diff --git a/CHANGELOG.md b/CHANGELOG.md
index edc75dc73..94a6ab902 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased 1.3.z]
 
+### Fixed
+ * Container processes will no longer inherit the CPU affinity of runc by
+   default. Instead, the default CPU affinity of container processes will be
+   the largest set of CPUs permitted by the container's cpuset cgroup and any
+   other system restrictions (such as isolated CPUs). (#4041, #4815, #4858)
+
 ## [1.3.0] - 2025-04-30
 
 > Mr. President, we must not allow a mine shaft gap!
diff --git a/libcontainer/process_linux.go b/libcontainer/process_linux.go
index a9354e516..cc50f278e 100644
--- a/libcontainer/process_linux.go
+++ b/libcontainer/process_linux.go
@@ -163,6 +163,46 @@ type setnsProcess struct {
 	initProcessPid  int
 }
 
+// tryResetCPUAffinity tries to reset the CPU affinity of the process
+// identified by pid to include all possible CPUs (notwithstanding cgroup
+// cpuset restrictions and isolated CPUs).
+func tryResetCPUAffinity(pid int) {
+	// When resetting the CPU affinity, we want to match the configured cgroup
+	// cpuset (or the default set of all CPUs, if no cpuset is configured)
+	// rather than some more restrictive affinity we were spawned in (such as
+	// one that may have been inherited from systemd). The cpuset cgroup used
+	// to reconfigure the cpumask automatically for joining processes, but
+	// kcommit da019032819a ("sched: Enforce user requested affinity") changed
+	// this behaviour in Linux 6.2.
+	//
+	// Parsing cpuset.cpus.effective is quite inefficient (and looking at
+	// things like /proc/stat would be wrong for most nested containers), but
+	// luckily sched_setaffinity(2) will implicitly:
+	//
+	//  * Clamp the cpumask so that it matches the current number of CPUs on
+	//    the system.
+	//  * Mask out any CPUs that are not a member of the target task's
+	//    configured cgroup cpuset.
+	//
+	// So we can just pass a very large array of set cpumask bits and the
+	// kernel will silently convert that to the correct value very cheaply.
+
+	// Ideally, we would just set the array to 0xFF...FF. Unfortunately, the
+	// size depends on the architecture. It is also a private newtype, so we
+	// can't use (^0) or generics since those require us to be able to name the
+	// type. However, we can just underflow the zero value instead.
+	// TODO: Once <https://golang.org/cl/698015> is merged, switch to that.
+	cpuset := unix.CPUSet{}
+	for i := range cpuset {
+		cpuset[i]-- // underflow to 0xFF..FF
+	}
+	if err := unix.SchedSetaffinity(pid, &cpuset); err != nil {
+		logrus.WithError(
+			os.NewSyscallError("sched_setaffinity", err),
+		).Warnf("resetting the CPU affinity of pid %d failed -- the container process may inherit runc's CPU affinity", pid)
+	}
+}
+
 // Starts setns process with specified initial CPU affinity.
 func (p *setnsProcess) startWithCPUAffinity() error {
 	aff := p.config.CPUAffinity
@@ -193,7 +233,13 @@ func (p *setnsProcess) startWithCPUAffinity() error {
 
