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

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Mar 18, 2020 at 04:14:26PM +0000, Vincenzo Frascino wrote:
> On 3/17/20 5:48 PM, Catalin Marinas wrote:
> > On Tue, Mar 17, 2020 at 04:40:48PM +0000, Vincenzo Frascino wrote:
> >> On 3/17/20 3:50 PM, Catalin Marinas wrote:
> >>> On Tue, Mar 17, 2020 at 03:04:01PM +0000, Vincenzo Frascino wrote:
> >>>> On 3/17/20 2:38 PM, Catalin Marinas wrote:
> >>>>> On Tue, Mar 17, 2020 at 12:22:12PM +0000, Vincenzo Frascino wrote:
> >>>>
> >>>> Can TASK_SIZE > UINTPTR_MAX on an arm64 system?
> >>>
> >>> TASK_SIZE yes on arm64 but not TASK_SIZE_32. I was asking about the
> >>> arm32 check where TASK_SIZE < UINTPTR_MAX. How does the vdsotest return
> >>> -EFAULT on arm32? Which code path causes this in the user vdso code?
[...]
> > So clock_gettime() on arm32 always falls back to the syscall?
> 
> This seems not what you asked, and I think I answered accordingly. Anyway, in
> the case of arm32 the error code path is handled via syscall fallback.
> 
> Look at the code below as an example (I am using getres because I know this
> email will be already too long, and I do not want to add pointless code, but the
> concept is the same for gettime and the others):
> 
> static __maybe_unused
> int __cvdso_clock_getres(clockid_t clock, struct __kernel_timespec *res)
> {
> 	int ret = __cvdso_clock_getres_common(clock, res);
> 
> 	if (unlikely(ret))
> 		return clock_getres_fallback(clock, res);
> 	return 0;
> }
> 
> When the return code of the "vdso" internal function returns an error the system
> call is triggered.

But when __cvdso_clock_getres_common() does *not* return an error, it
means that it handled the clock_getres() call without a fallback to the
syscall. I assume this is possible on arm32. When the clock_getres() is
handled directly (not as a syscall), why doesn't arm32 need the same
(res >= TASK_SIZE) check?

> In general arm32 has been ported to the unified vDSO library hence it has a
> proper implementation on par with all the other architectures supported by the
> unified library.

And that's what I do not fully understand. When the call doesn't fall
back to a syscall, why does arm32 and arm64 compat need to differ in the
implementation? I may be missing something here.

> >>> My guess is that on arm32 it only fails with -EFAULT in the syscall
> >>> fallback path since a copy_to_user() would fail the access_ok() check.
> >>> Does it always take the fallback path if ts > TASK_SIZE?
> >>
> >> Correct, it goes via fallback. The return codes for these syscalls are specified
> >> by the ABI [1]. Then I agree with you the way on which arm32 achieves it should
> >> be via access_ok() check.
> > 
> > "it should be" or "it is" on arm32?
[...]
> SYSCALL_DEFINE2(clock_gettime, const clockid_t, which_clock,
> 		struct __kernel_timespec __user *, tp)
[...]
> This is the syscall on which we fallback when the "vdso" internal function
> returns an error. The behavior of the vdso has to be exactly the same of the
> syscall otherwise we end up in an ABI breakage.

I agree. I just don't understand why on arm32 the vdso code doesn't need
to check for tp >= TASK_SIZE while the arm64 compat one does when it
does *not* fall back to a syscall. I understand the syscall fallback
case, that's caused by how we handle access_ok(), but this doesn't
explain the vdso-only case.

