linux-kernel - [BUG] arm64: an infinite loop in generic_perform

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Date:   Wed, 23 Jun 2021 10:39:31 +0800
From:   Chen Huang <chenhuang5@...wei.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Al Viro" <viro@...iv.linux.org.uk>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Catalin Marinas" <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
CC:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-mm <linux-mm@...ck.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: [BUG] arm64: an infinite loop in generic_perform_write()

When we access a device memory in userspace, then perform an unaligned write to a file.
For example, we register a uio device and mmap the device, then perform an write to a
file, like that:

	device_addr = mmap(device_fd);
	write(file_fd, device_addr + unaligned_num, size);

We found that the infinite loop happened in generic_perform_write function:

again:
	copied = copy_page_from_iter_atomic(); //copied = 0
	status = ops->write_end(); //status = 0
	if (status == 0)
		goto again;

In copy_page_from_iter_atomic, the copyin() function finally call
__arch_copy_from_user which create an exception table entry for 'insn'.
Then when kernel handles the alignment_fault, it will not panic. As the
arm64 memory model spec said, when the address is not a multiple of the
element size, the access is unaligned. Unaligned accesses are allowed to
addresses marked as Normal, but not to Device regions. An unaligned access
to a Device region will trigger an exception (alignment fault).

do_alignment_fault
    do_bad_area
	__do_kernel_fault
           fixup_exception

But that fixup cann't handle the unaligned copy, so the
copy_page_from_iter_atomic returns 0 and traps in loop.

Reported-by: Chen Huang <chenhuang5@...wei.com>