Re: [PATCH] x86: fix possible spectre-v1 in ptrace_get_debugreg()

[Thomas Gleixner]

On Fri, 24 May 2019, Dianzhang Chen wrote:

> Subject : [PATCH] x86: fix possible spectre-v1 in ptrace_get_debugreg()

Please use the proper prefix. Run git log on the file and you'll find
it. Also please start the short summary sentence after the prefix with an
upper case letter.

> The n in ptrace_get_debugreg() is indirectly controlled by userspace via syscall: ptrace(defined in kernel/ptrace.c), hence leading to a potential exploitation of the Spectre variant 1 vulnerability.
> The n can be controlled from: ptrace -> arch_ptrace -> ptrace_get_debugreg.
>

Please format the text proper with a line break around column 70.

Also please refrain from '(defined in kernel/ptrace.c)'. Use sys_ptrace()
which is entirely clear.

> Fix this by sanitizing n before using it to index thread->ptrace_bps.
> 
> Signed-off-by: Dianzhang Chen <dianzhangchen0@xxxxxxxxx>
> ---
>  arch/x86/kernel/ptrace.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
> index 4b8ee05..3f8f158 100644
> --- a/arch/x86/kernel/ptrace.c
> +++ b/arch/x86/kernel/ptrace.c
> @@ -24,6 +24,7 @@
>  #include <linux/rcupdate.h>
>  #include <linux/export.h>
>  #include <linux/context_tracking.h>
> +#include <linux/nospec.h>
>  
>  #include <linux/uaccess.h>
>  #include <asm/pgtable.h>
> @@ -644,7 +645,8 @@ static unsigned long ptrace_get_debugreg(struct task_struct *tsk, int n)
>  	unsigned long val = 0;
>  
>  	if (n < HBP_NUM) {
> -		struct perf_event *bp = thread->ptrace_bps[n];
> +		struct perf_event *bp =
> +			thread->ptrace_bps[array_index_nospec(n, HBP_NUM)];

Please use an intermediate variable to calculate the index instead of this
weird line break.

Thanks,

	tglx