Commit 6690e86b authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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sched/x86: Save [ER]FLAGS on context switch 



Effectively reverts commit:

  2c7577a7 ("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:

  5b24a7a2 ("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: default avatarJulien Thierry <julien.thierry@arm.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: default avatarAndy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@kernel.org
Fixes: 5b24a7a2 ("Add 'unsafe' user access functions for batched accesses")
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 5e7a8ca3

arch/x86/entry/entry_32.S

+2 −0
Original line number Diff line number Diff line
@@ -650,6 +650,7 @@ ENTRY(__switch_to_asm) 
	pushl	%ebx

	pushl	%edi

	pushl	%esi

	pushfl



	/* switch stack */

	movl	%esp, TASK_threadsp(%eax)
@@ -672,6 +673,7 @@ ENTRY(__switch_to_asm) 
#endif



	/* restore callee-saved registers */

	popfl

	popl	%esi

	popl	%edi

	popl	%ebx

arch/x86/entry/entry_64.S

+2 −0
Original line number Diff line number Diff line
@@ -291,6 +291,7 @@ ENTRY(__switch_to_asm) 
	pushq	%r13

	pushq	%r14

	pushq	%r15

	pushfq



	/* switch stack */

	movq	%rsp, TASK_threadsp(%rdi)
@@ -313,6 +314,7 @@ ENTRY(__switch_to_asm) 
#endif



	/* restore callee-saved registers */

	popfq

	popq	%r15

	popq	%r14

	popq	%r13

arch/x86/include/asm/switch_to.h

+1 −0
Original line number Diff line number Diff line
@@ -40,6 +40,7 @@ asmlinkage void ret_from_fork(void); 
 * order of the fields must match the code in __switch_to_asm().

 */

struct inactive_task_frame {

	unsigned long flags;

#ifdef CONFIG_X86_64

	unsigned long r15;

	unsigned long r14;

arch/x86/kernel/process_32.c

+7 −0
Original line number Diff line number Diff line
@@ -127,6 +127,13 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp, 
	struct task_struct *tsk;

	int err;



	/*

	 * 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;

arch/x86/kernel/process_64.c

+8 −0
Original line number Diff line number Diff line
@@ -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;

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