From 6690e86be83ac75832e461c141055b5d601c0a6d Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 14 Feb 2019 10:30:52 +0100 Subject: sched/x86: Save [ER]FLAGS on context switch Effectively reverts commit: 2c7577a75837 ("sched/x86_64: Don't save flags on context switch") Specifically because SMAP uses FLAGS.AC which invalidates the claim that the kernel has clean flags. In particular; while preemption from interrupt return is fine (the IRET frame on the exception stack contains FLAGS) it breaks any code that does synchonous scheduling, including preempt_enable(). This has become a significant issue ever since commit: 5b24a7a2aa20 ("Add 'unsafe' user access functions for batched accesses") provided for means of having 'normal' C code between STAC / CLAC, exposing the FLAGS.AC state. So far this hasn't led to trouble, however fix it before it comes apart. Reported-by: Julien Thierry Signed-off-by: Peter Zijlstra (Intel) Acked-by: Andy Lutomirski Cc: Borislav Petkov Cc: Josh Poimboeuf Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: stable@kernel.org Fixes: 5b24a7a2aa20 ("Add 'unsafe' user access functions for batched accesses") Signed-off-by: Ingo Molnar --- arch/x86/kernel/process_64.c | 8 ++++++++ 1 file changed, 8 insertions(+) (limited to 'arch/x86/kernel/process_64.c') diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c index 6a62f4af9fcf..026a43be9bd1 100644 --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -392,6 +392,14 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp, childregs = task_pt_regs(p); fork_frame = container_of(childregs, struct fork_frame, regs); frame = &fork_frame->frame; + + /* + * For a new task use the RESET flags value since there is no before. + * All the status flags are zero; DF and all the system flags must also + * be 0, specifically IF must be 0 because we context switch to the new + * task with interrupts disabled. + */ + frame->flags = X86_EFLAGS_FIXED; frame->bp = 0; frame->ret_addr = (unsigned long) ret_from_fork; p->thread.sp = (unsigned long) fork_frame; -- cgit v1.2.3