linux-kernel - [PATCH v2 1/2] x86/entry/32: Fix entry_INT80

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Message-Id: <dc09d9b574a5c1dcca996847875c73f8341ce0ad.1445035014.git.luto@kernel.org>
Date:	Fri, 16 Oct 2015 15:42:54 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v2 1/2] x86/entry/32: Fix entry_INT80_32 to expect interrupts to be on

When I rewrote entry_INT80_32, I thought that int80 was an interrupt
gate.  It's a trap gate.  *facepalm*

Thanks to Brian Gerst for pointing out that it's better to change
the entry code than to change the gate type.

Suggested-by: Brian Gerst <brgerst@...il.com>
Reported-by: Borislav Petkov <bp@...e.de>
Fixes: 150ac78d63af ("x86/entry/32: Switch INT80 to the new C syscall path")
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/entry/common.c          | 15 ++++++++++++---
 arch/x86/entry/entry_32.S        |  8 ++++----
 arch/x86/entry/entry_64_compat.S |  2 +-
 3 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index b53e04d301a3..09afb3b6acbb 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -351,7 +351,14 @@ __visible inline void syscall_return_slowpath(struct pt_regs *regs)
  * in workloads that use it, and it's usually called from
  * do_fast_syscall_32, so forcibly inline it to improve performance.
  */
-static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs)
+#ifdef CONFIG_X86_32
+/* 32-bit kernels use a trap gate for int80, and the asm code calls here. */
+__visible
+#else
+/* 64-bit kernels use do_syscall_32_irqs_off instead. */
+static
+#endif
+__always_inline void do_syscall_32_irqs_on(struct pt_regs *regs)
 {
 	struct thread_info *ti = pt_regs_to_thread_info(regs);
 	unsigned int nr = (unsigned int)regs->orig_ax;
@@ -386,12 +393,14 @@ static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs)
 	syscall_return_slowpath(regs);
 }
 
-/* Handles int $0x80 */
-__visible void do_int80_syscall_32(struct pt_regs *regs)
+#ifdef CONFIG_X86_64
+/* Handles int $0x80 on 64-bit kernels */
+__visible void do_syscall_32_irqs_off(struct pt_regs *regs)
 {
 	local_irq_enable();
 	do_syscall_32_irqs_on(regs);
 }
+#endif
 
 /* Returns 0 to return using IRET or 1 to return using SYSEXIT/SYSRETL. */
 __visible long do_fast_syscall_32(struct pt_regs *regs)
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 07a041b91af1..ba0dad937be9 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -346,13 +346,13 @@ ENTRY(entry_INT80_32)
 	SAVE_ALL pt_regs_ax=$-ENOSYS	/* save rest */
 
 	/*
-	 * User mode is traced as though IRQs are on, and the interrupt gate
-	 * turned them off.
+	 * User mode is traced as though IRQs are on.  Unlike the 64-bit
+	 * case, int80 is a trap gate on 32-bit kernels, so interrupts
+	 * are already on (unless user code is messing around with iopl).
 	 */
-	TRACE_IRQS_OFF
 
 	movl	%esp, %eax
-	call	do_int80_syscall_32
+	call	do_syscall_32_irqs_on
 .Lsyscall_32_done:
 
 restore_all:
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 78becafe60d1..dd160e4e2ef5 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -303,7 +303,7 @@ ENTRY(entry_INT80_compat)
 	TRACE_IRQS_OFF
 
 	movq	%rsp, %rdi
-	call	do_int80_syscall_32
+	call	do_syscall_32_irqs_off
 .Lsyscall_32_done:
 
 	/* Go back to user mode. */
-- 
2.4.3

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