2019-06-25 19:22:51

by Dianzhang Chen

Subject: [PATCH v2] x86/tls: Fix possible spectre-v1 in do_get_thread_area()

The index to access the threads tls array is controlled by userspace
via syscall: sys_ptrace(), hence leading to a potential exploitation
of the Spectre variant 1 vulnerability.
The idx can be controlled from:
ptrace -> arch_ptrace -> do_get_thread_area.

Fix this by sanitizing idx before using it to index p->thread.tls_array.

Signed-off-by: Dianzhang Chen <[email protected]>
---
arch/x86/kernel/tls.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index a5b802a..424cff5 100644
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -5,6 +5,7 @@
#include <linux/user.h>
#include <linux/regset.h>
#include <linux/syscalls.h>
+#include <linux/nospec.h>

#include <linux/uaccess.h>
#include <asm/desc.h>
@@ -220,6 +221,7 @@ int do_get_thread_area(struct task_struct *p, int idx,
struct user_desc __user *u_info)
{
struct user_desc info;
+ int index;

if (idx == -1 && get_user(idx, &u_info->entry_number))
return -EFAULT;
@@ -227,8 +229,9 @@ int do_get_thread_area(struct task_struct *p, int idx,
if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
return -EINVAL;

- fill_user_desc(&info, idx,
- &p->thread.tls_array[idx - GDT_ENTRY_TLS_MIN]);
+ index = idx - GDT_ENTRY_TLS_MIN;
+
+ fill_user_desc(&info, idx, &p->thread.tls_array[index]);

if (copy_to_user(u_info, &info, sizeof(info)))
return -EFAULT;
--
2.7.4

2019-06-25 19:30:29

by Thomas Gleixner

Subject: Re: [PATCH v2] x86/tls: Fix possible spectre-v1 in do_get_thread_area()

On Wed, 26 Jun 2019, Dianzhang Chen wrote:

> The index to access the threads tls array is controlled by userspace
> via syscall: sys_ptrace(), hence leading to a potential exploitation
> of the Spectre variant 1 vulnerability.
> The idx can be controlled from:
> ptrace -> arch_ptrace -> do_get_thread_area.
>
> Fix this by sanitizing idx before using it to index p->thread.tls_array.

Just that I can't find a place which sanitizes the value....

> +#include <linux/nospec.h>

and nothing which uses anything from this header file.

> #include <linux/uaccess.h>
> #include <asm/desc.h>
> @@ -220,6 +221,7 @@ int do_get_thread_area(struct task_struct *p, int idx,
> struct user_desc __user *u_info)
> {
> struct user_desc info;
> + int index;
>
> if (idx == -1 && get_user(idx, &u_info->entry_number))
> return -EFAULT;
> @@ -227,8 +229,9 @@ int do_get_thread_area(struct task_struct *p, int idx,
> if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
> return -EINVAL;
>
> - fill_user_desc(&info, idx,
> - &p->thread.tls_array[idx - GDT_ENTRY_TLS_MIN]);
> + index = idx - GDT_ENTRY_TLS_MIN;
> +
> + fill_user_desc(&info, idx, &p->thread.tls_array[index]);

So this is just a cosmetic change and the compiler will create probably
exactly the same binary.

Thanks,

tglx

2019-06-26 04:41:15

by Dianzhang Chen

Subject: Re: [PATCH v2] x86/tls: Fix possible spectre-v1 in do_get_thread_area()

On Wed, Jun 26, 2019 at 12:38 AM Thomas Gleixner <[email protected]> wrote:
>
> On Wed, 26 Jun 2019, Dianzhang Chen wrote:
>
> > The index to access the threads tls array is controlled by userspace
> > via syscall: sys_ptrace(), hence leading to a potential exploitation
> > of the Spectre variant 1 vulnerability.
> > The idx can be controlled from:
> > ptrace -> arch_ptrace -> do_get_thread_area.
> >
> > Fix this by sanitizing idx before using it to index p->thread.tls_array.
>
> Just that I can't find a place which sanitizes the value....
>
> > +#include <linux/nospec.h>
>
> and nothing which uses anything from this header file.
>
> > #include <linux/uaccess.h>
> > #include <asm/desc.h>
> > @@ -220,6 +221,7 @@ int do_get_thread_area(struct task_struct *p, int idx,
> > struct user_desc __user *u_info)
> > {
> > struct user_desc info;
> > + int index;
> >
> > if (idx == -1 && get_user(idx, &u_info->entry_number))
> > return -EFAULT;
> > @@ -227,8 +229,9 @@ int do_get_thread_area(struct task_struct *p, int idx,
> > if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
> > return -EINVAL;
> >
> > - fill_user_desc(&info, idx,
> > - &p->thread.tls_array[idx - GDT_ENTRY_TLS_MIN]);
> > + index = idx - GDT_ENTRY_TLS_MIN;
> > +
> > + fill_user_desc(&info, idx, &p->thread.tls_array[index]);
>
> So this is just a cosmetic change and the compiler will create probably
> exactly the same binary.
>
> Thanks,
>
> tglx
>

sorry for being careless, my bad.
And thanks for suggestion, i'll fix that in next version.