Re: [PATCH] fs: don't block i_writecount during exec

[Josef Bacik <josef@toxicpanda.com>]

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@kernel.org>

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef