped for a symlink to `/etc` | **TOCTOU** on a directory component | Defense: resolve + create with `openat`/`mkdirat` from an `O_NOFOLLOW|O_DIRECTORY` fd walk, then `realpath` the final dir **again** and re-assert containment. A component that is a symlink at walk time → `-32011`. |
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| A9 | `/home/u/Downloads`, `file<NUL>.iso` (`0x00` in the leaf) | NUL truncation — the write path sees `file`, logs/UI see more; CORE's fuzzer hit exactly this via `Content-Disposition` | NUL and every `< 0x20` byte and `0x7F` stripped from the leaf before use (mirrors `core/src/net/content_disposition.cpp` `sanitize_leaf`). If the leaf is empty after stripping → reject. |
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| A10 | `/home/u/Downloads`, `"\r\nSet-Cookie: x".iso` | CR/LF injection into logs / downstream | control bytes stripped as A9. |
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| A11 | `/home/u/Downloads`, `.` / `..` / `` (empty) | degenerate leaf | rejected. |
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| A12 | `/home/u/Downloads`, `con` / `aux` / `nul` | Windows device names | **allowed** on Linux — we are not Windows; do not over-reject. (Noted so a future "harden" pass doesn't add it thinking it was missed.) |
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| A13 | `/home/u/Downloads`, `<260 chars>` | overlong leaf, `ENAMETOOLONG` at `open` | leaf capped at 255 **bytes of UTF-8**, never splitting a codepoint (docs/04 §2). |
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| A14 | `/home/u/Downloads/<260 chars>/x`, `y` | overlong directory component | `mkdirat` / `realpath` returns `ENAMETOOLONG` → mapped `-32011`, not a crash. |
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| A15 | `saveDir` empty / null | no destination given | caller substitutes `saveTo.defaultDir`; `resolve_target` itself rejects an empty dir rather than defaulting silently. |
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| A16 | root `/home/u/Downloads` is itself a symlink to `/mnt/big/dl` | a symlinked root | `canonicalize_root` `realpath`s every configured root at load; the stored root is `/mnt/big/dl`, and a `saveDir` resolving there passes. A `saveDir` of the literal `/home/u/Downloads/x` also passes because it `realpath`s to `/mnt/big/dl/x`. |
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| A17 | `/home/u/Downloads` exists as a **file**, not a directory | destination is not a directory | `-32011` (`not_a_dir`), no write attempt. |
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| A18 | `/home/u/Downloads/新しい/フォルダ`, `映画.mkv` | non-ASCII, legitimate | **succeeds** — UTF-8 is fine; only control bytes and the structural checks apply. |
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| A19 | `/home/u/Downloads/./sub/.`, `x` | redundant `.` segments, no escape | normalized away; **succeeds** at `/home/u/Downloads/sub`. |
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| A20 | `/home/u/Downloads`, ` trailing-spaces.iso ` / `dots...` | trailing space/dot (Windows-hostile, and confuses "same file" checks) | trimmed: leading/trailing whitespace and trailing dots removed before use. Empty after trim → reject. |
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| A21 | relative `saveDir` (`Downloads/x`, `./x`, `x`) | a relative path has no well-defined base and invites cwd games | rejected — `resolve_target` requires an absolute `saveDir`. The GUI/CLI resolve against the default dir before calling. |
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## Implementation (`fs/safepath.cpp`, as built)
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1. **Sanitize the leaf first**, in isolation: strip `[0x00,0x20) ∪ {0x7F}`, trim
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whitespace, strip trailing dots and spaces, reject `.`/`..`/empty/`contains '/'`, cap
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255 UTF-8 bytes on a codepoint boundary. (A5, A6, A9–A13, A20)
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2. **Require `saveDir` absolute; reject any `..` component lexically.** A legitimate
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client never sends `..`; a web-origin path with `..` is an attack, so it does not even
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reach `realpath`. (A1, A2, A21)
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3. **If the directory already exists:** `realpath(saveDir)` — this follows every symlink,
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so a symlinked root or component resolves to where it *really* points — then assert the
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resolved path is inside a canonical root, component-wise (`d == root || d starts with
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root + "/"`). A symlink that escapes is caught here (A7); one that stays inside passes
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(A16). Open the resolved dir `O_PATH|O_DIRECTORY` for the leaf check. (A3, A4, A7, A16,
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A17, A19)
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4. **If a tail is missing (`mkdir -p` case):** find the deepest existing ancestor,
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`realpath` + root-check *that*, then create the missing components through an
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`openat/mkdirat` walk with `O_NOFOLLOW|O_DIRECTORY` from the ancestor's fd — the tail
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has no symlinks because it had no entries; a race that plants one trips `ELOOP` →
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`-32011`. Then re-derive the final dir's path from its fd (`/proc/self/fd/N`) and
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re-assert containment. (A8 for the created tail, A14)
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5. **Best-effort leaf check:** `fstatat(dir_fd, leaf, AT_SYMLINK_NOFOLLOW)` — refuse if it
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is already a symlink. This narrows, but does not close, the create-after-check race on
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the leaf: a symlink planted *after* this `fstatat` and *before* CORE opens the file is
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still followed. Closing it needs CORE to open with `O_NOFOLLOW` (plus `O_EXCL` on a
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fresh download). **Verified 2026-09-10: it does not yet** —
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`core/src/io/sparse_file.cpp:77` is `O_WRONLY | O_CREAT | O_CLOEXEC`. The flag change
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has been raised with CORE; until it lands this race is open, see the residual below.
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6. **Every failure is `-32011`, `data.path` = the *original* `saveDir`** — never the
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resolved path, which would leak where the roots actually live. The one exception is a
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`filename` that violates the schema's own `maxLength`, which is `-32602` at the param
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layer before this code runs.
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### Residual — currently OPEN, tracked
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Two TOCTOU gaps this code does not close on its own:
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1. **An existing intermediate directory** swapped for an out-of-root symlink between our
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`realpath` (step 3) and the write. Step 3 trusts `realpath` for the pre-existing
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prefix; a full `O_NOFOLLOW` chase would reject the legitimate symlinked directories
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A16 requires us to allow.
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2. **The leaf** swapped for a symlink between our `fstatat` (step 5) and CORE's `open`.
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Both are closed by CORE opening the file `O_NOFOLLOW` (and, for a fresh download,
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`O_EXCL`). **As verified on 2026-09-10 that is not yet the case** —
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`core/src/io/sparse_file.cpp:77` opens `O_WRONLY | O_CREAT | O_CLOEXEC`. The flag change has
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been raised with CORE; when it lands, update step 5 and this paragraph and re-verify the
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flags at that line.
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What limits the exposure *today*: the download directory lives under `~/.local/share` /
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`~/Downloads`, both `0700` — an attacker planting a symlink there already has write access
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to the user's account. The unqualified "you are covered" version of this claim does not
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hold until the CORE change lands; this is exactly the boundary where a reader stops
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checking, so it is spelled out.
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The post-M1 hardening for gap 1 is a per-step "resolve one component, re-validate the
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running path against the roots" chase (systemd's `chase_symlinks` shape).
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