[PATCH] Fix non-lazy GS handling in sys_vm86()

[PATCH] Fix non-lazy GS handling in sys_vm86()

[Lubomir Rintel]

This fixes a stack corruption panic or null dereference oops due to
a bad GS in resume_userspace() when returning from sys_vm86() and calling
lockdep_sys_exit().

Only a problem when CONFIG_LOCKDEP and CONFIG_CC_STACKPROTECTOR enabled.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
---
 arch/x86/kernel/vm86_32.c |   13 +++++++------
 1 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
index d7ac84e..177d976 100644
--- a/arch/x86/kernel/vm86_32.c
+++ b/arch/x86/kernel/vm86_32.c
@@ -287,10 +287,9 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
 	info->regs.pt.ds = 0;
 	info->regs.pt.es = 0;
 	info->regs.pt.fs = 0;
-
-/* we are clearing gs later just before "jmp resume_userspace",
- * because it is not saved/restored.
- */
+#ifndef CONFIG_X86_32_LAZY_GS
+	info->regs.pt.gs = 0;
+#endif
 
 /*
  * The flags register is also special: we cannot trust that the user
@@ -343,10 +342,12 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
 	__asm__ __volatile__(
 		"movl %0,%%esp\n\t"
 		"movl %1,%%ebp\n\t"
-		"mov  %2, %%gs\n\t"
+#ifdef CONFIG_X86_32_LAZY_GS
+		"mov  $0, %%gs\n\t"
+#endif
 		"jmp resume_userspace"
 		: /* no outputs */
-		:"r" (&info->regs), "r" (task_thread_info(tsk)), "r" (0));
+		:"r" (&info->regs), "r" (task_thread_info(tsk)));
 	/* we never return here */
 }
 

-- 
"Excuse all the blood" -- Dead

Re: [PATCH] Fix non-lazy GS handling in sys_vm86()

[Ingo Molnar]

* Lubomir Rintel <lkundrak@v3.sk> wrote:

> This fixes a stack corruption panic or null dereference oops due to
> a bad GS in resume_userspace() when returning from sys_vm86() and calling
> lockdep_sys_exit().
> 
> Only a problem when CONFIG_LOCKDEP and CONFIG_CC_STACKPROTECTOR enabled.
> 
> Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
> ---
>  arch/x86/kernel/vm86_32.c |   13 +++++++------
>  1 files changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
> index d7ac84e..177d976 100644
> --- a/arch/x86/kernel/vm86_32.c
> +++ b/arch/x86/kernel/vm86_32.c
> @@ -287,10 +287,9 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
>  	info->regs.pt.ds = 0;
>  	info->regs.pt.es = 0;
>  	info->regs.pt.fs = 0;
> -
> -/* we are clearing gs later just before "jmp resume_userspace",
> - * because it is not saved/restored.
> - */
> +#ifndef CONFIG_X86_32_LAZY_GS
> +	info->regs.pt.gs = 0;
> +#endif
>  
>  /*
>   * The flags register is also special: we cannot trust that the user
> @@ -343,10 +342,12 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
>  	__asm__ __volatile__(
>  		"movl %0,%%esp\n\t"
>  		"movl %1,%%ebp\n\t"
> -		"mov  %2, %%gs\n\t"
> +#ifdef CONFIG_X86_32_LAZY_GS
> +		"mov  $0, %%gs\n\t"
> +#endif

That bit looks rather untested - i dont think there's an immediate 
constant instruction variant for segment register moves ...

	Ingo

Re: [PATCH] Fix non-lazy GS handling in sys_vm86()

[Lubomir Rintel]

On Sun, 2009-06-07 at 09:56 +0200, Ingo Molnar wrote:
> * Lubomir Rintel <lkundrak@v3.sk> wrote:

> > -		"mov  %2, %%gs\n\t"
> > +#ifdef CONFIG_X86_32_LAZY_GS
> > +		"mov  $0, %%gs\n\t"
> > +#endif
> 
> That bit looks rather untested - i dont think there's an immediate 
> constant instruction variant for segment register moves ...

