Re: [REGRESSION] mm: process_vm_readv testcase no longer works after compat_prcoess_vm_readv removed

From: Jens Axboe <axboe@kernel.dk>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Kyle Huey <me@kylehuey.com>,
	open list <linux-kernel@vger.kernel.org>,
	Christoph Hellwig <hch@lst.de>,
	Robert O'Callahan <robert@ocallahan.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Arnd Bergmann <arnd@arndb.de>,
	David Howells <dhowells@redhat.com>,
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	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>,
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	linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
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	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [REGRESSION] mm: process_vm_readv testcase no longer works after compat_prcoess_vm_readv removed
Date: Mon, 26 Oct 2020 18:09:06 -0600	[thread overview]
Message-ID: <0127a542-3f93-7bd0-e00d-4a0e49846c8f@kernel.dk> (raw)
In-Reply-To: <20201027000521.GD3576660@ZenIV.linux.org.uk>

On 10/26/20 6:05 PM, Al Viro wrote:
> On Mon, Oct 26, 2020 at 05:56:11PM -0600, Jens Axboe wrote:
>> On 10/26/20 4:55 PM, Kyle Huey wrote:
>>> A test program from the rr[0] test suite, vm_readv_writev[1], no
>>> longer works on 5.10-rc1 when compiled as a 32 bit binary and executed
>>> on a 64 bit kernel. The first process_vm_readv call (on line 35) now
>>> fails with EFAULT. I have bisected this to
>>> c3973b401ef2b0b8005f8074a10e96e3ea093823.
>>>
>>> It should be fairly straightforward to extract the test case from our
>>> repository into a standalone program.
>>
>> Can you check with this applied?
>>
>> diff --git a/mm/process_vm_access.c b/mm/process_vm_access.c
>> index fd12da80b6f2..05676722d9cd 100644
>> --- a/mm/process_vm_access.c
>> +++ b/mm/process_vm_access.c
>> @@ -273,7 +273,8 @@ static ssize_t process_vm_rw(pid_t pid,
>>  		return rc;
>>  	if (!iov_iter_count(&iter))
>>  		goto free_iov_l;
>> -	iov_r = iovec_from_user(rvec, riovcnt, UIO_FASTIOV, iovstack_r, false);
>> +	iov_r = iovec_from_user(rvec, riovcnt, UIO_FASTIOV, iovstack_r,
>> +				in_compat_syscall());
> 
> _ouch_
> 
> There's a bug, all right, but I'm not sure that this is all there is
> to it. For now it's probably the right fix, but...  Consider the fun
> trying to use that from 32bit process to access the memory of 64bit
> one.  IOW, we might want to add an explicit flag for "force 64bit
> addresses/sizes in rvec".

Ouch yes good point, nice catch.

-- 
Jens Axboe