linux-kernel - [PATCH 1/3] x86/entry/64: Fix an IRQ state error on ptregs-using syscalls

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Date:	Sun, 31 Jan 2016 09:33:26 -0800
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org
Cc:	linux-kernel@...r.kernel.org, Brian Gerst <brgerst@...il.com>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH 1/3] x86/entry/64: Fix an IRQ state error on ptregs-using syscalls

I messed up the IRQ state when jumping off the fast path due to
invocation of a ptregs-using syscall.  This bug shouldn't have had
any impact yet, but it would have caused problems with subsequent
context tracking cleanups.

Fixes: 1e423bff959e x86/entry/64: ("Migrate the 64-bit syscall slow path to C")
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/entry/entry_64.S | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 567aa522ac0a..9f7bb808035e 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -191,8 +191,8 @@ entry_SYSCALL_64_fastpath:
 
 	/*
 	 * This call instruction is handled specially in stub_ptregs_64.
-	 * It might end up jumping to the slow path.  If it jumps, RAX is
-	 * clobbered.
+	 * It might end up jumping to the slow path.  If it jumps, RAX
+	 * and all argument registers are clobbered.
 	 */
 	call	*sys_call_table(, %rax, 8)
 .Lentry_SYSCALL_64_after_fastpath_call:
@@ -315,17 +315,24 @@ END(entry_SYSCALL_64)
 ENTRY(stub_ptregs_64)
 	/*
 	 * Syscalls marked as needing ptregs land here.
-	 * If we are on the fast path, we need to save the extra regs.
-	 * If we are on the slow path, the extra regs are already saved.
+	 * If we are on the fast path, we need to save the extra regs,
+	 * which we achieve by trying again on the slow path.  If we are on
+	 * the slow path, the extra regs are already saved.
 	 *
 	 * RAX stores a pointer to the C function implementing the syscall.
+	 * IRQs are on.
 	 */
 	cmpq	$.Lentry_SYSCALL_64_after_fastpath_call, (%rsp)
 	jne	1f
 
-	/* Called from fast path -- pop return address and jump to slow path */
+	/*
+	 * Called from fast path -- disable IRQs again, pop return address
+	 * and jump to slow path
+	 */
+	DISABLE_INTERRUPTS(CLBR_NONE)
+	TRACE_IRQS_OFF
 	popq	%rax
-	jmp	entry_SYSCALL64_slow_path	/* called from fast path */
+	jmp	entry_SYSCALL64_slow_path
 
 1:
 	/* Called from C */
-- 
2.5.0