 func (p *setnsProcess) setFinalCPUAffinity() error {
 	aff := p.config.CPUAffinity
-	if aff == nil || aff.Final == nil {
+	// If there was no affinity configured at all, we want to reset
+	// the affinity to make sure we don't inherit an unexpected one.
+	if aff == nil || aff.Final == nil && aff.Initial == nil {
+		tryResetCPUAffinity(p.pid())
+		return nil
+	}
+	if aff.Final == nil {
 		return nil
 	}
 	if err := unix.SchedSetaffinity(p.pid(), aff.Final); err != nil {
@@ -619,6 +665,9 @@ func (p *initProcess) start() (retErr error) {
 			return fmt.Errorf("unable to apply cgroup configuration: %w", err)
 		}
 	}
+	// Reset the CPU affinity after cgroups are configured to make sure it
+	// matches any configured cpuset.
+	tryResetCPUAffinity(p.pid())
 	if p.intelRdtManager != nil {
 		if err := p.intelRdtManager.Apply(p.pid()); err != nil {
 			return fmt.Errorf("unable to apply Intel RDT configuration: %w", err)
diff --git a/tests/integration/cpu_affinity.bats b/tests/integration/cpu_affinity.bats
index f6adfa2ae..1b4958a47 100644
--- a/tests/integration/cpu_affinity.bats
+++ b/tests/integration/cpu_affinity.bats
@@ -4,9 +4,14 @@
 
 load helpers
 
+INITIAL_CPU_MASK="$(grep -F Cpus_allowed_list: /proc/self/status | awk '{ print $2 }')"
+
 function setup() {
 	requires smp cgroups_cpuset
 	setup_busybox
+
+	echo "Initial CPU mask: $INITIAL_CPU_MASK" >&2
+	echo "---" >&2
 }
 
