linux-kernel - [PATCH 6/9] x86: Make user_mode work correctly if regs came from vm86 mode

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Message-Id: <0dd03eacb5f0a2b5ba0240de25347a31b493c289.1426728647.git.luto@kernel.org>
Date:	Wed, 18 Mar 2015 18:33:32 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	Brad Spengler <spender@...ecurity.net>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH 6/9] x86: Make user_mode work correctly if regs came from vm86 mode

user_mode is now identical to user_mode_vm.  Subsequent patches will
change all callers of user_mode_vm to user_mode and then delete
user_mode_vm.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/include/asm/ptrace.h | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index a60c59e977cc..6483525bb559 100644
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -96,11 +96,13 @@ static inline unsigned long regs_return_value(struct pt_regs *regs)
 }
 
 /*
- * user_mode_vm(regs) determines whether a register set came from user mode.
- * This is true if V8086 mode was enabled OR if the register set was from
- * protected mode with RPL-3 CS value.  This tricky test checks that with
- * one comparison.  Many places in the kernel can bypass this full check
- * if they have already ruled out V8086 mode, so user_mode(regs) can be used.
+ * user_mode(regs) determines whether a register set came from user
+ * mode.  On x86_32, this is true if V8086 mode was enabled OR if the
+ * register set was from protected mode with RPL-3 CS value.  This
+ * tricky test checks that with one comparison.
+ *
+ * On x86_64, vm86 mode is mercifully nonexistent, and we don't need
+ * the extra check.
  */
 static inline int user_mode(struct pt_regs *regs)
 {
@@ -113,12 +115,7 @@ static inline int user_mode(struct pt_regs *regs)
 
 static inline int user_mode_vm(struct pt_regs *regs)
 {
-#ifdef CONFIG_X86_32
-	return ((regs->cs & SEGMENT_RPL_MASK) | (regs->flags & X86_VM_MASK)) >=
-		USER_RPL;
-#else
 	return user_mode(regs);
-#endif
 }
 
 /*
-- 
2.3.0

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