| Message ID | 20181119160410.ne7oiq2gkwt6jiqg@linutronix.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2] x86/fpu: Disable BH while while loading FPU registers in __fpu__restore_sig() | expand |
On 11/19/18 8:04 AM, Sebastian Andrzej Siewior wrote:
> v1…v2: A more verbose commit as message.
I was really hoping for code comments. :)
On 2018-11-19 09:02:45 [-0800], Dave Hansen wrote: > On 11/19/18 8:04 AM, Sebastian Andrzej Siewior wrote: > > v1…v2: A more verbose commit as message. > > I was really hoping for code comments. :) I though we agreed to make those in the larger series because those comments in __fpu__restore_sig() would be removed anyway (as part of the series). Sebastian
On Mon, Nov 19, 2018 at 06:11:29PM +0100, Sebastian Andrzej Siewior wrote: > On 2018-11-19 09:02:45 [-0800], Dave Hansen wrote: > > On 11/19/18 8:04 AM, Sebastian Andrzej Siewior wrote: > > > v1…v2: A more verbose commit as message. > > > > I was really hoping for code comments. :) > > I though we agreed to make those in the larger series because those > comments in __fpu__restore_sig() would be removed anyway (as part of the > series). Also, over local_bh_disable() does not really fit as this is generic code and Sebastian said ARM does the whole thing a bit differently, for one. We probably should put that comment somewhere prominent in arch/x86/kernel/fpu/core.c or similar - somewhere people poking at FPU stuff will see it.
On 2018-11-19 18:27:43 [+0100], Borislav Petkov wrote: > On Mon, Nov 19, 2018 at 06:11:29PM +0100, Sebastian Andrzej Siewior wrote: > > On 2018-11-19 09:02:45 [-0800], Dave Hansen wrote: > > > On 11/19/18 8:04 AM, Sebastian Andrzej Siewior wrote: > > > > v1…v2: A more verbose commit as message. > > > > > > I was really hoping for code comments. :) > > > > I though we agreed to make those in the larger series because those > > comments in __fpu__restore_sig() would be removed anyway (as part of the > > series). > > Also, over local_bh_disable() does not really fit as this is generic > code and Sebastian said ARM does the whole thing a bit differently, for > one. > > We probably should put that comment somewhere prominent in > arch/x86/kernel/fpu/core.c or similar - somewhere people poking at FPU > stuff will see it. I though about __fpregs_changes_begin() in the last patch of the commit: https://git.kernel.org/pub/scm/linux/kernel/git/bigeasy/staging.git/commit/?h=x86_fpu_rtu_v4 Sebastian
On 11/19/18 9:27 AM, Borislav Petkov wrote: >>> I was really hoping for code comments. :) >> I though we agreed to make those in the larger series because those >> comments in __fpu__restore_sig() would be removed anyway (as part of the >> series). > Also, over local_bh_disable() does not really fit as this is generic > code and Sebastian said ARM does the whole thing a bit differently, for > one. > > We probably should put that comment somewhere prominent in > arch/x86/kernel/fpu/core.c or similar - somewhere people poking at FPU > stuff will see it. Ahh, got it. arch/x86/kernel/fpu/core.c would be a fine spot.
On Mon, Nov 19, 2018 at 06:31:36PM +0100, Sebastian Andrzej Siewior wrote: > I though about __fpregs_changes_begin() in the last patch of the commit: > https://git.kernel.org/pub/scm/linux/kernel/git/bigeasy/staging.git/commit/?h=x86_fpu_rtu_v4 Also a good spot - api.h talks about preemption already so sure, why not. Thx.
