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Message-ID: <debf5db0-d4b8-5fdd-f798-b7b41e316342@arm.com>
Date:   Thu, 7 Apr 2022 16:53:41 +0100
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Subject: Re: [RFC PATCH -next V2 7/7] arm64: add pagecache reading to machine
 check safe
Content-Language: en-GB
To:     Tong Tiangen <tongtiangen@huawei.com>,
        Mark Rutland <mark.rutland@arm.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
References: <20220406091311.3354723-1-tongtiangen@huawei.com>
 <20220406091311.3354723-8-tongtiangen@huawei.com>
 <Yk15Fex1+fg6ZQrX@FVFF77S0Q05N>
 <afd5285b-9d8b-3c58-111e-095d1e5b74f8@huawei.com>
From:   Robin Murphy <robin.murphy@arm.com>
In-Reply-To: <afd5285b-9d8b-3c58-111e-095d1e5b74f8@huawei.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk

On 2022-04-07 15:56, Tong Tiangen wrote:
> 
> 
> 在 2022/4/6 19:27, Mark Rutland 写道:
>> On Wed, Apr 06, 2022 at 09:13:11AM +0000, Tong Tiangen wrote:
>>> When user process reading file, the data is cached in pagecache and
>>> the data belongs to the user process, When machine check error is
>>> encountered during pagecache reading, killing the user process and
>>> isolate the user page with hardware memory errors is a more reasonable
>>> choice than kernel panic.
>>>
>>> The __arch_copy_mc_to_user() in copy_to_user_mc.S is largely borrows
>>> from __arch_copy_to_user() in copy_to_user.S and the main difference
>>> is __arch_copy_mc_to_user() add the extable entry to support machine
>>> check safe.
>>
>> As with prior patches, *why* is the distinction necessary?
>>
>> This patch adds a bunch of conditional logic, but *structurally* it 
>> doesn't
>> alter the handling to be substantially different for the MC and non-MC 
>> cases.
>>
>> This seems like pointless duplication that just makes it harder to 
>> maintain
>> this code.
>>
>> Thanks,
>> Mark.
> 
> Agreed, The implementation here looks a little ugly and harder to maintain.
> 
> The purpose of my doing this is not all copy_to_user can be recovered.
> 
> A memory error is consumed when reading pagecache using copy_to_user.
> I think in this scenario, only the process is affected because it can't 
> read
> pagecache data correctly. Just kill the process and don't need the whole
> kernel panic.
> 
> So I need two different copy_to_user implementation, one is existing 
> __arch_copy_to_user,
> this function will panic when consuming memory errors. The other one is 
> this new helper
> __arch_copy_mc_to_user, this interface is used when reading pagecache. 
> It can recover from
> consume memory error.

OK, but do we really need two almost-identical implementations of every 
function where the only difference is how the exception table entries 
are annotated? Could the exception handler itself just figure out who 
owns the page where the fault occurred and decide what action to take as 
appropriate?

Robin.

> 
> In future, if find a scenario use copy_to_user can also be recovered, we 
> can also use this mc
> safe helper instead it.
> 
> Thanks,
> Tong.
> 
>>
>>> In _copy_page_to_iter(), machine check safe only considered ITER_IOVEC
>>> which is used by pagecache reading.
>>>
>>> Signed-off-by: Tong Tiangen <tongtiangen@huawei.com>
>>> ---
>>>   arch/arm64/include/asm/uaccess.h | 15 ++++++
>>>   arch/arm64/lib/Makefile          |  2 +-
>>>   arch/arm64/lib/copy_to_user_mc.S | 78 +++++++++++++++++++++++++++++
>>>   include/linux/uio.h              |  9 +++-
>>>   lib/iov_iter.c                   | 85 +++++++++++++++++++++++++-------
>>>   5 files changed, 170 insertions(+), 19 deletions(-)
>>>   create mode 100644 arch/arm64/lib/copy_to_user_mc.S
>>>
>>> diff --git a/arch/arm64/include/asm/uaccess.h 
>>> b/arch/arm64/include/asm/uaccess.h
>>> index 24b662407fbd..f0d5e811165a 100644
