wings/server/filesystem/path.go

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package filesystem
import (
"context"
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iofs "io/fs"
"os"
"path/filepath"
"strings"
"sync"
"emperror.dev/errors"
"golang.org/x/sync/errgroup"
)
// Checks if the given file or path is in the server's file denylist. If so, an Error
// is returned, otherwise nil is returned.
func (fs *Filesystem) IsIgnored(paths ...string) error {
for _, p := range paths {
sp, err := fs.SafePath(p)
if err != nil {
return err
}
if fs.denylist.MatchesPath(sp) {
return errors.WithStack(&Error{code: ErrCodeDenylistFile, path: p, resolved: sp})
}
}
return nil
}
// Normalizes a directory being passed in to ensure the user is not able to escape
// from their data directory. After normalization if the directory is still within their home
// path it is returned. If they managed to "escape" an error will be returned.
//
// This logic is actually copied over from the SFTP server code. Ideally that eventually
// either gets ported into this application, or is able to make use of this package.
func (fs *Filesystem) SafePath(p string) (string, error) {
// Start with a cleaned up path before checking the more complex bits.
r := fs.unsafeFilePath(p)
// At the same time, evaluate the symlink status and determine where this file or folder
// is truly pointing to.
ep, err := filepath.EvalSymlinks(r)
if err != nil && !os.IsNotExist(err) {
return "", errors.Wrap(err, "server/filesystem: failed to evaluate symlink")
} else if os.IsNotExist(err) {
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// The target of one of the symlinks (EvalSymlinks is recursive) does not exist.
// So we get what target path does not exist and check if it's within the data
// directory. If it is, we return the original path, otherwise we return an error.
pErr, ok := err.(*iofs.PathError)
if !ok {
return "", errors.Wrap(err, "server/filesystem: failed to evaluate symlink")
}
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ep = pErr.Path
}
// If the requested directory from EvalSymlinks begins with the server root directory go
// ahead and return it. If not we'll return an error which will block any further action
// on the file.
if fs.unsafeIsInDataDirectory(ep) {
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// Returning the original path here instead of the resolved path ensures that
// whatever the user is trying to do will work as expected. If we returned the
// resolved path, the user would be unable to know that it is in fact a symlink.
return r, nil
}
return "", NewBadPathResolution(p, r)
}
// Generate a path to the file by cleaning it up and appending the root server path to it. This
// DOES NOT guarantee that the file resolves within the server data directory. You'll want to use
// the fs.unsafeIsInDataDirectory(p) function to confirm.
func (fs *Filesystem) unsafeFilePath(p string) string {
// Calling filepath.Clean on the joined directory will resolve it to the absolute path,
// removing any ../ type of resolution arguments, and leaving us with a direct path link.
//
// This will also trim the existing root path off the beginning of the path passed to
// the function since that can get a bit messy.
return filepath.Clean(filepath.Join(fs.Path(), strings.TrimPrefix(p, fs.Path())))
}
// Check that that path string starts with the server data directory path. This function DOES NOT
// validate that the rest of the path does not end up resolving out of this directory, or that the
// targeted file or folder is not a symlink doing the same thing.
func (fs *Filesystem) unsafeIsInDataDirectory(p string) bool {
return strings.HasPrefix(strings.TrimSuffix(p, "/")+"/", strings.TrimSuffix(fs.Path(), "/")+"/")
}
// Executes the fs.SafePath function in parallel against an array of paths. If any of the calls
// fails an error will be returned.
func (fs *Filesystem) ParallelSafePath(paths []string) ([]string, error) {
var cleaned []string
// Simple locker function to avoid racy appends to the array of cleaned paths.
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m := new(sync.Mutex)
push := func(c string) {
m.Lock()
cleaned = append(cleaned, c)
m.Unlock()
}
// Create an error group that we can use to run processes in parallel while retaining
// the ability to cancel the entire process immediately should any of it fail.
g, ctx := errgroup.WithContext(context.Background())
// Iterate over all of the paths and generate a cleaned path, if there is an error for any
// of the files, abort the process.
for _, p := range paths {
// Create copy so we can use it within the goroutine correctly.
pi := p
// Recursively call this function to continue digging through the directory tree within
// a separate goroutine. If the context is canceled abort this process.
g.Go(func() error {
select {
case <-ctx.Done():
return ctx.Err()
default:
// If the callback returns true, go ahead and keep walking deeper. This allows
// us to programmatically continue deeper into directories, or stop digging
// if that pathway knows it needs nothing else.
if c, err := fs.SafePath(pi); err != nil {
return err
} else {
push(c)
}
return nil
}
})
}
// Block until all of the routines finish and have returned a value.
return cleaned, g.Wait()
}