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Subject[PATCH] x86/fpu: Fix FPU register read access to the current task

* Bobby Powers <bobbypowers@gmail.com> wrote:

> Hello,
>
> Ingo Molnar <mingo@kernel.org> wrote:
> > Please have a look.
>
> I've been running this for ~ 2 weeks. I've only seen one issue, when
> emerging mesa 10.5.6:
>
> [May26 20:41] traps: aclocal-1.15[27452] trap invalid opcode
> ip:7f6331031ab0 sp:7ffe73ece880 error:0 in
> libperl.so.5.20.2[7f6330f18000+19e000]
> [ +0.000051] ------------[ cut here ]------------
> [ +0.000005] WARNING: CPU: 0 PID: 27452 at
> arch/x86/kernel/fpu/core.c:324 fpu__activate_stopped+0x8a/0xa0()

Good catch, this shows a bug in the new FPU code.

Could you check whether the fix below solves the problem?

Thanks,

Ingo

====================>
From 47f01e8cc23f3d041f6b9fb97627369eaf75ba7f Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@kernel.org>
Date: Wed, 27 May 2015 12:22:29 +0200
Subject: [PATCH] x86/fpu: Fix FPU register read access to the current task

Bobby Powers reported the following FPU warning during ELF coredumping:

WARNING: CPU: 0 PID: 27452 at arch/x86/kernel/fpu/core.c:324 fpu__activate_stopped+0x8a/0xa0()

This warning unearthed an invalid assumption about fpu__activate_stopped()
that I added in:

67e97fc2ec57 ("x86/fpu: Rename init_fpu() to fpu__unlazy_stopped() and add debugging check")

the old init_fpu() function had an (intentional but obscure) side effect:
when FPU registers are accessed for the current task, for reading, then
it synchronized live in-register FPU state with the fpstate by saving it.

So fix this bug by saving the FPU if we are the current task. We'll
still warn in fpu__save() if this is called for not yet stopped
child tasks, so the debugging check is still preserved.

Also rename the function to fpu__activate_fpstate(), because it's not
exclusively used for stopped tasks, but for the current task as well.

( Note that this bug calls for a cleaner separation of access-for-read
and access-for-modification FPU methods, but we'll do that in separate
patches. )

Reported-by: Bobby Powers <bobbypowers@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/include/asm/fpu/internal.h | 2 +-
arch/x86/kernel/fpu/core.c | 43 +++++++++++++++++++++----------------
arch/x86/kernel/fpu/regset.c | 12 +++++------
3 files changed, 32 insertions(+), 25 deletions(-)

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 99690bed920a..62d13d515f95 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -22,7 +22,7 @@
* High level FPU state handling functions:
*/
extern void fpu__activate_curr(struct fpu *fpu);
-extern void fpu__activate_stopped(struct fpu *fpu);
+extern void fpu__activate_fpstate(struct fpu *fpu);
extern void fpu__save(struct fpu *fpu);
extern void fpu__restore(struct fpu *fpu);
extern int fpu__restore_sig(void __user *buf, int ia32_frame);
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 01a15503c3be..b41049247cfa 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -296,40 +296,47 @@ void fpu__activate_curr(struct fpu *fpu)
EXPORT_SYMBOL_GPL(fpu__activate_curr);