>>> --- a/arch/arm64/include/asm/uaccess.h
>>> +++ b/arch/arm64/include/asm/uaccess.h
>>> @@ -448,6 +448,21 @@ extern long strncpy_from_user(char *dest, const 
>>> char __user *src, long count);
>>>   extern __must_check long strnlen_user(const char __user *str, long n);
>>> +#ifdef CONFIG_ARCH_HAS_COPY_MC
>>> +extern unsigned long __must_check __arch_copy_mc_to_user(void __user 
>>> *to,
>>> +                             const void *from, unsigned long n);
>>> +static inline unsigned long __must_check
>>> +copy_mc_to_user(void __user *to, const void *from, unsigned long n)
>>> +{
>>> +    uaccess_ttbr0_enable();
>>> +    n = __arch_copy_mc_to_user(__uaccess_mask_ptr(to), from, n);
>>> +    uaccess_ttbr0_disable();
>>> +
>>> +    return n;
>>> +}
>>> +#define copy_mc_to_user copy_mc_to_user
>>> +#endif
>>> +
>>>   #ifdef CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE
>>>   struct page;
>>>   void memcpy_page_flushcache(char *to, struct page *page, size_t 
>>> offset, size_t len);
>>> diff --git a/arch/arm64/lib/Makefile b/arch/arm64/lib/Makefile
>>> index 29c578414b12..9b3571227fb4 100644
>>> --- a/arch/arm64/lib/Makefile
>>> +++ b/arch/arm64/lib/Makefile
>>> @@ -23,4 +23,4 @@ obj-$(CONFIG_ARM64_MTE) += mte.o
>>>   obj-$(CONFIG_KASAN_SW_TAGS) += kasan_sw_tags.o
>>> -obj-$(CONFIG_ARCH_HAS_CPY_MC) += copy_page_mc.o
>>> +obj-$(CONFIG_ARCH_HAS_COPY_MC) += copy_page_mc.o copy_to_user_mc.o
>>> diff --git a/arch/arm64/lib/copy_to_user_mc.S 
>>> b/arch/arm64/lib/copy_to_user_mc.S
>>> new file mode 100644
>>> index 000000000000..9d228ff15446
>>> --- /dev/null
>>> +++ b/arch/arm64/lib/copy_to_user_mc.S
>>> @@ -0,0 +1,78 @@
>>> +/* SPDX-License-Identifier: GPL-2.0-only */
>>> +/*
>>> + * Copyright (C) 2012 ARM Ltd.
>>> + */
>>> +
>>> +#include <linux/linkage.h>
>>> +
>>> +#include <asm/asm-uaccess.h>
>>> +#include <asm/assembler.h>
>>> +#include <asm/cache.h>
>>> +
>>> +/*
>>> + * Copy to user space from a kernel buffer (alignment handled by the 
>>> hardware)
>>> + *
>>> + * Parameters:
>>> + *    x0 - to
>>> + *    x1 - from
>>> + *    x2 - n
>>> + * Returns:
>>> + *    x0 - bytes not copied
>>> + */
>>> +    .macro ldrb1 reg, ptr, val
>>> +    1000: ldrb  \reg, [\ptr], \val;
>>> +    _asm_extable_mc 1000b, 9998f;
>>> +    .endm
>>> +
>>> +    .macro strb1 reg, ptr, val
>>> +    user_ldst_mc 9998f, sttrb, \reg, \ptr, \val
>>> +    .endm
>>> +
>>> +    .macro ldrh1 reg, ptr, val
>>> +    1001: ldrh  \reg, [\ptr], \val;
>>> +    _asm_extable_mc 1001b, 9998f;
>>> +    .endm
>>> +
>>> +    .macro strh1 reg, ptr, val
>>> +    user_ldst_mc 9997f, sttrh, \reg, \ptr, \val
>>> +    .endm
>>> +
>>> +    .macro ldr1 reg, ptr, val
>>> +    1002: ldr \reg, [\ptr], \val;
>>> +    _asm_extable_mc 1002b, 9998f;
>>> +    .endm
>>> +
>>> +    .macro str1 reg, ptr, val
>>> +    user_ldst_mc 9997f, sttr, \reg, \ptr, \val
>>> +    .endm
>>> +
>>> +    .macro ldp1 reg1, reg2, ptr, val
>>> +    1003: ldp \reg1, \reg2, [\ptr], \val;
>>> +    _asm_extable_mc 1003b, 9998f;
>>> +    .endm
>>> +
>>> +    .macro stp1 reg1, reg2, ptr, val
>>> +    user_stp 9997f, \reg1, \reg2, \ptr, \val
>>> +    .endm
>>> +
>>> +end    .req    x5
>>> +srcin    .req    x15
>>> +SYM_FUNC_START(__arch_copy_mc_to_user)
>>> +    add    end, x0, x2
>>> +    mov    srcin, x1
>>> +#include "copy_template.S"
>>> +    mov    x0, #0
>>> +    ret
>>> +
>>> +    // Exception fixups
>>> +9997:    cbz    x0, 9998f            // Check machine check exception
>>> +    cmp    dst, dstin
>>> +    b.ne    9998f
>>> +    // Before being absolutely sure we couldn't copy anything, try 
>>> harder
>>> +    ldrb    tmp1w, [srcin]
>>> +USER(9998f, sttrb tmp1w, [dst])
>>> +    add    dst, dst, #1
>>> +9998:    sub    x0, end, dst            // bytes not copied
>>> +    ret
