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Create escape hatch for supplementary group sandboxing woes #8307

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Create escape hatch for supplementary group sandboxing woes #8307

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6d1aa52
Create escape hatch for supplementary group sandboxing woes
Ericson2314 May 8, 2023
2524a21
Update src/libstore/build/local-derivation-goal.cc
Ericson2314 May 15, 2023
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13 changes: 4 additions & 9 deletions src/libstore/build/local-derivation-goal.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -907,15 +907,10 @@ void LocalDerivationGoal::startBuilder()
openSlave();

/* Drop additional groups here because we can't do it
after we've created the new user namespace. FIXME:
this means that if we're not root in the parent
namespace, we can't drop additional groups; they will
be mapped to nogroup in the child namespace. There does
not seem to be a workaround for this. (But who can tell
from reading user_namespaces(7)?)
See also https://lwn.net/Articles/621612/. */
if (getuid() == 0 && setgroups(0, 0) == -1)
throw SysError("setgroups failed");
after we've created the new user namespace. */
if (settings.dropSupplementaryGroups)
if (setgroups(0, 0) == -1)
throw SysError("setgroups failed. Set the drop-supplementary-groups option to false to skip this step.");

ProcessOptions options;
options.cloneFlags = CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWIPC | CLONE_NEWUTS | CLONE_PARENT | SIGCHLD;
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19 changes: 19 additions & 0 deletions src/libstore/globals.hh
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Expand Up @@ -515,6 +515,25 @@ public:
Setting<bool> sandboxFallback{this, true, "sandbox-fallback",
"Whether to disable sandboxing when the kernel doesn't allow it."};

Setting<bool> dropSupplementaryGroups{this, getuid() == 0, "drop-supplementary-groups",
R"(
Whether to drop supplementary groups when building with sandboxing.
This is normally a good idea if we are root and have the capability to
do so.

But if this "root" is mapped from a non-root user in a larger
namespace, we won't be able drop additional groups; they will be
mapped to nogroup in the child namespace. There does not seem to be a
workaround for this.

(But who can tell from reading user_namespaces(7)? See also https://lwn.net/Articles/621612/.)

TODO: It might be good to create a middle ground option that allows
`setgroups` to fail if all additional groups are "nogroup" / the value
of `/proc/sys/fs/overflowuid`. This would handle the common
nested-sandboxing case identified above.
)"};

#if __linux__
Setting<std::string> sandboxShmSize{
this, "50%", "sandbox-dev-shm-size",
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