[PATCH 1/3] arm64: ptrace: Avoid setting compat FP[SC]R to garbage if get_user fails

[Will Deacon]

On Thu, Jun 29, 2017 at 03:25:47PM +0100, Dave Martin wrote:
> If get_user() fails when reading the new FPSCR value from userspace
> in compat_vfp_get(), then garbage* will be written to the task's
> FPSR and FPCR registers.
> 
> This patch prevents this by checking the return from get_user()
> first.
> 
> [*] Actually, zero, due to the behaviour of get_user() on error, but
> that's still not what userspace expects.

On the other hand, I don't think userspace can expect that if ptrace returns
an error then none of the state has been updated, can it?

Given that we don't propagate the return value from __copy_from_user,
I don't see what we're really fixing here and what userspace can now rely
on that it couldn't rely on before.

Will

> 
> Fixes: 478fcb2cdb23 ("arm64: Debugging support")
> Signed-off-by: Dave Martin <Dave.Martin at arm.com>
> ---
>  arch/arm64/kernel/ptrace.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index 35846f1..4c068dc 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -947,8 +947,10 @@ static int compat_vfp_set(struct task_struct *target,
>  
>  	if (count && !ret) {
>  		ret = get_user(fpscr, (compat_ulong_t *)ubuf);
> -		uregs->fpsr = fpscr & VFP_FPSCR_STAT_MASK;
> -		uregs->fpcr = fpscr & VFP_FPSCR_CTRL_MASK;
> +		if (!ret) {
> +			uregs->fpsr = fpscr & VFP_FPSCR_STAT_MASK;
> +			uregs->fpcr = fpscr & VFP_FPSCR_CTRL_MASK;
> +		}
>  	}
>  
>  	fpsimd_flush_task_state(target);
> -- 
> 2.1.4
>