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Allow unshare(CLONE_NEWUSER|CLONE_NEWNS|CLONE_NEWUTS) syscall #4

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Allow unshare(CLONE_NEWUSER|CLONE_NEWNS|CLONE_NEWUTS) syscall #4

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28 changes: 28 additions & 0 deletions seccomp/default_linux.go
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Original file line number Diff line number Diff line change
Expand Up @@ -646,6 +646,34 @@ func DefaultProfile() *Seccomp {
Arches: []string{"s390", "s390x"},
},
},
{
LinuxSyscall: specs.LinuxSyscall{
Names: []string{
"unshare",
},
Action: specs.ActAllow,
Args: []specs.LinuxSeccompArg{
{
Index: 0,
Value: unix.CLONE_NEWNS,
ValueTwo: 0,
Op: specs.OpMaskedEqual,
},
{
Index: 0,
Value: unix.CLONE_NEWUTS,
ValueTwo: 0,
Op: specs.OpMaskedEqual,
},
{
Index: 0,
Value: unix.CLONE_NEWUSER,
ValueTwo: 0,
Op: specs.OpMaskedEqual,
},
},
},
},
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@AkihiroSuda AkihiroSuda Oct 2, 2025

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CLONE_NEWUSER does not seem covered in this PR.

Anyway, it might be still scary to allow CLONE_NEWUSER by default, due to its several vulnerabilities in the past (CVE-2023–0386, etc.)

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@adamnovak adamnovak Oct 2, 2025

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The Docker documentation lists unshare as a thing that is blocked by default, but doesn't provide an example of how an authorized user would unblock it or an argument to it for a particular workload. It just says you can pass a seccomp profile (which it seems to expect you to already be competent to create based on the default one) or you can use --security-opt seccomp=unconfined.

Given the number of people who actually know how to write a seccomp profile, and that there isn't a slightly-more-permissive one included as an available option, probably the vast majority of workloads that need user namespaces are currently running completely unconfined.

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@slonopotamus slonopotamus Oct 2, 2025

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CLONE_NEWUSER does not seem covered in this PR.

Whoops, I somehow messed up PR and did CLONE_UTS twice.

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@slonopotamus slonopotamus Oct 2, 2025 •
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Anyway, it might be still scary to allow CLONE_NEWUSER by default, due to its several vulnerabilities in the past (CVE-2023–0386, etc.)

There is a long discussion in moby/moby#42441 about safety of these. I just wanted to make a PR that (if this feature is declared safe) would implement the change.

And I said, my specific usecase is to make Buildah work. Building with Buildah inside a container will always be at least as safe (and normally much safer) than building with docker build outside of container, even if there will be a future vulnerability in unshare.

I agree with @adamnovak that writing a custom seccomp policy is too hard.

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@sbrivio-rh sbrivio-rh Oct 8, 2025

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probably the vast majority of workloads that need user namespaces are currently running completely unconfined

Or, worse yet, use custom builds of applications with sandboxing disabled (https://bugs.passt.top/show_bug.cgi?id=116#c6).

{
LinuxSyscall: specs.LinuxSyscall{
Names: []string{
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