From: Borislav Petkov <bp@alien8.de>
To: Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
"H. Peter Anvin" <hpa@zytor.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
Tony Luck <tony.luck@intel.com>,
Andy Lutomirski <luto@kernel.org>,
Rik van Riel <riel@surriel.com>,
"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Fenghua Yu <fenghua.yu@intel.com>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for __fpu__restore_sig()
Date: Fri, 21 Feb 2020 18:58:59 +0100 [thread overview]
Message-ID: <20200221175859.GL25747@zn.tnic> (raw)
In-Reply-To: <20200121201843.12047-9-yu-cheng.yu@intel.com>
On Tue, Jan 21, 2020 at 12:18:43PM -0800, Yu-cheng Yu wrote:
> When returning from a signal, there is user state in a userspace memory
> buffer and supervisor state in registers. The in-kernel buffer has neither
> valid user or supervisor state. To restore both, save supervisor fpregs
The correct formulation is "neither ... nor ... "
> first (and protect them across context switches), then restore it along
s/it/them/
> with user state.
>
> This patch introduces saving and restoring of supervisor xstates for a
Avoid having "This patch" or "This commit" in the commit message. It is
tautologically useless.
Also, do
$ git grep 'This patch' Documentation/process
for more details.
> sigreturn in the following steps:
>
> - Preserve supervisor register values by saving the whole fpu xstates.
> Saving the whole is necessary because doing XSAVES with a partial
> requested-feature bitmap (RFBM) changes xcomp_bv. Since user xstates are
> to be restored from a user buffer, saved user xstates are discarded.
>
> - Set TIF_NEED_FPU_LOAD, and do __fpu_invalidate_fpregs_state().
> This prevents a context switch from corrupting the saved xstates,
> and xstate is considered to be loaded again on return to userland.
s/considered/going to/
> - Under fpregs_lock(), restore user xstates (from the user buffer), and
> then supervisor xstates (from previously saved content).
>
> - When both parts of the restoration succeed, mark fpregs as valid.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
> ---
> arch/x86/kernel/fpu/signal.c | 26 +++++++++++++++++++++-----
> 1 file changed, 21 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index e3781a4a52a8..0d3e06a772b0 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -327,14 +327,22 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
> }
>
> /*
> - * The current state of the FPU registers does not matter. By setting
> - * TIF_NEED_FPU_LOAD unconditionally it is ensured that the our xstate
> - * is not modified on context switch and that the xstate is considered
> + * Supervisor xstates are not modified by user space input, and
> + * need to be saved and restored. Save the whole because doing
> + * partial XSAVES changes xcomp_bv.
> + * By setting TIF_NEED_FPU_LOAD it is ensured that our xstate is
> + * not modified on context switch and that the xstate is considered
Reflow those comments to 80 cols. There's room to the right.
> * to be loaded again on return to userland (overriding last_cpu avoids
> * the optimisation).
> */
> - set_thread_flag(TIF_NEED_FPU_LOAD);
> + fpregs_lock();
> + if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
> + if (xfeatures_mask_supervisor())
> + copy_xregs_to_kernel(&fpu->state.xsave);
> + set_thread_flag(TIF_NEED_FPU_LOAD);
So the code sets TIF_NEED_FPU_LOAD unconditionally, why are you changing
this?
Why don't you simply do:
set_thread_flag(TIF_NEED_FPU_LOAD);
fpregs_lock();
if (xfeatures_mask_supervisor())
copy_xregs_to_kernel(&fpu->state.xsave);
fpregs_unlock();
> + }
> __fpu_invalidate_fpregs_state(fpu);
> + fpregs_unlock();
>
> if ((unsigned long)buf_fx % 64)
> fx_only = 1;
> @@ -360,6 +368,9 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
> ret = copy_user_to_fpregs_zeroing(buf_fx, xfeatures_user, fx_only);
> pagefault_enable();
<--- comment here why you're doing this. That function is an abomination
and needs commenting at least.
