linux-kernel - Re: [PATCH] fs/proc/kcore: reinstate bounce buffer for KCORE

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Message-ID: <b6cb8d7f-f3f3-93c3-3ea0-4c184109a4db@redhat.com>
Date:   Tue, 1 Aug 2023 18:34:26 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Baoquan He <bhe@...hat.com>,
        Uladzislau Rezki <urezki@...il.com>,
        linux-fsdevel@...r.kernel.org, Jiri Olsa <olsajiri@...il.com>,
        Will Deacon <will@...nel.org>, Mike Galbraith <efault@....de>,
        Mark Rutland <mark.rutland@....com>,
        wangkefeng.wang@...wei.com, catalin.marinas@....com,
        ardb@...nel.org,
        Linux regression tracking <regressions@...mhuis.info>,
        regressions@...ts.linux.dev, Matthew Wilcox <willy@...radead.org>,
        Liu Shixin <liushixin2@...wei.com>,
        Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        stable@...r.kernel.org
Subject: Re: [PATCH] fs/proc/kcore: reinstate bounce buffer for KCORE_TEXT
 regions

On 01.08.23 18:33, Lorenzo Stoakes wrote:
> On Tue, Aug 01, 2023 at 11:05:40AM +0200, David Hildenbrand wrote:
>> On 31.07.23 23:50, Lorenzo Stoakes wrote:
>>> Some architectures do not populate the entire range categorised by
>>> KCORE_TEXT, so we must ensure that the kernel address we read from is
>>> valid.
>>>
>>> Unfortunately there is no solution currently available to do so with a
>>> purely iterator solution so reinstate the bounce buffer in this instance so
>>> we can use copy_from_kernel_nofault() in order to avoid page faults when
>>> regions are unmapped.
>>>
>>> This change partly reverts commit 2e1c0170771e ("fs/proc/kcore: avoid
>>> bounce buffer for ktext data"), reinstating the bounce buffer, but adapts
>>> the code to continue to use an iterator.
>>>
>>> Fixes: 2e1c0170771e ("fs/proc/kcore: avoid bounce buffer for ktext data")
>>> Reported-by: Jiri Olsa <olsajiri@...il.com>
>>> Closes: https://lore.kernel.org/all/ZHc2fm+9daF6cgCE@krava
>>> Cc: stable@...r.kernel.org
>>> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
>>> ---
>>>    fs/proc/kcore.c | 26 +++++++++++++++++++++++++-
>>>    1 file changed, 25 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
>>> index 9cb32e1a78a0..3bc689038232 100644
>>> --- a/fs/proc/kcore.c
>>> +++ b/fs/proc/kcore.c
>>> @@ -309,6 +309,8 @@ static void append_kcore_note(char *notes, size_t *i, const char *name,
>>>    static ssize_t read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter)
>>>    {
>>> +	struct file *file = iocb->ki_filp;
>>> +	char *buf = file->private_data;
>>>    	loff_t *fpos = &iocb->ki_pos;
>>>    	size_t phdrs_offset, notes_offset, data_offset;
>>>    	size_t page_offline_frozen = 1;
>>> @@ -554,11 +556,22 @@ static ssize_t read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter)
>>>    			fallthrough;
>>>    		case KCORE_VMEMMAP:
>>>    		case KCORE_TEXT:
>>> +			/*
>>> +			 * Sadly we must use a bounce buffer here to be able to
>>> +			 * make use of copy_from_kernel_nofault(), as these
>>> +			 * memory regions might not always be mapped on all
>>> +			 * architectures.
>>> +			 */
>>> +			if (copy_from_kernel_nofault(buf, (void *)start, tsz)) {
>>> +				if (iov_iter_zero(tsz, iter) != tsz) {
>>> +					ret = -EFAULT;
>>> +					goto out;
>>> +				}
>>>    			/*
>>>    			 * We use _copy_to_iter() to bypass usermode hardening
>>>    			 * which would otherwise prevent this operation.
>>>    			 */
>>
>> Having a comment at this indentation level looks for the else case looks
>> kind of weird.
> 
> Yeah, but having it indented again would be weird and seem like it doesn't
> apply to the block below, there's really no good spot for it and
> checkpatch.pl doesn't mind so I think this is ok :)
> 
>>
>> (does that comment still apply?)
> 
> Hm good point, actually, now we're using the bounce buffer we don't need to
> avoid usermode hardening any more.
> 
> However since we've established a bounce buffer ourselves its still
> appropriate to use _copy_to_iter() as we know the source region is good to
> copy from.
> 
> To make life easy I'll just respin with an updated comment :)

I'm not too picky this time, no need to resend if everybody else is fine :P

-- 
Cheers,

David / dhildenb