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 <20211030031832.165457-3-liaochang1@huawei.com> <87ee83goju.fsf@disp2133>
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From:   =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn.topel@gmail.com>
Date:   Sun, 31 Oct 2021 12:14:33 +0100
Message-ID: <CAJ+HfNjaBQrBtOuMvTccbb2K-Ua=T1eZk0+70hp0_aOAc83A-Q@mail.gmail.com>
Subject: Re: [PATCH 2/3] RISC-V: use memcpy for kexec_file mode
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Liao Chang <liaochang1@huawei.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Nick Kossifidis <mick@ics.forth.gr>, jszhang@kernel.org,
        guoren@linux.alibaba.com, Pekka Enberg <penberg@kernel.org>,
        sunnanyong@huawei.com, wangkefeng.wang@huawei.com,
        changbin.du@intel.com, Alex Ghiti <alex@ghiti.fr>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, kexec@lists.infradead.org
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On Sat, 30 Oct 2021 at 05:51, Eric W. Biederman <ebiederm@xmission.com> wro=
te:
>
> Liao Chang <liaochang1@huawei.com> writes:
>
> > The pointer to buffer loading kernel binaries is in kernel space for
> > kexec_fil mode, When copy_from_user copies data from pointer to a block
> > of memory, it checkes that the pointer is in the user space range, on
> > RISCV-V that is:
> >
> > static inline bool __access_ok(unsigned long addr, unsigned long size)
> > {
> >       return size <=3D TASK_SIZE && addr <=3D TASK_SIZE - size;
> > }
> >
> > and TASK_SIZE is 0x4000000000 for 64-bits, which now causes
> > copy_from_user to reject the access of the field 'buf' of struct
> > kexec_segment that is in range [CONFIG_PAGE_OFFSET - VMALLOC_SIZE,
> > CONFIG_PAGE_OFFSET), is invalid user space pointer.
> >
> > This patch fixes this issue by skipping access_ok(), use mempcy() inste=
ad.
>
> I am a bit confused.
>
> Why is machine_kexec ever calling copy_from_user?  That seems wrong in
> all cases.
>

It's not machine_kexec -- it's machine_kexec_prepare, which pulls out
the FDT from the image. It looks like MIPS does it similarly.

(Caveat: I might be confused as well! ;-))


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