Mel Gorman 36b038791e x86/fpu: Drop fpregs lock before inheriting FPU permissions ... Mike Galbraith reported the following against an old fork of preempt-rt but the same issue also applies to the current preempt-rt tree. BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:46 in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 1, name: systemd preempt_count: 1, expected: 0 RCU nest depth: 0, expected: 0 Preemption disabled at: fpu_clone CPU: 6 PID: 1 Comm: systemd Tainted: G E (unreleased) Call Trace: <TASK> dump_stack_lvl ? fpu_clone __might_resched rt_spin_lock fpu_clone ? copy_thread ? copy_process ? shmem_alloc_inode ? kmem_cache_alloc ? kernel_clone ? __do_sys_clone ? do_syscall_64 ? __x64_sys_rt_sigprocmask ? syscall_exit_to_user_mode ? do_syscall_64 ? syscall_exit_to_user_mode ? do_syscall_64 ? syscall_exit_to_user_mode ? do_syscall_64 ? exc_page_fault ? entry_SYSCALL_64_after_hwframe </TASK> Mike says: The splat comes from fpu_inherit_perms() being called under fpregs_lock(), and us reaching the spin_lock_irq() therein due to fpu_state_size_dynamic() returning true despite static key __fpu_state_size_dynamic having never been enabled. Mike's assessment looks correct. fpregs_lock on a PREEMPT_RT kernel disables preemption so calling spin_lock_irq() in fpu_inherit_perms() is unsafe. This problem exists since commit 9e798e9aa1 ("x86/fpu: Prepare fpu_clone() for dynamically enabled features"). Even though the original bug report should not have enabled the paths at all, the bug still exists. fpregs_lock is necessary when editing the FPU registers or a task's FP state but it is not necessary for fpu_inherit_perms(). The only write of any FP state in fpu_inherit_perms() is for the new child which is not running yet and cannot context switch or be borrowed by a kernel thread yet. Hence, fpregs_lock is not protecting anything in the new child until clone() completes and can be dropped earlier. The siglock still needs to be acquired by fpu_inherit_perms() as the read of the parent's permissions has to be serialised. [ bp: Cleanup splat. ] Fixes: 9e798e9aa1 ("x86/fpu: Prepare fpu_clone() for dynamically enabled features") Reported-by: Mike Galbraith <efault@gmx.de> Signed-off-by: Mel Gorman <mgorman@techsingularity.net> Signed-off-by: Borislav Petkov <bp@suse.de> Reviewed-by: Thomas Gleixner <tglx@linutronix.de> Cc: <stable@vger.kernel.org> Link: https://lore.kernel.org/r/20221110124400.zgymc2lnwqjukgfh@techsingularity.net		2022-11-10 16:57:38 +01:00
..
bugs.c	x86/fpu: Replace the includes of fpu/internal.h	2021-10-20 15:27:29 +02:00
context.h	x86/fpu: Update XFD state where required	2021-10-26 10:53:02 +02:00
core.c	x86/fpu: Drop fpregs lock before inheriting FPU permissions	2022-11-10 16:57:38 +01:00
init.c	x86/fpu: Configure init_fpstate attributes orderly	2022-10-17 15:44:25 +02:00
internal.h	x86/fpu/signal: Prepare for variable sigframe length	2021-10-26 10:18:09 +02:00
legacy.h	x86/fpu: Remove .fixup usage	2021-12-11 09:09:48 +01:00
Makefile	treewide: Add SPDX license identifier - Makefile/Kconfig	2019-05-21 10:50:46 +02:00
regset.c	x86/ptrace: Fix xfpregs_set()'s incorrect xmm clearing	2022-02-18 11:23:21 +01:00
signal.c	x86/fpu/signal: Initialize sw_bytes in save_xstate_epilog()	2021-11-30 15:13:47 -08:00
xstate.c	x86/fpu: Fix copy_xstate_to_uabi() to copy init states correctly	2022-10-21 15:22:09 -07:00
xstate.h	x86/fpu/xsave: Support XSAVEC in the kernel	2022-04-25 15:05:37 +02:00