 function teardown() {
@@ -99,3 +104,107 @@ function cpus_to_mask() {
 	[[ "$output" == *"nsexec"*": affinity: $mask"* ]]
 	[[ "$output" == *"Cpus_allowed_list:	$final"* ]] # Mind the literal tab.
 }
+
+@test "runc run [CPU affinity should reset]" {
+	# We need to use RUNC_CMDLINE since taskset requires a proper binary, not a
+	# bash function (which is what runc and __runc are).
+	setup_runc_cmdline
+
+	first="$(first_cpu)"
+
+	# Running without cpuset should result in an affinity for all CPUs.
+	update_config '.process.args = [ "/bin/grep", "-F", "Cpus_allowed_list:", "/proc/self/status" ]'
+	update_config 'del(.linux.resources.cpu)'
+	sane_run taskset -c "$first" "${RUNC_CMDLINE[@]}" run ctr
+	[ "$status" -eq 0 ]
+	[[ "$output" != $'Cpus_allowed_list:\t'"$first" ]]
+	[[ "$output" == $'Cpus_allowed_list:\t'"$INITIAL_CPU_MASK" ]]
+}
+
+@test "runc run [CPU affinity should reset to cgroup cpuset]" {
+	[ $EUID -ne 0 ] && requires rootless_cgroup
+	set_cgroups_path
+
+	# We need to use RUNC_CMDLINE since taskset requires a proper binary, not a
+	# bash function (which is what runc and __runc are).
+	setup_runc_cmdline
+
+	first="$(first_cpu)"
+	second="$((first + 1))" # Hacky; might not work in all environments.
+
+	# Running with a cpuset should result in an affinity that matches.
+	update_config '.process.args = [ "/bin/grep", "-F", "Cpus_allowed_list:", "/proc/self/status" ]'
+	update_config '.linux.resources.cpu = {"mems": "0", "cpus": "'"$first-$second"'"}'
+	sane_run taskset -c "$first" "${RUNC_CMDLINE[@]}" run ctr
+	[ "$status" -eq 0 ]
+	[[ "$output" != $'Cpus_allowed_list:\t'"$first" ]]
+	# XXX: For some reason, systemd-cgroup leads to us using the all-set
+	#      cpumask rather than the cpuset we configured?
+	[ -v RUNC_USE_SYSTEMD ] || [[ "$output" == $'Cpus_allowed_list:\t'"$first-$second" ]]
+
+	# Ditto for a cpuset that has no overlap with the original cpumask.
+	update_config '.linux.resources.cpu = {"mems": "0", "cpus": "'"$second"'"}'
+	sane_run taskset -c "$first" "${RUNC_CMDLINE[@]}" run ctr
+	[ "$status" -eq 0 ]
+	[[ "$output" != $'Cpus_allowed_list:\t'"$first" ]]
+	# XXX: For some reason, systemd-cgroup leads to us using the all-set
+	#      cpumask rather than the cpuset we configured?
+	[ -v RUNC_USE_SYSTEMD ] || [[ "$output" == $'Cpus_allowed_list:\t'"$second" ]]
+}
+
+@test "runc exec [default CPU affinity should reset]" {
+	# We need to use RUNC_CMDLINE since taskset requires a proper binary, not a
+	# bash function (which is what runc and __runc are).
+	setup_runc_cmdline
+
+	first="$(first_cpu)"
+
+	# Running without cpuset should result in an affinity for all CPUs.
+	update_config '.process.args = [ "/bin/sleep", "infinity" ]'
+	update_config 'del(.linux.resources.cpu)'
+	sane_run taskset -c "$first" "${RUNC_CMDLINE[@]}" run -d --console-socket "$CONSOLE_SOCKET" ctr3
+	[ "$status" -eq 0 ]
+	sane_run taskset -c "$first" "${RUNC_CMDLINE[@]}" exec ctr3 grep -F Cpus_allowed_list: /proc/self/status
+	[ "$status" -eq 0 ]
+	[[ "$output" != $'Cpus_allowed_list:\t'"$first" ]]
+	[[ "$output" == $'Cpus_allowed_list:\t'"$INITIAL_CPU_MASK" ]]
+}
+
+@test "runc exec [default CPU affinity should reset to cgroup cpuset]" {
+	[ $EUID -ne 0 ] && requires rootless_cgroup
+	set_cgroups_path
+
+	# We need to use RUNC_CMDLINE since taskset requires a proper binary, not a
+	# bash function (which is what runc and __runc are).
+	setup_runc_cmdline
+
+	first="$(first_cpu)"
+	second="$((first + 1))" # Hacky; might not work in all environments.
+
+	# Running with a cpuset should result in an affinity that matches.
+	update_config '.process.args = [ "/bin/sleep", "infinity" ]'
+	update_config '.linux.resources.cpu = {"mems": "0", "cpus": "'"$first-$second"'"}'
+	sane_run taskset -c "$first" "${RUNC_CMDLINE[@]}" run -d --console-socket "$CONSOLE_SOCKET" ctr
+	[ "$status" -eq 0 ]
+	sane_run taskset -c "$first" "${RUNC_CMDLINE[@]}" exec ctr grep -F Cpus_allowed_list: /proc/self/status
+	[ "$status" -eq 0 ]
+	[[ "$output" != $'Cpus_allowed_list:\t'"$first" ]]
+	# XXX: For some reason, systemd-cgroup leads to us using the all-set
+	#      cpumask rather than the cpuset we configured?
+	[ -v RUNC_USE_SYSTEMD ] || [[ "$output" == $'Cpus_allowed_list:\t'"$first-$second" ]]
+
+	# Stop the container so we can reconfigure it.
+	runc delete -f ctr
+	[ "$status" -eq 0 ]
+
+	# Ditto for a cpuset that has no overlap with the original cpumask.
+	update_config '.linux.resources.cpu = {"mems": "0", "cpus": "'"$second"'"}'
+	sane_run taskset -c "$first" "${RUNC_CMDLINE[@]}" run -d --console-socket "$CONSOLE_SOCKET" ctr
+	[ "$status" -eq 0 ]
+	sane_run taskset -c "$first" "${RUNC_CMDLINE[@]}" exec ctr grep -F Cpus_allowed_list: /proc/self/status
+	[ "$status" -eq 0 ]
+	[[ "$output" != $'Cpus_allowed_list:\t'"$first" ]]
+	# XXX: For some reason, systemd-cgroup leads to us using the all-set
+	#      cpumask rather than the cpuset we configured?
+	[ -v RUNC_USE_SYSTEMD ] || [[ "$output" == $'Cpus_allowed_list:\t'"$second" ]]
+}