>>> +SYM_FUNC_END(__arch_copy_mc_to_user)
>>> +EXPORT_SYMBOL(__arch_copy_mc_to_user)
>>> diff --git a/include/linux/uio.h b/include/linux/uio.h
>>> index 739285fe5a2f..539d9ee9b032 100644
>>> --- a/include/linux/uio.h
>>> +++ b/include/linux/uio.h
>>> @@ -147,10 +147,17 @@ size_t _copy_to_iter(const void *addr, size_t 
>>> bytes, struct iov_iter *i);
>>>   size_t _copy_from_iter(void *addr, size_t bytes, struct iov_iter *i);
>>>   size_t _copy_from_iter_nocache(void *addr, size_t bytes, struct 
>>> iov_iter *i);
>>> +#ifdef CONFIG_ARCH_HAS_COPY_MC
>>> +size_t copy_mc_page_to_iter(struct page *page, size_t offset, size_t 
>>> bytes,
>>> +                struct iov_iter *i);
>>> +#else
>>> +#define copy_mc_page_to_iter copy_page_to_iter
>>> +#endif
>>> +
>>>   static inline size_t copy_folio_to_iter(struct folio *folio, size_t 
>>> offset,
>>>           size_t bytes, struct iov_iter *i)
>>>   {
>>> -    return copy_page_to_iter(&folio->page, offset, bytes, i);
>>> +    return copy_mc_page_to_iter(&folio->page, offset, bytes, i);
>>>   }
>>>   static __always_inline __must_check
>>> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
>>> index 6dd5330f7a99..2c5f3bb6391d 100644
>>> --- a/lib/iov_iter.c
>>> +++ b/lib/iov_iter.c
>>> @@ -157,6 +157,19 @@ static int copyout(void __user *to, const void 
>>> *from, size_t n)
>>>       return n;
>>>   }
>>> +#ifdef CONFIG_ARCH_HAS_COPY_MC
>>> +static int copyout_mc(void __user *to, const void *from, size_t n)
>>> +{
>>> +    if (access_ok(to, n)) {
>>> +        instrument_copy_to_user(to, from, n);
>>> +        n = copy_mc_to_user((__force void *) to, from, n);
>>> +    }
>>> +    return n;
>>> +}
>>> +#else
>>> +#define copyout_mc copyout
>>> +#endif
>>> +
>>>   static int copyin(void *to, const void __user *from, size_t n)
>>>   {
>>>       if (should_fail_usercopy())
>>> @@ -169,7 +182,7 @@ static int copyin(void *to, const void __user 
>>> *from, size_t n)
>>>   }
>>>   static size_t copy_page_to_iter_iovec(struct page *page, size_t 
>>> offset, size_t bytes,
>>> -             struct iov_iter *i)
>>> +             struct iov_iter *i, bool mc_safe)
>>>   {
>>>       size_t skip, copy, left, wanted;
>>>       const struct iovec *iov;
>>> @@ -194,7 +207,10 @@ static size_t copy_page_to_iter_iovec(struct 
>>> page *page, size_t offset, size_t b
>>>           from = kaddr + offset;
>>>           /* first chunk, usually the only one */
>>> -        left = copyout(buf, from, copy);
>>> +        if (mc_safe)
>>> +            left = copyout_mc(buf, from, copy);
>>> +        else
>>> +            left = copyout(buf, from, copy);
>>>           copy -= left;
>>>           skip += copy;
>>>           from += copy;
>>> @@ -204,7 +220,10 @@ static size_t copy_page_to_iter_iovec(struct 
>>> page *page, size_t offset, size_t b
>>>               iov++;
>>>               buf = iov->iov_base;
>>>               copy = min(bytes, iov->iov_len);
>>> -            left = copyout(buf, from, copy);
>>> +            if (mc_safe)
>>> +                left = copyout_mc(buf, from, copy);
>>> +            else
>>> +                left = copyout(buf, from, copy);
>>>               copy -= left;
>>>               skip = copy;
>>>               from += copy;
>>> @@ -223,7 +242,10 @@ static size_t copy_page_to_iter_iovec(struct 
>>> page *page, size_t offset, size_t b
>>>       kaddr = kmap(page);
>>>       from = kaddr + offset;
>>> -    left = copyout(buf, from, copy);
>>> +    if (mc_safe)
>>> +        left = copyout_mc(buf, from, copy);
>>> +    else
>>> +        left = copyout(buf, from, copy);
>>>       copy -= left;
>>>       skip += copy;
>>>       from += copy;
>>> @@ -232,7 +254,10 @@ static size_t copy_page_to_iter_iovec(struct 
>>> page *page, size_t offset, size_t b
>>>           iov++;
>>>           buf = iov->iov_base;
