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From:   Lai Jiangshan <laijs@linux.alibaba.com>
To:     linux-kernel@vger.kernel.org
Cc:     Lai Jiangshan <laijs@linux.alibaba.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Jann Horn <jannh@google.com>,
        Dave Hansen <dave.hansen@linux.intel.com>
Subject: [PATCH 5/5] x86/entry: don't copy to tmp in fixup_bad_iret
Date:   Wed, 27 May 2020 07:31:07 +0000
Message-Id: <20200527073107.2127-6-laijs@linux.alibaba.com>
In-Reply-To: <20200527073107.2127-1-laijs@linux.alibaba.com>
References: <20200527073107.2127-1-laijs@linux.alibaba.com>
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It is safe to do memcpy() in fixup_bad_iret() now.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/kernel/traps.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 8291f84933ff..6fe72c771745 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -657,17 +657,23 @@ struct bad_iret_stack *fixup_bad_iret(struct bad_iret_stack *s)
 	 * (rather than just below the IRET frame) and we want to
 	 * pretend that the exception came from the IRET target.
 	 */
-	struct bad_iret_stack tmp, *new_stack =
+	struct bad_iret_stack *new_stack =
 		(struct bad_iret_stack *)__this_cpu_read(cpu_current_top_of_stack) - 1;
 
-	/* Copy the IRET target to the temporary storage. */
-	memcpy(&tmp.regs.ip, (void *)s->regs.sp, 5*8);
+	/*
+	 * Both the IRET frame and the saved pt_regs must be on the
+	 * entry stacks since iret to user is only issued on the
+	 * entry stacks. So they don't overlap with kernel stack and
+	 * memcpy() is safe to copy them.
+	 */
+	BUG_ON(((unsigned long)s - (unsigned long)new_stack) < PAGE_SIZE ||
+	       ((unsigned long)new_stack - (unsigned long)s) < PAGE_SIZE);
 
-	/* Copy the remainder of the stack from the current stack. */
-	memcpy(&tmp, s, offsetof(struct bad_iret_stack, regs.ip));
+	/* Copy the IRET target to the new stack. */
+	memcpy(&new_stack->regs.ip, (void *)s->regs.sp, 5*8);
 
-	/* Update the entry stack */
-	memcpy(new_stack, &tmp, sizeof(tmp));
+	/* Copy the remainder of the stack from the current stack. */
+	memcpy(new_stack, s, offsetof(struct bad_iret_stack, regs.ip));
 
 	BUG_ON(!user_mode(&new_stack->regs));
 	return new_stack;
-- 
2.20.1