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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 13 Sep 2021 09:45:51 -0700
Message-ID: <CAKwvOd=gO=7MjL9bQR_+xUYCMVNVbsCGV0WCb0hK-rSiJ3kpbA@mail.gmail.com>
Subject: Re: [PATCH] x86/uaccess: Fix 32-bit __get_user_asm_u64() when CC_HAS_ASM_GOTO_OUTPUT=y
To:     Will Deacon <will@kernel.org>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        Bill Wendling <morbo@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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On Mon, Sep 13, 2021 at 9:36 AM Will Deacon <will@kernel.org> wrote:
>
> Commit 865c50e1d279 ("x86/uaccess: utilize CONFIG_CC_HAS_ASM_GOTO_OUTPUT")
> added an optimised version of __get_user_asm() for x86 using 'asm goto'.
>
> Like the non-optimised code, the 32-bit implementation of 64-bit get_user()
> expands to a pair of 32-bit accesses. Unlike the non-optimised code, the
> _original_ pointer is incremented to copy the high word instead of loading
> through a new pointer explicitly constructed to point at a 32-bit type.
> Consequently, if the pointer points at a 64-bit type then we end up
> loading the wrong data for the upper 32-bits.
>
> This was observed as a mount() failure in Android targetting i686 after

s/targetting/targeting/

> b0cfcdd9b967 ("d_path: make 'prepend()' fill up the buffer exactly on
> overflow") because the call to copy_from_kernel_nofault() from
> prepend_copy() ends up in __get_kernel_nofault() and casts the source
> pointer to a 'u64 __user *'. An attempt to mount at "/debug_ramdisk"
> therefore ends up failing trying to mount "/debumdismdisk".
>
> Use the existing '__gu_ptr' source pointer to unsigned int for 32-bit
> __get_user_asm_u64() instead of the original pointer.
>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Bill Wendling <morbo@google.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Reported-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Fixes: 865c50e1d279 ("x86/uaccess: utilize CONFIG_CC_HAS_ASM_GOTO_OUTPUT")
> Signed-off-by: Will Deacon <will@kernel.org>

Sorry I missed this; I think -Wunused-variable would have helped here.
Thanks for debugging+fixing.

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>  arch/x86/include/asm/uaccess.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index c9fa7be3df82..5c95d242f38d 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -301,8 +301,8 @@ do {                                                                        \
>         unsigned int __gu_low, __gu_high;                               \
>         const unsigned int __user *__gu_ptr;                            \
>         __gu_ptr = (const void __user *)(ptr);                          \
> -       __get_user_asm(__gu_low, ptr, "l", "=r", label);                \
> -       __get_user_asm(__gu_high, ptr+1, "l", "=r", label);             \
> +       __get_user_asm(__gu_low, __gu_ptr, "l", "=r", label);           \
> +       __get_user_asm(__gu_high, __gu_ptr+1, "l", "=r", label);        \
>         (x) = ((unsigned long long)__gu_high << 32) | __gu_low;         \
>  } while (0)
>  #else
> --
> 2.33.0.309.g3052b89438-goog
>


-- 
Thanks,
~Nick Desaulniers