/*
- * This function must be called before we modify a stopped child's
- * fpstate.
+ * This function must be called before we read or write a task's fpstate.
*
- * If the child has not used the FPU before then initialize its
+ * If the task has not used the FPU before then initialize its
* fpstate.
*
- * If the child has used the FPU before then unlazy it.
+ * If the task has used the FPU before then save and unlazy it.
*
- * [ After this function call, after registers in the fpstate are
+ * [ If this function is used for non-current child tasks, then
+ * after this function call, after registers in the fpstate are
* modified and the child task has woken up, the child task will
* restore the modified FPU state from the modified context. If we
* didn't clear its lazy status here then the lazy in-registers
* state pending on its former CPU could be restored, corrupting
- * the modifications. ]
+ * the modifications.
*
- * This function is also called before we read a stopped child's
- * FPU state - to make sure it's initialized if the child has
- * no active FPU state.
+ * This function can be used for the current task as well, but
+ * only for reading the fpstate. Modifications to the fpstate
+ * will be lost on eagerfpu systems. ]
*
* TODO: A future optimization would be to skip the unlazying in
* the read-only case, it's not strictly necessary for
* read-only access to the context.
*/
-void fpu__activate_stopped(struct fpu *child_fpu)
+void fpu__activate_fpstate(struct fpu *fpu)
{
- WARN_ON_FPU(child_fpu == &current->thread.fpu);
-
- if (child_fpu->fpstate_active) {
- child_fpu->last_cpu = -1;
+ /*
+ * If fpregs are active (in the current CPU), then
+ * copy them to the fpstate:
+ */
+ if (fpu->fpregs_active) {
+ fpu__save(fpu);
} else {
- fpstate_init(&child_fpu->state);
-
- /* Safe to do for stopped child tasks: */
- child_fpu->fpstate_active = 1;
+ if (fpu->fpstate_active) {
+ /* Invalidate any lazy state: */
+ fpu->last_cpu = -1;
+ } else {
+ fpstate_init(&fpu->state);
+
+ /* Safe to do for current and for stopped child tasks: */
+ fpu->fpstate_active = 1;
+ }
}
}

diff --git a/arch/x86/kernel/fpu/regset.c b/arch/x86/kernel/fpu/regset.c
index 297b3da8e4c4..a1f97d9d6a45 100644
--- a/arch/x86/kernel/fpu/regset.c
+++ b/arch/x86/kernel/fpu/regset.c
@@ -33,7 +33,7 @@ int xfpregs_get(struct task_struct *target, const struct user_regset *regset,
if (!cpu_has_fxsr)
return -ENODEV;

- fpu__activate_stopped(fpu);
+ fpu__activate_fpstate(fpu);
fpstate_sanitize_xstate(fpu);

return user_regset_copyout(&pos, &count, &kbuf, &ubuf,
@@ -50,7 +50,7 @@ int xfpregs_set(struct task_struct *target, const struct user_regset *regset,
if (!cpu_has_fxsr)
return -ENODEV;

- fpu__activate_stopped(fpu);
+ fpu__activate_fpstate(fpu);
fpstate_sanitize_xstate(fpu);

ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
@@ -82,7 +82,7 @@ int xstateregs_get(struct task_struct *target, const struct user_regset *regset,
if (!cpu_has_xsave)
return -ENODEV;

- fpu__activate_stopped(fpu);
+ fpu__activate_fpstate(fpu);

xsave = &fpu->state.xsave;

@@ -111,7 +111,7 @@ int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
if (!cpu_has_xsave)
return -ENODEV;

- fpu__activate_stopped(fpu);
+ fpu__activate_fpstate(fpu);

xsave = &fpu->state.xsave;

@@ -273,7 +273,7 @@ int fpregs_get(struct task_struct *target, const struct user_regset *regset,
struct fpu *fpu = &target->thread.fpu;
struct user_i387_ia32_struct env;

- fpu__activate_stopped(fpu);
+ fpu__activate_fpstate(fpu);

if (!static_cpu_has(X86_FEATURE_FPU))
return fpregs_soft_get(target, regset, pos, count, kbuf, ubuf);
@@ -303,7 +303,7 @@ int fpregs_set(struct task_struct *target, const struct user_regset *regset,
struct user_i387_ia32_struct env;
int ret;

- fpu__activate_stopped(fpu);
+ fpu__activate_fpstate(fpu);
fpstate_sanitize_xstate(fpu);

if (!static_cpu_has(X86_FEATURE_FPU))

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