[PATCH] vt_ioctl: fix potential spectre v1 in VT_DISALLOCATE

[PATCH] vt_ioctl: fix potential spectre v1 in VT_DISALLOCATE

[Xiaomeng Tong]

In VT_ACTIVATE an almost identical code path has been patched
with array_index_nospec. In the VT_DISALLOCATE path, the arg is
the user input from a system call argument and lately used as a index
for vc_cons[index].d access, which can be reached through path like
vt_disallocate->vc_busy or vt_disallocate->vc_deallocate.
For consistency both code paths should have the same mitigations
applied.

Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
---
 drivers/tty/vt/vt_ioctl.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
index 580136986..acd53c1c9 100644
--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -898,10 +898,13 @@ int vt_ioctl(struct tty_struct *tty,
 		if (arg > MAX_NR_CONSOLES)
 			return -ENXIO;
 
-		if (arg == 0)
+		if (arg == 0) {
 			vt_disallocate_all();
-		else
-			return vt_disallocate(--arg);
+		} else {
+			--arg;
+			arg = array_index_nospec(arg, MAX_NR_CONSOLES);
+			return vt_disallocate(arg);
+		}
 		break;
 
 	case VT_RESIZE:
-- 
2.17.1

Re: [PATCH] vt_ioctl: fix potential spectre v1 in VT_DISALLOCATE

[Jiri Slaby]

On 22. 02. 22, 4:56, Xiaomeng Tong wrote:
> In VT_ACTIVATE an almost identical code path has been patched
> with array_index_nospec. In the VT_DISALLOCATE path, the arg is
> the user input from a system call argument and lately used as a index
> for vc_cons[index].d access, which can be reached through path like
> vt_disallocate->vc_busy or vt_disallocate->vc_deallocate.
> For consistency both code paths should have the same mitigations
> applied.
> 
> Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
> ---
>   drivers/tty/vt/vt_ioctl.c | 9 ++++++---
>   1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
> index 580136986..acd53c1c9 100644
> --- a/drivers/tty/vt/vt_ioctl.c
> +++ b/drivers/tty/vt/vt_ioctl.c
> @@ -898,10 +898,13 @@ int vt_ioctl(struct tty_struct *tty,
>   		if (arg > MAX_NR_CONSOLES)
>   			return -ENXIO;
>   
> -		if (arg == 0)
> +		if (arg == 0) {
>   			vt_disallocate_all();

Could you add a break; here and remove the else branching completely?

> -		else
> -			return vt_disallocate(--arg);
> +		} else {
> +			--arg;
> +			arg = array_index_nospec(arg, MAX_NR_CONSOLES);
You could simply do:
   arg = array_index_nospec(arg - 1, MAX_NR_CONSOLES);
and remove
   --arg;
completely.

> +			return vt_disallocate(arg);
> +		}
>   		break;

When you do the above, this break is superfluous.

thanks,
-- 
js
suse labs

Re: [PATCH] vt_ioctl: fix potential spectre v1 in VT_DISALLOCATE

[Xiaomeng Tong]

On Tue, 22 Feb 2022 07:56:13 +0100, Jiri Slaby wrote:
> Could you add a break; here and remove the else branching completely?
> ...
> You could simply do:
>   arg = array_index_nospec(arg - 1, MAX_NR_CONSOLES);
> and remove
>    --arg;
> completely.

Already adjusted as suggested in Patch v2, thanks!

Best regards,
--
Xiaomeng Tong