>>>           copy = min(bytes, iov->iov_len);
>>> -        left = copyout(buf, from, copy);
>>> +        if (mc_safe)
>>> +            left = copyout_mc(buf, from, copy);
>>> +        else
>>> +            left = copyout(buf, from, copy);
>>>           copy -= left;
>>>           skip = copy;
>>>           from += copy;
>>> @@ -674,15 +699,6 @@ size_t _copy_to_iter(const void *addr, size_t 
>>> bytes, struct iov_iter *i)
>>>   EXPORT_SYMBOL(_copy_to_iter);
>>>   #ifdef CONFIG_ARCH_HAS_COPY_MC
>>> -static int copyout_mc(void __user *to, const void *from, size_t n)
>>> -{
>>> -    if (access_ok(to, n)) {
>>> -        instrument_copy_to_user(to, from, n);
>>> -        n = copy_mc_to_user((__force void *) to, from, n);
>>> -    }
>>> -    return n;
>>> -}
>>> -
>>>   static size_t copy_mc_pipe_to_iter(const void *addr, size_t bytes,
>>>                   struct iov_iter *i)
>>>   {
>>> @@ -846,10 +862,10 @@ static inline bool page_copy_sane(struct page 
>>> *page, size_t offset, size_t n)
>>>   }
>>>   static size_t __copy_page_to_iter(struct page *page, size_t offset, 
>>> size_t bytes,
>>> -             struct iov_iter *i)
>>> +             struct iov_iter *i, bool mc_safe)
>>>   {
>>>       if (likely(iter_is_iovec(i)))
>>> -        return copy_page_to_iter_iovec(page, offset, bytes, i);
>>> +        return copy_page_to_iter_iovec(page, offset, bytes, i, 
>>> mc_safe);
>>>       if (iov_iter_is_bvec(i) || iov_iter_is_kvec(i) || 
>>> iov_iter_is_xarray(i)) {
>>>           void *kaddr = kmap_local_page(page);
>>>           size_t wanted = _copy_to_iter(kaddr + offset, bytes, i);
>>> @@ -878,7 +894,7 @@ size_t copy_page_to_iter(struct page *page, 
>>> size_t offset, size_t bytes,
>>>       offset %= PAGE_SIZE;
>>>       while (1) {
>>>           size_t n = __copy_page_to_iter(page, offset,
>>> -                min(bytes, (size_t)PAGE_SIZE - offset), i);
>>> +                min(bytes, (size_t)PAGE_SIZE - offset), i, false);
>>>           res += n;
>>>           bytes -= n;
>>>           if (!bytes || !n)
>>> @@ -893,6 +909,41 @@ size_t copy_page_to_iter(struct page *page, 
>>> size_t offset, size_t bytes,
>>>   }
>>>   EXPORT_SYMBOL(copy_page_to_iter);
>>> +#ifdef CONFIG_ARCH_HAS_COPY_MC
>>> +/**
>>> + * copy_mc_page_to_iter - copy page to iter with source memory error 
>>> exception handling.
>>> + *
>>> + * The filemap_read deploys this for pagecache reading and the main 
>>> differences between
>>> + * this and typical copy_page_to_iter() is call __copy_page_to_iter 
>>> with mc_safe true.
>>> + *
>>> + * Return: number of bytes copied (may be %0)
>>> + */
>>> +size_t copy_mc_page_to_iter(struct page *page, size_t offset, size_t 
>>> bytes,
>>> +             struct iov_iter *i)
>>> +{
>>> +    size_t res = 0;
>>> +
>>> +    if (unlikely(!page_copy_sane(page, offset, bytes)))
>>> +        return 0;
>>> +    page += offset / PAGE_SIZE; // first subpage
>>> +    offset %= PAGE_SIZE;
>>> +    while (1) {
>>> +        size_t n = __copy_page_to_iter(page, offset,
>>> +                min(bytes, (size_t)PAGE_SIZE - offset), i, true);
>>> +        res += n;
>>> +        bytes -= n;
>>> +        if (!bytes || !n)
>>> +            break;
>>> +        offset += n;
>>> +        if (offset == PAGE_SIZE) {
>>> +            page++;
>>> +            offset = 0;
>>> +        }
>>> +    }
>>> +    return res;
>>> +}
>>> +#endif
>>> +
>>>   size_t copy_page_from_iter(struct page *page, size_t offset, size_t 
>>> bytes,
>>>                struct iov_iter *i)
>>>   {
>>> -- 
>>> 2.18.0.huawei.25
>>>
>>>
>>> _______________________________________________
>>> linux-arm-kernel mailing list
>>> linux-arm-kernel@lists.infradead.org
>>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>> .
> 
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