On Mon, Nov 19, 2018 at 05:04:10PM +0100, Sebastian Andrzej Siewior wrote: > The sequence > fpu->initialized = 1; /* step A */ > preempt_disable(); /* step B */ > fpu__restore(fpu); > preempt_enable(); > > is racy in regard to a context switch. > For 32bit frames __fpu__restore_sig() prepares the FPU state within > fpu->state. To ensure that a context switch (switch_fpu_prepare() in > particular) does not modify fpu->state it uses fpu__drop() which sets > fpu->initializes to 0. "... ->initialized to 0." Also, a new line here pls. > With this change the CPU's FPU state is not saved ^ comma: , Also, instead of "with this change" I think you mean: "After ->initialized is cleared, the CPU's FPU state..." > to fpu->state during a context switch. > It then loads the state to fpu->state from userland and ensures it > sane. "... and ensures it is sane." > The new state is loaded via fpu__restore(). The code sets then > fpu->initializes to 1 in order to avoid fpu__initialize() doing fpu->initialized > anything (overwrite the new state) which is part of fpu__restore(). <---- newline here. > A context switch between step A and B would save CPU's current FPU > registers to fpu->state and overwrite the newly prepared state. This > looks like tiny race window but the Kernel Test Robot reported this back > in 2016 while we had lazy FPU support. Borislav Petkov made the link > between that report and another patch that has been posted. > Since the removal of the lazy FPU support, this race goes unnoticed > because the warning has been removed. > > Use local_bh_disable() around the restore sequence to avoid the race. BH Let's write it out once: "Bottom halves need to be... " > needs to be disabled because BH is allowed to run (even with preemption > disabled) and might invoke kernel_fpu_begin(). ... and let's put the potential example here with IPsec and softirq. > Link: https://lkml.kernel.org/r/20160226074940.GA28911@pd.tnic > Cc: stable@vger.kernel.org > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > --- > v1…v2: A more verbose commit as message. Very much needed, thanks! > arch/x86/kernel/fpu/signal.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c > index 61a949d84dfa5..d99a8ee9e185e 100644 > --- a/arch/x86/kernel/fpu/signal.c > +++ b/arch/x86/kernel/fpu/signal.c > @@ -344,10 +344,10 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size) > sanitize_restored_xstate(tsk, &env, xfeatures, fx_only); > } > > + local_bh_disable(); > fpu->initialized = 1; > - preempt_disable(); > fpu__restore(fpu); > - preempt_enable(); > + local_bh_enable(); > > return err; > } else { > --
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c index 61a949d84dfa5..d99a8ee9e185e 100644 --- a/arch/x86/kernel/fpu/signal.c +++ b/arch/x86/kernel/fpu/signal.c @@ -344,10 +344,10 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size) sanitize_restored_xstate(tsk, &env, xfeatures, fx_only); } + local_bh_disable(); fpu->initialized = 1; - preempt_disable(); fpu__restore(fpu); - preempt_enable(); + local_bh_enable(); return err; } else {
The sequence fpu->initialized = 1; /* step A */ preempt_disable(); /* step B */ fpu__restore(fpu); preempt_enable(); is racy in regard to a context switch. For 32bit frames __fpu__restore_sig() prepares the FPU state within fpu->state. To ensure that a context switch (switch_fpu_prepare() in particular) does not modify fpu->state it uses fpu__drop() which sets fpu->initializes to 0. With this change the CPU's FPU state is not saved to fpu->state during a context switch. It then loads the state to fpu->state from userland and ensures it sane. The new state is loaded via fpu__restore(). The code sets then fpu->initializes to 1 in order to avoid fpu__initialize() doing anything (overwrite the new state) which is part of fpu__restore(). A context switch between step A and B would save CPU's current FPU registers to fpu->state and overwrite the newly prepared state. This looks like tiny race window but the Kernel Test Robot reported this back in 2016 while we had lazy FPU support. Borislav Petkov made the link between that report and another patch that has been posted. Since the removal of the lazy FPU support, this race goes unnoticed because the warning has been removed. Use local_bh_disable() around the restore sequence to avoid the race. BH needs to be disabled because BH is allowed to run (even with preemption disabled) and might invoke kernel_fpu_begin(). Link: https://lkml.kernel.org/r/20160226074940.GA28911@pd.tnic Cc: stable@vger.kernel.org Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> --- v1…v2: A more verbose commit as message. arch/x86/kernel/fpu/signal.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)