Re: [PATCH] x86/entry/64: Fix context tracking state warning when load_gs_index fails

[Andy Lutomirski]

On Mon, Sep 26, 2016 at 4:49 AM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>
>  WARNING: CPU: 0 PID: 3331 at arch/x86/entry/common.c:45 enter_from_user_mode+0x32/0x50
>  CPU: 0 PID: 3331 Comm: ldt_gdt_64 Not tainted 4.8.0-rc7+ #13
>  Call Trace:
>   dump_stack+0x99/0xd0
>   __warn+0xd1/0xf0
>   warn_slowpath_null+0x1d/0x20
>   enter_from_user_mode+0x32/0x50
>   error_entry+0x6d/0xc0
>   ? general_protection+0x12/0x30
>   ? native_load_gs_index+0xd/0x20
>   ? do_set_thread_area+0x19c/0x1f0
>   SyS_set_thread_area+0x24/0x30
>   do_int80_syscall_32+0x7c/0x220
>   entry_INT80_compat+0x38/0x50
>
> This can be reproduced by running the GS testcase of ldt_gdt test unit in
> selftests.
>
> do_int80_syscall_32() will call enter_form_user_mode() to convert context
> tracking state from user state to kernel state. The load_gs_index can fail
> with user gsbase, gsbase will be fixed up and proceed if this happen.
> However, enter_from_user_mode() will be called again in the fixed up path
> though it is context tracking kernel state currently.
>
> This patch fix it by just fixing up gsbase and telling lockdep that IRQs
> are off once load_gs_index failed with user gsbase.
>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
>  arch/x86/entry/entry_64.S | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index d172c61..dc1ec23 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -1045,7 +1045,9 @@ ENTRY(error_entry)
>          * gsbase and proceed.  We'll fix up the exception and land in
>          * .Lgs_change's error handler with kernel gsbase.
>          */
> -       jmp     .Lerror_entry_from_usermode_swapgs
> +       SWAPGS
> +       TRACE_IRQS_OFF

Let's make this more readable: can you change this to:

SWAPGS
jmp .Lerror_entry_done

and remove the .Lerror_entry_from_usermode_swapgs label as well?

Thanks,
Andy



-- 
Andy Lutomirski
AMA Capital Management, LLC