Messages in this thread Patch in this message | | | Date | Thu, 22 Apr 2021 17:55:45 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH 2/3] arm64: syscall.h: Add sign extension handling in syscall_get_return_value for compat |
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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.he@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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