Re: [PATCH v4 18/26] arm64: vdso32: Replace TASK_SIZE_32 check in vgettimeofday

[Will Deacon <will@kernel.org>]

On Thu, Mar 19, 2020 at 04:58:00PM +0000, Vincenzo Frascino wrote:
> On 3/19/20 3:49 PM, Andy Lutomirski wrote:
> > On Tue, Mar 17, 2020 at 7:38 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
> >>
> >> On Tue, Mar 17, 2020 at 12:22:12PM +0000, Vincenzo Frascino wrote:
> >>> diff --git a/arch/arm64/kernel/vdso32/vgettimeofday.c b/arch/arm64/kernel/vdso32/vgettimeofday.c
> >>> index 54fc1c2ce93f..91138077b073 100644
> >>> --- a/arch/arm64/kernel/vdso32/vgettimeofday.c
> >>> +++ b/arch/arm64/kernel/vdso32/vgettimeofday.c
> >>> @@ -8,11 +8,14 @@
> >>>  #include <linux/time.h>
> >>>  #include <linux/types.h>
> >>>
> >>> +#define VALID_CLOCK_ID(x) \
> >>> +     ((x >= 0) && (x < VDSO_BASES))
> >>> +
> >>>  int __vdso_clock_gettime(clockid_t clock,
> >>>                        struct old_timespec32 *ts)
> >>>  {
> >>>       /* The checks below are required for ABI consistency with arm */
> >>> -     if ((u32)ts >= TASK_SIZE_32)
> >>> +     if ((u32)ts > UINTPTR_MAX - sizeof(*ts) + 1)
> >>>               return -EFAULT;
> >>>
> >>>       return __cvdso_clock_gettime32(clock, ts);
> >>
> >> I probably miss something but I can't find the TASK_SIZE check in the
> >> arch/arm/vdso/vgettimeofday.c code. Is this done elsewhere?
> >>
> > 
> > Can you not just remove the TASK_SIZE_32 check entirely?  If you pass
> > a garbage address to the vDSO, you are quite likely to get SIGSEGV.
> > Why does this particular type of error need special handling?
> > 
> 
> In this particular case the system call and the vDSO library as it stands
> returns -EFAULT on all the architectures that support the vdso library except on
> arm64 compat. The reason why it does not return the correct error code, as I was
> discussing with Catalin, it is because arm64 uses USER_DS (addr_limit set
> happens in arch/arm64/kernel/entry.S), which is defined as (1 << VA_BITS), as
> access_ok() validation even on compat tasks and since arm64 supports up to 52bit
> VA, this does not detect the end of the user address space for a 32 bit task.
> Hence when we fall back on the system call we get the wrong error code out of it.
> 
> According to me to have ABI consistency we need this check, but if we say that
> we can make an ABI exception in this case, I am fine with that either if it has
> enough consensus.
> 
> Please let me know your thoughts.

I don't agree with your reasoning -- letting the thing SEGV is perfectly
fine and we don't need to perform additional checking in userspace here.
If you treat the vDSO more as being part of libc then part of the kernel
then I think it makes perfect sense.

There are other system calls that will SEGV in libc if they are passed dodgy
pointers before the kernel has a chance to return -EFAULT.

Will