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Filipe Casal
41f411d6e9
A checked receiver-side directory option whose leaf does not exist can
be created as attacker-controlled transfer content and then consumed by
the same transfer: --backup-dir=a, with "a" arriving in-band as a
symlink pointing out of the restricted directory. The same shape works
against --copy-dest, where it reads an outside file and delivers it to
the client. Passing the leaf under the parent's /proc/self/fd pin is
not enough: the peer's symlink wins the race to the name, and rsync
creates or reads through it.
The answer is to hand rsync an inode rather than a name -- but what
inode depends on what rsync does with the option, so the policy is per
option rather than per type ("type 2" means "check when receiving", not
"is a directory"):
--backup-dir, --partial-dir rsync creates them on demand. rrsync
creates them instead, walking down from
the restricted dir one component at a
time with O_NOFOLLOW (nested names too:
make_bak_dir() builds a hierarchy), and
pins the result. The partial dir is
made 0700, as rsync makes it.
--temp-dir rsync requires it to exist, so a missing
one stays an error.
--link-dest, --compare-dest, rsync only reads through these, and a
--copy-dest missing one is the ordinary first-run
case that must keep working. rrsync
pins an empty directory it then unlinks:
the transfer behaves as with a missing
one, and there is no name left for the
peer to take over. Not quite identical:
rsync prints "--link-dest arg does not
exist" for a genuinely missing basis and
the placeholder suppresses that.
If the transfer later replaces a created name, the held inode is merely
detached -- the backup fails, it does not escape.
Without /proc/self/fd there is no way to name an inode, so on those
platforms every missing type-2 option path is refused instead -- the six
above, not the type-3 paths covered at the end.
That is the same fail-closed behaviour this change originally had
everywhere; the pinning is what buys back first use where it can.
An earlier version refused every missing type-2 leaf on every platform.
That closed the pivot but broke first-use --backup-dir and --partial-dir
-- including --partial-dir=.rsync-partial, the documented resumable-
upload idiom -- and would have broken first-run --link-dest, which is
how every rotating-snapshot script starts.
The regressions run their security assertions BEFORE their controls, so
an environment where a control fails for an unrelated reason cannot mask
the escape check by aborting first -- which is exactly what happened to
one reviewer. The alt-dest one also has to beat a race: the generator
runs ahead of the receiver, so sorting the pivot symlink first does not
guarantee it is installed before the basis lookup, and a run where the
generator won would pass vacuously. A few thousand files in between
give the receiver the head start, and the test asserts the symlink was
really installed rather than trusting the ordering. Measured 5/5 RED on
the parent, 3/3 GREEN here.
The regressions cover each branch of the policy: the backup-dir pivot,
the --copy-dest read escape (its own test), first-use --backup-dir,
nested first-use --backup-dir, first-use --partial-dir including its
mode, a refused missing --temp-dir, an accepted first-run --link-dest
with no placeholder left behind, and an existing basis still working.
Each requires the specific mechanism rather than just "the outside file
was left alone", which any unrelated failure would satisfy, and each
takes the refusal branch where the pin primitive is unavailable.
Behaviour worth knowing about, since rrsync now creates these rather
than rsync: they are created while the ARGUMENTS are parsed, so they
appear even under --dry-run, where rsync's own make_path() deliberately
does not mkdir; and one is left behind if the transfer then fails. A
pinned --partial-dir is also one directory rather than one per
destination directory, and rsync will not auto-remove it -- which is not
new, rrsync already rewrote an existing relative partial dir to its pin.
A nested --partial-dir now behaves differently from plain rsync, which
documents creating "just the last directory -- not the whole path". With
--partial-dir=a/b and "a" missing, rsync creates nothing and rrsync
creates both. Containment is unaffected -- each component is made
beneath the fd already held, O_NOFOLLOW -- but rrsync accepts a shape
rsync would not honour, and since rsync only removes the last component
of a relative partial dir, an empty parent is left behind.
Not fixed here, and NOT claimed to be: this covers the six type-2
directory options. --files-from, --log-file and receiver positional
paths are type 3; where the leaf and its parent are both missing, or on
a platform without /proc/self/fd, those still reach rsync unpinned, as
SECURITY.md describes. That is the rest of the issue, not this one.
The wrapper is handed a shim rather than RSYNC directly: RSYNC is a
multi-word command whenever the runner forces --protocol=N, and rrsync
execlp()s its RSYNC as a single executable name, so this test died
before reaching the policy under test. A fleet run caught it on the
protocol columns of three targets.