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Date:   Wed, 15 Jul 2020 17:29:12 +0200
From:   Marco Elver <elver@google.com>
To:     syzbot <syzbot+0f1e470df6a4316e0a11@syzkaller.appspotmail.com>
Cc:     akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, syzkaller-bugs@googlegroups.com,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Will Deacon <will@kernel.org>
Subject: Re: KCSAN: data-race in generic_file_buffered_read /
 generic_file_buffered_read
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On Wed, Jul 15, 2020 at 08:16AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    e9919e11 Merge branch 'for-linus' of git://git.kernel.org/..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1217a83b100000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=570eb530a65cd98e
> dashboard link: https://syzkaller.appspot.com/bug?extid=0f1e470df6a4316e0a11
> compiler:       clang version 11.0.0 (https://github.com/llvm/llvm-project.git ca2dcbd030eadbf0aa9b660efe864ff08af6e18b)
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+0f1e470df6a4316e0a11@syzkaller.appspotmail.com
> 
> ==================================================================
> BUG: KCSAN: data-race in generic_file_buffered_read / generic_file_buffered_read

Our guess is that this is either misuse of an API from userspace, or a
bug. Can someone clarify?

Below are the snippets of code around these accesses.

> write to 0xffff8880968747b0 of 8 bytes by task 6336 on cpu 0:
>  generic_file_buffered_read+0x18be/0x19e0 mm/filemap.c:2246

	...
	would_block:
		error = -EAGAIN;
	out:
		ra->prev_pos = prev_index;
		ra->prev_pos <<= PAGE_SHIFT;
2246)		ra->prev_pos |= prev_offset;

		*ppos = ((loff_t)index << PAGE_SHIFT) + offset;
		file_accessed(filp);
		return written ? written : error;
	}
	EXPORT_SYMBOL_GPL(generic_file_buffered_read);
	...

>  generic_file_read_iter+0x7d/0x3e0 mm/filemap.c:2326
>  ext4_file_read_iter+0x2d6/0x420 fs/ext4/file.c:74
>  call_read_iter include/linux/fs.h:1902 [inline]
>  generic_file_splice_read+0x22a/0x310 fs/splice.c:312
>  do_splice_to fs/splice.c:870 [inline]
>  splice_direct_to_actor+0x2a8/0x660 fs/splice.c:950
>  do_splice_direct+0xf2/0x170 fs/splice.c:1059
>  do_sendfile+0x562/0xb10 fs/read_write.c:1540
>  __do_sys_sendfile64 fs/read_write.c:1601 [inline]
>  __se_sys_sendfile64 fs/read_write.c:1587 [inline]
>  __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1587
>  do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> read to 0xffff8880968747b0 of 8 bytes by task 6334 on cpu 1:
>  generic_file_buffered_read+0x11e/0x19e0 mm/filemap.c:2011

	...
	index = *ppos >> PAGE_SHIFT;
	prev_index = ra->prev_pos >> PAGE_SHIFT;
2011)	prev_offset = ra->prev_pos & (PAGE_SIZE-1);
	last_index = (*ppos + iter->count + PAGE_SIZE-1) >> PAGE_SHIFT;
	offset = *ppos & ~PAGE_MASK;
	...

>  generic_file_read_iter+0x7d/0x3e0 mm/filemap.c:2326
>  ext4_file_read_iter+0x2d6/0x420 fs/ext4/file.c:74
>  call_read_iter include/linux/fs.h:1902 [inline]
>  generic_file_splice_read+0x22a/0x310 fs/splice.c:312
>  do_splice_to fs/splice.c:870 [inline]
>  splice_direct_to_actor+0x2a8/0x660 fs/splice.c:950
>  do_splice_direct+0xf2/0x170 fs/splice.c:1059
>  do_sendfile+0x562/0xb10 fs/read_write.c:1540
>  __do_sys_sendfile64 fs/read_write.c:1601 [inline]
>  __se_sys_sendfile64 fs/read_write.c:1587 [inline]
>  __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1587
>  do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 PID: 6334 Comm: syz-executor.0 Not tainted 5.8.0-rc5-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> ==================================================================

Thanks,
-- Marco