On Wed, Apr 21, 2021 at 06:41:05PM +0100, Mark Rutland wrote:
> 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.
Looking at this with fresh eyes, I think we can more closely mirror
syscall_get_error(), and do something like:
static inline long syscall_get_return_value(struct task_struct *task,
struct pt_regs *regs)
{
long val = regs->regs[0];
long error = val;
if (is_compat_thread(task_thread_info(task)))
error = sign_extend64(error, 31);
return IS_ERR_VALUE(error) ? error : val;
}
Thanks,
Mark.
>
> Mark.
>
> >
> > Signed-off-by: He Zhe <zhe...@windriver.com>
> > ---
> > 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
> >
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