linux-kernel - Re: [PATCH v4 18/26] arm64: vdso32: Replace TASK_SIZE

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Date:   Tue, 17 Mar 2020 17:48:06 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        linux-mips@...r.kernel.org, x86@...nel.org,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Russell King <linux@...linux.org.uk>,
        Paul Burton <paul.burton@...s.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Collingbourne <pcc@...gle.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Andrei Vagin <avagin@...nvz.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Mark Rutland <Mark.Rutland@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v4 18/26] arm64: vdso32: Replace TASK_SIZE_32 check in
 vgettimeofday

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?
> 
> Sorry I got confused because you referred to arch/arm/vdso/vgettimeofday.c which
> is the arm64 implementation, not the compat one :)

You figured out (in your subsequent reply) that I was indeed talking
about arm32 ;).

> In the case of arm32 everything is handled via syscall fallback.

So clock_gettime() on arm32 always falls back to the syscall?

> > 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?

If, on arm32, clock_gettime() is (would be?) handled in the vdso
entirely, who checks for the pointer outside the accessible address
space (as per the clock_gettime man page)?

I'm fine with such check as long as it is consistent across arm32 and
arm64 compat. Or even on arm64 native between syscall fallback and vdso
execution. I haven't figured out yet whether this is the 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.
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).

-- 
Catalin