Right. Sorry, I only tested (dosbox & svgalib) with stackprotector
enabled. This time I ensured it compiles and runs dosbox & mode3 (from
svgalib) both with and without stackprotector:

>From 8bdb7e27766be7bf2902cf6d6938f3e5868d0aac Mon Sep 17 00:00:00 2001
From: Lubomir Rintel <lkundrak@v3.sk>
Date: Sat, 6 Jun 2009 20:09:17 +0200
Subject: [PATCH] Fix non-lazy GS handling in sys_vm86()

This fixes a stack corruption panic or null dereference oops due to
a bad GS in resume_userspace() when returning from sys_vm86() and calling
lockdep_sys_exit().

Only a problem when CONFIG_LOCKDEP and CONFIG_CC_STACKPROTECTOR enabled.
---
 arch/x86/kernel/vm86_32.c |    9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
index d7ac84e..6a17769 100644
--- a/arch/x86/kernel/vm86_32.c
+++ b/arch/x86/kernel/vm86_32.c
@@ -287,10 +287,9 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
 	info->regs.pt.ds = 0;
 	info->regs.pt.es = 0;
 	info->regs.pt.fs = 0;
-
-/* we are clearing gs later just before "jmp resume_userspace",
- * because it is not saved/restored.
- */
+#ifndef CONFIG_X86_32_LAZY_GS
+	info->regs.pt.gs = 0;
+#endif
 
 /*
  * The flags register is also special: we cannot trust that the user
@@ -343,7 +342,9 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
 	__asm__ __volatile__(
 		"movl %0,%%esp\n\t"
 		"movl %1,%%ebp\n\t"
+#ifdef CONFIG_X86_32_LAZY_GS
 		"mov  %2, %%gs\n\t"
+#endif
 		"jmp resume_userspace"
 		: /* no outputs */
 		:"r" (&info->regs), "r" (task_thread_info(tsk)), "r" (0));
-- 
1.6.2.2

-- 
"Excuse all the blood" -- Dead

[tip:x86/urgent] x86: Fix non-lazy GS handling in sys_vm86()

[tip-bot for Lubomir Rintel]

Commit-ID:  3aa6b186f86c5d06d6d92d14311ffed51f091f40
Gitweb:     http://git.kernel.org/tip/3aa6b186f86c5d06d6d92d14311ffed51f091f40
Author:     Lubomir Rintel <lkundrak@v3.sk>
AuthorDate: Sun, 7 Jun 2009 16:23:48 +0200
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Sun, 7 Jun 2009 16:31:23 +0200

x86: Fix non-lazy GS handling in sys_vm86()

This fixes a stack corruption panic or null dereference oops
due to a bad GS in resume_userspace() when returning from
sys_vm86() and calling lockdep_sys_exit().

Only a problem when CONFIG_LOCKDEP and CONFIG_CC_STACKPROTECTOR
enabled.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Cc: H. Peter Anvin <hpa@zytor.com>
LKML-Reference: <1244384628.2323.4.camel@bimbo>
Signed-off-by: Ingo Molnar <mingo@elte.hu>


---
 arch/x86/kernel/vm86_32.c |    9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
index d7ac84e..6a17769 100644
--- a/arch/x86/kernel/vm86_32.c
+++ b/arch/x86/kernel/vm86_32.c
@@ -287,10 +287,9 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
 	info->regs.pt.ds = 0;
 	info->regs.pt.es = 0;
 	info->regs.pt.fs = 0;
-
-/* we are clearing gs later just before "jmp resume_userspace",
- * because it is not saved/restored.
- */
+#ifndef CONFIG_X86_32_LAZY_GS
+	info->regs.pt.gs = 0;
+#endif
 
 /*
  * The flags register is also special: we cannot trust that the user
@@ -343,7 +342,9 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
 	__asm__ __volatile__(
 		"movl %0,%%esp\n\t"
 		"movl %1,%%ebp\n\t"
+#ifdef CONFIG_X86_32_LAZY_GS
 		"mov  %2, %%gs\n\t"
+#endif
 		"jmp resume_userspace"
 		: /* no outputs */
 		:"r" (&info->regs), "r" (task_thread_info(tsk)), "r" (0));