archive: cache parent directory fd during tar extraction#26
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The Unpack and UnpackLayer functions validated tar entry paths using a pure string check (filepath.Join + filepath.Rel). This does not follow symlinks, so a malicious archive could plant a symlink pointing outside the extraction root and then write files through it, bypassing the check. On Linux this is mitigated by chrootarchive wrapping extraction in a chroot(2) call. On platforms without chroot support (Windows) and for callers that bypass chrootarchive (e.g. BuildKit ADD --unpack), the extraction root is not enforced. Introduce safeResolve (ported from containerd/continuity/fs.RootPath), which walks each path component with os.Lstat and resolves symlinks within the extraction root, bounding absolute targets and relative targets that would escape root back inside it. Apply safeResolve to: - Unpack: main path computation and the deferred directory chtimes loop - UnpackLayer: same - createTarFile TypeLink: hardlink target resolution Add a regression test for the symlink-chain bypass: a within-dest symlink (go_up -> "..") used to redirect a second symlink outside dest (escape -> "../victim"), which the static check missed. Fixes ART-225. Signed-off-by: Cesar Talledo <cesar.talledo@docker.com>
os.Root, used to bound all tar extraction operations within the extraction root at the kernel level using openat(2) semantics, was introduced in Go 1.24. Update go.mod and the CI test matrix to require 1.24 as the oldest supported version. Signed-off-by: Cesar Talledo <cesar.talledo@docker.com>
Replace the path-string-based extraction with os.Root, which bounds all file operations within the extraction root at the kernel level using openat(2) semantics. This is the primary defence against tar path-traversal attacks (ART-226). Key changes: - Unpack and UnpackLayer open an os.Root for dest at entry. - createTarFile takes *os.Root and a root-relative name instead of an absolute path and extractDir. - TypeDir, TypeReg, TypeLink, Lchown, Chmod, and Chtimes all go through root.* methods. - For operations os.Root does not yet support (mknod, xattrs, lchtimes on symlinks), absPath is derived via safeResolve so paths remain bounded within the root. - overlayWhiteoutConverter.ConvertRead makes direct unix.Setxattr and unix.Mknod syscalls and requires an absolute path; pass safeResolve'd absPath rather than the root-relative hdr.Name. - safepath.go is retained: safeResolve is still used for absPath derivation. - testBreakout updated to skip symlinks: correct under the os.Root model, where symlink nodes may have out-of-root targets but traversal through them via root.* is rejected by the kernel. Signed-off-by: Cesar Talledo <cesar.talledo@docker.com>
Each root.*(path) call re-walks every path component via doInRoot, costing 2D syscalls per call at path depth D. A typical file entry triggers ~5 root.* calls, so at depth 4 that is ~40 syscalls in path traversal alone. Add a dirCache that keeps one parent-directory fd open between entries. Consecutive entries in the same directory reuse the cached fd for all *at(2) operations, reducing path-traversal cost to ~5 syscalls per entry regardless of depth. Signed-off-by: Cesar Talledo <cesar.talledo@docker.com>
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Summary
Depends on #25.
Each
root.*(path)call re-walks every path component viadoInRoot, costing 2D syscalls per call at path depth D. A typical file entry triggers ~5root.*calls, so at depth 4 that is ~40 syscalls in path traversal alone.This PR adds a
dirCachethat keeps one parent-directory fd open between entries. Consecutive entries in the same directory reuse the cached fd for all*at(2)operations, reducing path-traversal cost to ~5 syscalls per entry regardless of depth.Test plan
go test ./...passes