linux-kernel - Re: [PATCH v2 6/7] x86/mce: Recover from poison found while copying from user space

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Message-ID: <20201005163247.GE21151@zn.tnic>
Date:   Mon, 5 Oct 2020 18:32:47 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Tony Luck <tony.luck@...el.com>
Cc:     Youquan Song <youquan.song@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/7] x86/mce: Recover from poison found while copying
 from user space

On Wed, Sep 30, 2020 at 04:26:10PM -0700, Tony Luck wrote:
> From: Youquan Song <youquan.song@...el.com>
> 
> Existing kernel code can only recover from a machine check on code that
> is tagged in the exception table with a fault handling recovery path.
> 
> Two new fields in the task structure to pass information from machine
> check handler to the "task_work" that is queued to run before the task
> returns to user mode:
> 
> + mce_vaddr: will be initialized to the user virtual address of the fault
>   in the case where the fault occurred in the kernel copying data from
>   a user address.  This is so that kill_me_maybe() can provide that
>   information to the user SIGBUS handler.
> 
> + mce_kflags: copy of the struct mce.kflags needed by kill_me_maybe()
>   to determine if mce_vaddr is applicable to this error.
> 
> Add code to recover from a machine check while copying data from user
> space to the kernel. Action for this case is the same as if the user
> touched the poison directly; unmap the page and send a SIGBUS to the task.
> 
> Refactor the recovery code path to share common code between the "fault
> in user mode" case and the "fault while copying from user" case.
> 
> New code paths will be activated by the next patch which sets
> MCE_IN_KERNEL_COPYIN.
> 
> Signed-off-by: Youquan Song <youquan.song@...el.com>
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
>  arch/x86/kernel/cpu/mce/core.c | 33 +++++++++++++++++++++------------
>  include/linux/sched.h          |  2 ++
>  2 files changed, 23 insertions(+), 12 deletions(-)

Isn't that just simpler?

diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
index 4d2cf08820af..dc6c83aa2ec1 100644
--- a/arch/x86/kernel/cpu/mce/core.c
+++ b/arch/x86/kernel/cpu/mce/core.c
@@ -1261,6 +1261,21 @@ static void kill_me_maybe(struct callback_head *cb)
 	kill_me_now(cb);
 }
 
+static inline void queue_task_work(struct mce *m, int kill_it)
+{
+	current->mce_addr = m->addr;
+	current->mce_kflags = m->kflags;
+	current->mce_ripv = !!(m->mcgstatus & MCG_STATUS_RIPV);
+	current->mce_whole_page = whole_page(m);
+
+	if (kill_it)
+		current->mce_kill_me.func = kill_me_now;
+	else
+		current->mce_kill_me.func = kill_me_maybe;
+
+	task_work_add(current, &current->mce_kill_me, true);
+}
+
 /*
  * The actual machine check handler. This only handles real
  * exceptions when something got corrupted coming in through int 18.
@@ -1402,13 +1417,8 @@ noinstr void do_machine_check(struct pt_regs *regs)
 		/* If this triggers there is no way to recover. Die hard. */
 		BUG_ON(!on_thread_stack() || !user_mode(regs));
 
-		current->mce_addr = m.addr;
-		current->mce_ripv = !!(m.mcgstatus & MCG_STATUS_RIPV);
-		current->mce_whole_page = whole_page(&m);
-		current->mce_kill_me.func = kill_me_maybe;
-		if (kill_it)
-			current->mce_kill_me.func = kill_me_now;
-		task_work_add(current, &current->mce_kill_me, true);
+		queue_task_work(&m, kill_it);
+
 	} else {
 		/*
 		 * Handle an MCE which has happened in kernel space but from
@@ -1423,6 +1433,9 @@ noinstr void do_machine_check(struct pt_regs *regs)
 			if (!fixup_exception(regs, X86_TRAP_MC, 0, 0))
 				mce_panic("Failed kernel mode recovery", &m, msg);
 		}
+
+		if (m.kflags & MCE_IN_KERNEL_COPYIN)
+			queue_task_work(&m, kill_it);
 	}
 out:
 	mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index e7f65573dde3..d383cf09e78f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1308,6 +1308,8 @@ struct task_struct {
 #endif
 
 #ifdef CONFIG_X86_MCE
+	void __user			*mce_vaddr;
+	__u64				mce_kflags;
 	u64				mce_addr;
 	__u64				mce_ripv : 1,
 					mce_whole_page : 1,
-- 
Regards/Gruss,
    Boris.

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