> >>>>> This last check needs an explanation. If the clock_id is invalid but res
> >>>>> is not NULL, we allow it. I don't see where the compatibility issue is,
> >>>>> arm32 doesn't have such check.
> >>>>
> >>>> The case that you are describing has to return -EPERM per ABI spec. This case
> >>>> has to return -EINVAL.
> >>>>
> >>>> The first case is taken care from the generic code. But if we don't do this
> >>>> check before on arm64 compat we end up returning the wrong error code.
> >>>
> >>> I guess I have the same question as above. Where does the arm32 code
> >>> return -EINVAL for that case? Did it work correctly before you removed
> >>> the TASK_SIZE_32 check?
> >>
> >> I repeated the test and seems that it was failing even before I removed
> >> TASK_SIZE_32. For reasons I can't explain I did not catch it before.
> >>
> >> The getres syscall should return -EINVAL in the cases specified in [1].
> > 
> > It states 'clk_id specified is not supported on this system'. Fair
> > enough but it doesn't say that it returns -EINVAL only if res == NULL.
> 
> Actually it does, the description of getres() starts with:
> 
> 'The function clock_getres() finds the resolution (precision) of the
> specified clock clk_id, and, if res is *non-NULL*, stores it in the
> struct timespec pointed to by res.'
> 
> Please refer to the system call below of which we mimic the behavior in the vdso
> (kernel/time/posix-timers.c):
> 
> SYSCALL_DEFINE2(clock_getres_time32, clockid_t, which_clock,
> 		struct old_timespec32 __user *, tp)
> {
> 	const struct k_clock *kc = clockid_to_kclock(which_clock);
> 	struct timespec64 ts;
> 	int err;
> 
> 	if (!kc)
> 		return -EINVAL;
> 
> 	err = kc->clock_getres(which_clock, &ts);
> 	if (!err && tp && put_old_timespec32(&ts, tp))
> 		return -EFAULT;
> 
> 	return err;
> }
> 
> If the clock is bogus and res == NULL we are supposed to return -EINVAL and not
> -EFAULT or something else.

If the clock is bogus, the above returns 'err' irrespective of the value
of 'tp'. I presume 'err' is -EINVAL in this case. But there is no
condition that tp == NULL above.

What the above tries to achieve is that if there is no error (err == 0)
and tp != NULL, try to write the timespec to the user tp pointer. If
put_old_timespec32() fails, that's when we return -EFAULT.

> This is what the test is trying to verify. If the check below is not
> in place on arm64 compat, I get error report from the test suite.
> 	if (!VALID_CLOCK_ID(clock_id) && res == NULL)
> 		return -EINVAL;

I really don't get where you deduced that you need to check for res ==
NULL. The above should be:

	if (!VALID_CLOCK_ID(clock_id))
		return -EINVAL;

Furthermore, my assumption is that __cvdso_clock_getres_common() should
handle this case already and we don't need it in the arch vdso code.

> > You also don't explain why __cvdso_clock_getres_time32() doesn't already
> > detect an invalid clk_id on arm64 compat (but does it on arm32).
> > 
> 
> Thanks for asking to me this question, if I would not have spent the day trying
> to explain it, I would not have found a bug in the getres() fallback:
> 
> diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
> index 1dd22da1c3a9..803039d504de 100644
> --- a/arch/arm64/include/asm/unistd.h
> +++ b/arch/arm64/include/asm/unistd.h
> @@ -25,8 +25,8 @@
>  #define __NR_compat_gettimeofday       78
>  #define __NR_compat_sigreturn          119
>  #define __NR_compat_rt_sigreturn       173
> -#define __NR_compat_clock_getres       247
>  #define __NR_compat_clock_gettime      263
> +#define __NR_compat_clock_getres       264
>  #define __NR_compat_clock_gettime64    403
>  #define __NR_compat_clock_getres_time64        406
> 
> In particular compat getres is mis-numbered and that is what causes the issue.
> 
> I am going to add a patch to my v5 that addresses the issue (or probably a
> separate one and cc stable since it fixes a bug) and in this patch I will remove
> the check on VALID_CLOCK_ID.

Please send this as a separate patch that should be merged as a fix, cc
stable.

> I hope that this long email helps you to have a clearer picture of what is going
> on. Please let me know if there is still something missing.

Not entirely. Sorry.

-- 
Catalin

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