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libstore/linux: compile the seccomp BPF explicitly

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This is a preparation for precompiling the filter, which is done separately.
The behaviour should be unchanged for now.

Change-Id: I899aa7242962615949208597aca88913feba1cb8
This commit is contained in:
alois31 2024-08-06 16:38:32 +02:00
parent 741d3b441c
commit 403fa9e2b6
Signed by untrusted user: alois31
GPG key ID: E0F59EA5E5216914
1 changed files with 31 additions and 11 deletions
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42
src/libstore/platform/linux.cc
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@ -11,6 +11,7 @@
#include <sys/prctl.h>
#if HAVE_SECCOMP
#include <linux/filter.h>
#include <seccomp.h>
#endif
@ -146,11 +147,8 @@ static void allowSyscall(scmp_filter_ctx ctx, int syscall) {
seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EPERM), syscall, 1, SCMP_A##modePos(SCMP_CMP_MASKED_EQ, S_ISGID, S_ISGID)) != 0) \
throw SysError("unable to add seccomp rule");
#endif
void LinuxLocalDerivationGoal::setupSyscallFilter()
static std::vector<struct sock_filter> compileSyscallFilter()
{
#if HAVE_SECCOMP
scmp_filter_ctx ctx;
// Pretend that syscalls we don't yet know about don't exist.
@ -703,18 +701,40 @@ void LinuxLocalDerivationGoal::setupSyscallFilter()
seccomp_rule_add(ctx, SCMP_ACT_ERRNO(ENOTSUP), SCMP_SYS(fsetxattr), 0) != 0)
throw SysError("unable to add seccomp rule");
Pipe filterPipe;
filterPipe.create();
auto filterBytes_ = std::async([&]() {
return drainFD(filterPipe.readSide.get());
});
if (seccomp_export_bpf(ctx, filterPipe.writeSide.get()) != 0)
throw SysError("unable to compile seccomp BPF program");
filterPipe.writeSide.close();
auto filterBytes = filterBytes_.get();
assert(filterBytes.size() % sizeof(struct sock_filter) == 0);
std::vector<struct sock_filter> filter(filterBytes.size() / sizeof(struct sock_filter));
std::memcpy(filter.data(), filterBytes.data(), filterBytes.size());
return filter;
}
#endif
void LinuxLocalDerivationGoal::setupSyscallFilter()
{
// Set the NO_NEW_PRIVS prctl flag.
// This both makes loading seccomp filters work for unprivileged users,
// and is an additional security measure in its own right.
if (seccomp_attr_set(ctx, SCMP_FLTATR_CTL_NNP, 1) != 0)
throw SysError("unable to set 'no new privileges' seccomp attribute");
if (seccomp_load(ctx) != 0)
throw SysError("unable to load seccomp BPF program");
#else
// Still set the no-new-privileges flag if libseccomp is not available.
if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1)
throw SysError("PR_SET_NO_NEW_PRIVS failed");
#if HAVE_SECCOMP
auto seccompBPF = compileSyscallFilter();
assert(seccompBPF.size() <= std::numeric_limits<unsigned short>::max());
struct sock_fprog fprog = {
.len = static_cast<unsigned short>(seccompBPF.size()),
.filter = seccompBPF.data(),
};
if (syscall(SYS_seccomp, SECCOMP_SET_MODE_FILTER, 0, &fprog) != 0)
throw SysError("unable to load seccomp BPF program");
#endif
}