On Fri, May 31, 2024 at 03:01:43PM +0200, Christian Brauner wrote: > Back in 2021 we already discussed removing deny_write_access() for > executables. Back then I was hesistant because I thought that this might > cause issues in userspace. But even back then I had started taking some > notes on what could potentially depend on this and I didn't come up with > a lot so I've changed my mind and I would like to try this. > > Here are some of the notes that I took: > > (1) The deny_write_access() mechanism is causing really pointless issues > such as [1]. If a thread in a thread-group opens a file writable, > then writes some stuff, then closing the file descriptor and then > calling execve() they can fail the execve() with ETXTBUSY because > another thread in the thread-group could have concurrently called > fork(). Multi-threaded libraries such as go suffer from this. > > (2) There are userspace attacks that rely on overwriting the binary of a > running process. These attacks are _mitigated_ but _not at all > prevented_ from ocurring by the deny_write_access() mechanism. > > I'll go over some details. The clearest example of such attacks was > the attack against runC in CVE-2019-5736 (cf. [3]). > > An attack could compromise the runC host binary from inside a > _privileged_ runC container. The malicious binary could then be used > to take over the host. > > (It is crucial to note that this attack is _not_ possible with > unprivileged containers. IOW, the setup here is already insecure.) > > The attack can be made when attaching to a running container or when > starting a container running a specially crafted image. For example, > when runC attaches to a container the attacker can trick it into > executing itself. > > This could be done by replacing the target binary inside the > container with a custom binary pointing back at the runC binary > itself. As an example, if the target binary was /bin/bash, this > could be replaced with an executable script specifying the > interpreter path #!/proc/self/exe. > > As such when /bin/bash is executed inside the container, instead the > target of /proc/self/exe will be executed. That magic link will > point to the runc binary on the host. The attacker can then proceed > to write to the target of /proc/self/exe to try and overwrite the > runC binary on the host. > > However, this will not succeed because of deny_write_access(). Now, > one might think that this would prevent the attack but it doesn't. > > To overcome this, the attacker has multiple ways: > * Open a file descriptor to /proc/self/exe using the O_PATH flag and > then proceed to reopen the binary as O_WRONLY through > /proc/self/fd/<nr> and try to write to it in a busy loop from a > separate process. Ultimately it will succeed when the runC binary > exits. After this the runC binary is compromised and can be used > to attack other containers or the host itself. > * Use a malicious shared library annotating a function in there with > the constructor attribute making the malicious function run as an > initializor. The malicious library will then open /proc/self/exe > for creating a new entry under /proc/self/fd/<nr>. It'll then call > exec to a) force runC to exit and b) hand the file descriptor off > to a program that then reopens /proc/self/fd/<nr> for writing > (which is now possible because runC has exited) and overwriting > that binary. > > To sum up: the deny_write_access() mechanism doesn't prevent such > attacks in insecure setups. It just makes them minimally harder. > That's all. > > The only way back then to prevent this is to create a temporary copy > of the calling binary itself when it starts or attaches to > containers. So what I did back then for LXC (and Aleksa for runC) > was to create an anonymous, in-memory file using the memfd_create() > system call and to copy itself into the temporary in-memory file, > which is then sealed to prevent further modifications. This sealed, > in-memory file copy is then executed instead of the original on-disk > binary. > > Any compromising write operations from a privileged container to the > host binary will then write to the temporary in-memory binary and > not to the host binary on-disk, preserving the integrity of the host > binary. Also as the temporary, in-memory binary is sealed, writes to > this will also fail. > > The point is that deny_write_access() is uselss to prevent these > attacks. > > (3) Denying write access to an inode because it's currently used in an > exec path could easily be done on an LSM level. It might need an > additional hook but that should be about it. > > (4) The MAP_DENYWRITE flag for mmap() has been deprecated a long time > ago so while we do protect the main executable the bigger portion of > the things you'd think need protecting such as the shared libraries > aren't. IOW, we let anyone happily overwrite shared libraries. > > (5) We removed all remaining uses of VM_DENYWRITE in [2]. That means: > (5.1) We removed the legacy uselib() protection for preventing > overwriting of shared libraries. Nobody cared in 3 years. > (5.2) We allow write access to the elf interpreter after exec > completed treating it on a par with shared libraries. > > Yes, someone in userspace could potentially be relying on this. It's not > completely out of the realm of possibility but let's find out if that's > actually the case and not guess. > > Link: https://github.com/golang/go/issues/22315 [1] > Link: 49624efa65ac ("Merge tag 'denywrite-for-5.15' of git://github.com/davidhildenbrand/linux") [2] > Link: https://unit42.paloaltonetworks.com/breaking-docker-via-runc-explaining-cve-2019-5736 [3] > Link: https://lwn.net/Articles/866493 > Link: https://github.com/golang/go/issues/22220 > Link: https://github.com/golang/go/blob/5bf8c0cf09ee5c7e5a37ab90afcce154ab716a97/src/cmd/go/internal/work/buildid.go#L724 > Link: https://github.com/golang/go/blob/5bf8c0cf09ee5c7e5a37ab90afcce154ab716a97/src/cmd/go/internal/work/exec.go#L1493 > Link: https://github.com/golang/go/blob/5bf8c0cf09ee5c7e5a37ab90afcce154ab716a97/src/cmd/go/internal/script/cmds.go#L457 > Link: https://github.com/golang/go/blob/5bf8c0cf09ee5c7e5a37ab90afcce154ab716a97/src/cmd/go/internal/test/test.go#L1557 > Link: https://github.com/golang/go/blob/5bf8c0cf09ee5c7e5a37ab90afcce154ab716a97/src/os/exec/lp_linux_test.go#L61 > Link: https://github.com/buildkite/agent/pull/2736 > Link: https://github.com/rust-lang/rust/issues/114554 > Link: https://bugs.openjdk.org/browse/JDK-8068370 > Link: https://github.com/dotnet/runtime/issues/58964 > Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx> Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx> Thanks, Josef
