linux-kernel - [tip: x86/urgent] x86/fpu: Exclude dynamic states from init

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Message-ID: <166601445015.401.6980293789787013355.tip-bot2@tip-bot2>
Date:   Mon, 17 Oct 2022 13:47:30 -0000
From:   "tip-bot2 for Chang S. Bae" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Lin X Wang <lin.x.wang@...el.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/urgent] x86/fpu: Exclude dynamic states from init_fpstate

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     a401f45e38754953c9d402f8b3bc965707eecc91
Gitweb:        https://git.kernel.org/tip/a401f45e38754953c9d402f8b3bc965707eecc91
Author:        Chang S. Bae <chang.seok.bae@...el.com>
AuthorDate:    Wed, 24 Aug 2022 12:12:23 -07:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Mon, 17 Oct 2022 15:44:25 +02:00

x86/fpu: Exclude dynamic states from init_fpstate

== Background ==

The XSTATE init code initializes all enabled and supported components.
Then, the init states are saved in the init_fpstate buffer that is
statically allocated in about one page.

The AMX TILE_DATA state is large (8KB) but its init state is zero. And the
feature comes only with the compacted format with these established
dependencies: AMX->XFD->XSAVES. So this state is excludable from
init_fpstate.

== Problem ==

But the buffer is formatted to include that large state. Then, this can be
the cause of a noisy splat like the below.

This came from XRSTORS for the task with init_fpstate in its XSAVE buffer.
It is reproducible on AMX systems when the running kernel is built with
CONFIG_DEBUG_PAGEALLOC=y and CONFIG_DEBUG_PAGEALLOC_ENABLE_DEFAULT=y:

 Bad FPU state detected at restore_fpregs_from_fpstate+0x57/0xd0, reinitializing FPU registers.
 ...
 RIP: 0010:restore_fpregs_from_fpstate+0x57/0xd0
  ? restore_fpregs_from_fpstate+0x45/0xd0
  switch_fpu_return+0x4e/0xe0
  exit_to_user_mode_prepare+0x17b/0x1b0
  syscall_exit_to_user_mode+0x29/0x40
  do_syscall_64+0x67/0x80
  ? do_syscall_64+0x67/0x80
  ? exc_page_fault+0x86/0x180
  entry_SYSCALL_64_after_hwframe+0x63/0xcd

== Solution ==

Adjust init_fpstate to exclude dynamic states. XRSTORS from init_fpstate
still initializes those states when their bits are set in the
requested-feature bitmap.

Fixes: 2308ee57d93d ("x86/fpu/amx: Enable the AMX feature in 64-bit mode")
Reported-by: Lin X Wang <lin.x.wang@...el.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Lin X Wang <lin.x.wang@...el.com>
Link: https://lore.kernel.org/r/20220824191223.1248-4-chang.seok.bae@intel.com

---
 arch/x86/kernel/fpu/xstate.c |  9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index f5ef786..e77cabf 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -857,9 +857,12 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
 	update_regset_xstate_info(fpu_user_cfg.max_size,
 				  fpu_user_cfg.max_features);

-	/* Bring init_fpstate size and features up to date */
-	init_fpstate.size		= fpu_kernel_cfg.max_size;
-	init_fpstate.xfeatures		= fpu_kernel_cfg.max_features;
+	/*
+	 * init_fpstate excludes dynamic states as they are large but init
+	 * state is zero.
+	 */
+	init_fpstate.size		= fpu_kernel_cfg.default_size;
+	init_fpstate.xfeatures		= fpu_kernel_cfg.default_features;

 	if (init_fpstate.size > sizeof(init_fpstate.regs)) {
 		pr_warn("x86/fpu: init_fpstate buffer too small (%zu < %d), disabling XSAVE\n",