set_cpus_allowed_common() passes a remote task's p->mm to mm_update_cpus_allowed() while holding p->pi_lock, but that lock did not stabilize the pointer against MM replacement. A same-owner sched_setaffinity() caller can therefore retain the old pointer while exec replaces and frees it. On architectures which shoot down rather than refcount lazy TLB MMs, exit_mm() and kthread_unuse_mm() have the same lifetime gap. BUG: KASAN: slab-use-after-free in set_cpus_allowed_common Read of size 4 at addr ffff888123155e4c by task poc/5132 Call Trace: set_cpus_allowed_common (kernel/sched/core.c:10835) set_cpus_allowed_fair (kernel/sched/fair.c:10046) __set_cpus_allowed_ptr_locked (kernel/sched/core.c:3224) __set_cpus_allowed_ptr (kernel/sched/core.c:3258) __sched_setaffinity (kernel/sched/syscalls.c:1161) sched_setaffinity (kernel/sched/syscalls.c:1239) __x64_sys_sched_setaffinity (kernel/sched/syscalls.c:1275) do_syscall_64 (arch/x86/entry/syscall_64.c:84) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121) Freed by task 5127: kmem_cache_free (mm/slub.c:6757) __mmput (kernel/fork.c:1204) mmput (kernel/fork.c:1215) setup_new_exec (fs/exec.c:1395) load_elf_binary (fs/binfmt_elf.c:1024) bprm_execve (fs/exec.c:1865) do_execveat_common.isra.0 (fs/exec.c:1963) __x64_sys_execveat (fs/exec.c:2047) do_syscall_64 (arch/x86/entry/syscall_64.c:84) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121) Kernel panic - not syncing: KASAN: panic_on_warn set ... Affinity paths hold p->pi_lock across the p->mm load and its complete use. After publishing a replacement or NULL p->mm, take and release that same lock before dropping the old MM reference. For an updater that is already using the old pointer, the ordering is: affinity updater MM teardown ---------------- ----------- raw_spin_lock(&p->pi_lock) mm = p->mm /* old_mm */ p->mm = new_mm or NULL raw_spin_lock(&p->pi_lock) /* waits */ mm_update_cpus_allowed(mm, mask) raw_spin_unlock(&p->pi_lock) /* release */ /* acquire succeeds */ raw_spin_unlock(&p->pi_lock) ... mmput/mmdrop(old_mm) The teardown-side acquire cannot complete until an updater ordered before it has released p->pi_lock, so old_mm remains alive throughout that updater's access. If teardown is ordered first instead, its unlock is a release that orders the preceding p->mm store before a later updater's lock acquire. The later updater therefore observes new_mm or NULL. An updater that acquired the lock before the store but has not loaded p->mm yet is still ordered before the teardown-side acquisition; it may observe either pointer, but teardown waits for its complete use before dropping old_mm. All runtime callers of set_cpus_allowed_common() hold p->pi_lock across this access. The runqueue lock is not part of the p->mm lifetime ordering, so the grace period only needs p->pi_lock. Place it after publication and before the old reference drop in exec_mmap(), exit_mm(), and kthread_unuse_mm(). The last two paths matter on architectures which shoot down rather than refcount lazy TLB MMs. This change was developed with assistance from an LLM. Fixes: 7e019dcc470f ("sched: Improve cache locality of RSEQ concurrency IDs for intermittent workloads") Cc: stable@vger.kernel.org Reported-by: Signed-off-by: Weiming Shi --- Changes in v3: - Document the p->pi_lock grace period and acquire/release ordering. - Use p->pi_lock directly and cover exit_mm() and kthread_unuse_mm(). - Refresh the KASAN trace and provide a standalone reproducer in reply. Changes in v2: - Replace the schedule-in deferral with an exec-side runqueue-lock barrier. - Keep affinity expansions synchronous, including expansions narrowed before the next schedule-in. - Avoid changes to the MM-CID fork, exit, and scheduling state machine. fs/exec.c | 1 + include/linux/sched.h | 2 ++ kernel/exit.c | 1 + kernel/kthread.c | 1 + kernel/sched/core.c | 9 +++++++++ 5 files changed, 14 insertions(+) diff --git a/fs/exec.c b/fs/exec.c index 8d2b14e1d455..3196421cb81d 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -905,6 +905,7 @@ static int exec_mmap(struct linux_binprm *bprm) BUG_ON(active_mm != old_mm); /* Defer teardown to setup_new_exec(), outside the exec locks. */ bprm->old_mm = old_mm; + sched_mm_cid_mm_barrier(tsk); } else { mmdrop_lazy_tlb(active_mm); } diff --git a/include/linux/sched.h b/include/linux/sched.h index 1c93da75226e..f88ab0445be3 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -2358,6 +2358,7 @@ static __always_inline void alloc_tag_restore(struct alloc_tag *tag, struct allo #ifdef CONFIG_SCHED_MM_CID void sched_mm_cid_before_execve(struct task_struct *t); void sched_mm_cid_after_execve(struct task_struct *t); +void sched_mm_cid_mm_barrier(struct task_struct *t); void sched_mm_cid_exit(struct task_struct *t); static __always_inline int task_mm_cid(struct task_struct *t) { @@ -2366,6 +2367,7 @@ static __always_inline int task_mm_cid(struct task_struct *t) #else static inline void sched_mm_cid_before_execve(struct task_struct *t) { } static inline void sched_mm_cid_after_execve(struct task_struct *t) { } +static inline void sched_mm_cid_mm_barrier(struct task_struct *t) { } static inline void sched_mm_cid_exit(struct task_struct *t) { } static __always_inline int task_mm_cid(struct task_struct *t) { diff --git a/kernel/exit.c b/kernel/exit.c index d5e7bc2af804..5f9979d6216c 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -582,6 +582,7 @@ static void exit_mm(void) local_irq_enable(); task_unlock(current); mmap_read_unlock(mm); + sched_mm_cid_mm_barrier(current); mm_update_next_owner(mm); mmput(mm); if (test_thread_flag(TIF_MEMDIE)) diff --git a/kernel/kthread.c b/kernel/kthread.c index fdb8fb07f40b..e7b393cfdcee 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -1682,6 +1682,7 @@ void kthread_unuse_mm(struct mm_struct *mm) enter_lazy_tlb(mm, tsk); local_irq_enable(); task_unlock(tsk); + sched_mm_cid_mm_barrier(tsk); mmdrop(mm); } diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 04b62c16fa8c..d78a174b5405 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -11128,6 +11128,15 @@ void sched_mm_cid_after_execve(struct task_struct *t) sched_mm_cid_fork(t); } +/* + * Pair with set_cpus_allowed_common()'s p->pi_lock critical section. Call + * after publishing p->mm and before dropping the old MM reference. + */ +void sched_mm_cid_mm_barrier(struct task_struct *t) +{ + guard(raw_spinlock_irq)(&t->pi_lock); +} + static void mm_cid_work_fn(struct work_struct *work) { struct mm_struct *mm = container_of(work, struct mm_struct, mm_cid.work); -- 2.56.0