Re: [PATCH 2/3] arm64: syscall.h: Add sign extension handling in syscall_get_return_value for compat

From: Mark Rutland
Date: Wed Apr 21 2021 - 13:41:15 EST


On Fri, Apr 16, 2021 at 03:55:32PM +0800, He Zhe wrote:
> Add sign extension handling in syscall_get_return_value so that it can
> handle 32-bit compatible case and can be used by for example audit, just
> like what syscall_get_error does.

If a compat syscall can ever legitimately return a non-error value with
bit 31 set, and this sign-extends it, is that ever going to reach
userspace as a 64-bit value?

IIUC things like mmap() can return pointers above 2GiB for a compat
task, so I'm a bit uneasy that we'd handle those wrong. I can't see a
way of preventing that unless we keep the upper 32 bits for errors.

Mark.

>
> Signed-off-by: He Zhe <zhe.he@xxxxxxxxxxxxx>
> ---
> arch/arm64/include/asm/syscall.h | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
> index cfc0672013f6..cd7a22787aeb 100644
> --- a/arch/arm64/include/asm/syscall.h
> +++ b/arch/arm64/include/asm/syscall.h
> @@ -44,7 +44,12 @@ static inline long syscall_get_error(struct task_struct *task,
> static inline long syscall_get_return_value(struct task_struct *task,
> struct pt_regs *regs)
> {
> - return regs->regs[0];
> + long val = regs->regs[0];
> +
> + if (is_compat_thread(task_thread_info(task)))
> + val = sign_extend64(val, 31);
> +
> + return val;
> }
>
> static inline void syscall_set_return_value(struct task_struct *task,
> --
> 2.17.1
>