> if (!ret) {
> + if (xfeatures_mask_supervisor())
> + copy_kernel_to_xregs(&fpu->state.xsave,
> + xfeatures_mask_supervisor());
> fpregs_mark_activate();
> fpregs_unlock();
> return 0;
> @@ -389,7 +400,12 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
> fpregs_lock();
> if (unlikely(init_bv))
> copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
> - ret = copy_kernel_to_xregs_err(&fpu->state.xsave, xfeatures_user);
> + /*
> + * Restore previously saved supervisor xstates along with
> + * copied-in user xstates.
> + */
> + ret = copy_kernel_to_xregs_err(&fpu->state.xsave,
> + xfeatures_user | xfeatures_mask_supervisor());
>
> } else if (use_fxsr()) {
> ret = __copy_from_user(&fpu->state.fxsave, buf_fx, state_size);
> --
> 2.21.0
>
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
next prev parent reply other threads:[~2020-02-21 17:59 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-21 20:18 [PATCH v2 0/8] Support XSAVES supervisor states Yu-cheng Yu
2020-01-21 20:18 ` [PATCH v2 1/8] x86/fpu/xstate: Define new macros for supervisor and user xstates Yu-cheng Yu
2020-02-20 11:47 ` Borislav Petkov
2020-02-20 20:23 ` Yu-cheng Yu
2020-01-21 20:18 ` [PATCH v2 2/8] x86/fpu/xstate: Separate user and supervisor xfeatures mask Yu-cheng Yu
2020-02-21 10:34 ` Borislav Petkov
2020-01-21 20:18 ` [PATCH v2 3/8] x86/fpu/xstate: Introduce XSAVES supervisor states Yu-cheng Yu
2020-01-21 20:18 ` [PATCH v2 4/8] x86/fpu/xstate: Define new functions for clearing fpregs and xstates Yu-cheng Yu
2020-02-21 14:04 ` Borislav Petkov
2020-01-21 20:18 ` [PATCH v2 5/8] x86/fpu/xstate: Rename validate_xstate_header() to validate_xstate_header_from_user() Yu-cheng Yu
2020-02-21 14:13 ` Borislav Petkov
2020-01-21 20:18 ` [PATCH v2 6/8] x86/fpu/xstate: Update sanitize_restored_xstate() for supervisor xstates Yu-cheng Yu
2020-02-21 14:30 ` Borislav Petkov
2020-01-21 20:18 ` [PATCH v2 7/8] x86/fpu/xstate: Update copy_kernel_to_xregs_err() for XSAVES supervisor states Yu-cheng Yu
2020-01-21 20:18 ` [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for __fpu__restore_sig() Yu-cheng Yu
2020-02-21 17:58 ` Borislav Petkov [this message]
2020-02-27 22:52 ` Yu-cheng Yu
2020-02-28 12:17 ` Borislav Petkov
2020-02-28 12:51 ` Sebastian Andrzej Siewior
2020-02-28 15:53 ` Yu-cheng Yu
2020-02-28 16:23 ` Borislav Petkov
2020-02-28 16:20 ` Yu-cheng Yu
2020-02-28 16:50 ` Sebastian Andrzej Siewior
2020-02-28 16:54 ` Yu-cheng Yu
2020-02-28 17:22 ` Borislav Petkov
2020-02-28 18:11 ` Yu-cheng Yu
2020-02-28 18:31 ` Borislav Petkov
2020-02-28 21:22 ` Yu-cheng Yu
2020-02-28 21:47 ` Borislav Petkov
2020-02-28 22:13 ` Yu-cheng Yu
2020-02-29 14:36 ` Borislav Petkov
2020-03-02 18:09 ` Yu-cheng Yu
2020-03-04 18:18 ` Yu-cheng Yu
2020-03-06 20:50 ` Borislav Petkov
2020-03-10 20:36 ` Yu-cheng Yu
2020-03-10 21:16 ` Thomas Gleixner
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