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sched/x86: Save [ER]FLAGS on context switch

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Effectively reverts commit:

  2c7577a758 ("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:

  5b24a7a2aa ("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 <julien.thierry@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Andy 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: 5b24a7a2aa ("Add 'unsafe' user access functions for batched accesses")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2019-02-14 10:30:52 +01:00 committed by Ingo Molnar
parent 5e7a8ca319
commit 6690e86be8
5 changed files with 20 additions and 0 deletions
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2
arch/x86/entry/entry_32.S
View File

@ -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

2
arch/x86/entry/entry_64.S
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@ -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

1
arch/x86/include/asm/switch_to.h
View File

@ -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;

7
arch/x86/kernel/process_32.c
View File

@ -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;

8
arch/x86/kernel/process_64.c
View File

@ -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;