hardcoresushi/libgocryptfs
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fusefrontend: -allow_other: set file mode *after* chown in Create()

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Reported by @slackner at https://github.com/rfjakob/gocryptfs/issues/327 :

Possible race-conditions between file creation and Fchownat

* Assume a system contains a gocryptfs mount as root user
  with -allow_other
* As a regular user create a new file with mode containing
  the SUID flag and write access for other users
* Before gocryptfs executes the Fchownat call, try to open
  the file again, write some exploit code to it, and try to run it.

For a short time, the file is owned by root and has the SUID flag, so
this is pretty dangerous.
This commit is contained in:
Jakob Unterwurzacher 2019-01-08 21:50:10 +01:00
parent 4170ef00f3
commit b22cc03c75
1 changed files with 20 additions and 2 deletions
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22
internal/fusefrontend/fs.go
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@ -239,6 +239,14 @@ func (fs *FS) Create(path string, flags uint32, mode uint32, context *fuse.Conte
return nil, fuse.ToStatus(err)
}
defer syscall.Close(dirfd)
// Don't set full mode before we have set the correct owner. Files with SUID/SGID
// mode belonging to the wrong owner would be a security risk. Even for other
// modes, we don't want anyone else to open the file in the meantime: the fd would
// stay open and could later be used to read the file.
origMode := mode
if fs.args.PreserveOwner {
mode = 0000
}
fd := -1
// Handle long file name
if !fs.args.PlaintextNames && nametransform.IsLongContent(cName) {
@ -253,7 +261,6 @@ func (fs *FS) Create(path string, flags uint32, mode uint32, context *fuse.Conte
nametransform.DeleteLongNameAt(dirfd, cName)
return nil, fuse.ToStatus(err)
}
} else {
// Create content, normal (short) file name
fd, err = syscallcompat.Openat(dirfd, cName, newFlags|syscall.O_CREAT|syscall.O_EXCL, mode)
@ -271,7 +278,18 @@ func (fs *FS) Create(path string, flags uint32, mode uint32, context *fuse.Conte
if fs.args.PreserveOwner {
err = syscall.Fchown(fd, int(context.Owner.Uid), int(context.Owner.Gid))
if err != nil {
tlog.Warn.Printf("Create: Fchown() failed: %v", err)
tlog.Warn.Printf("Create %q: Fchown %d:%d failed: %v", cName, context.Owner.Uid, context.Owner.Gid, err)
// In case of a failure, we don't want to proceed setting more
// permissive modes.
syscall.Close(fd)
return nil, fuse.ToStatus(err)
}
}
// Set mode
if mode != origMode {
err = syscall.Fchmod(fd, origMode)
if err != nil {
tlog.Warn.Printf("Create %q: Fchmod %#o -> %#o failed: %v", cName, mode, origMode, err)
}
}
f := os.NewFile(uintptr(fd), cName)