Commit 459ed28f authored Mar 03, 2022 by Theodore Ts'o Committed by Lee Jones Apr 20, 2022

BACKPORT: ext4: don't BUG if someone dirty pages without asking ext4 first

[ Upstream commit cc5095747edfb054ca2068d01af20be3fcc3634f ]

[un]pin_user_pages_remote is dirtying pages without properly warning
the file system in advance.  A related race was noted by Jan Kara in
2018[1]; however, more recently instead of it being a very hard-to-hit
race, it could be reliably triggered by process_vm_writev(2) which was
discovered by Syzbot[2].

This is technically a bug in mm/gup.c, but arguably ext4 is fragile in
that if some other kernel subsystem dirty pages without properly
notifying the file system using page_mkwrite(), ext4 will BUG, while
other file systems will not BUG (although data will still be lost).

So instead of crashing with a BUG, issue a warning (since there may be
potential data loss) and just mark the page as clean to avoid
unprivileged denial of service attacks until the problem can be
properly fixed.  More discussion and background can be found in the
thread starting at [2].

[1] https://lore.kernel.org/linux-mm/20180103100430.GE4911@quack2.suse.cz
[2] https://lore.kernel.org/r/Yg0m6IjcNmfaSokM@google.com



Reported-by:  <syzbot+d59332e2db681cf18f0318a06e994ebbb529a8db@syzkaller.appspotmail.com>
Reported-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/YiDS9wVfq4mM2jGK@mit.edu


Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Change-Id: I894c3ada3eadfb9dce596d07d2ec496d28d0ecfe

parent a894c2e4

fs/ext4/inode.c

+25 −0

Original line number	Diff line number	Diff line
		@@ -2174,6 +2174,15 @@ static int ext4_writepage(struct page *page,
		else
		len = PAGE_SIZE;

		/* Should never happen but for bugs in other kernel subsystems */
		if (!page_has_buffers(page)) {
		ext4_warning_inode(inode,
		"page %lu does not have buffers attached", page->index);
		ClearPageDirty(page);
		unlock_page(page);
		return 0;
		}

		page_bufs = page_buffers(page);
		/*
		* We cannot do block allocation or other extent handling in this
		@@ -2723,6 +2732,22 @@ static int mpage_prepare_extent_to_map(struct mpage_da_data *mpd)
		wait_on_page_writeback(page);
		BUG_ON(PageWriteback(page));

		/*
		* Should never happen but for buggy code in
		* other subsystems that call
		* set_page_dirty() without properly warning
		* the file system first. See [1] for more
		* information.
		*
		* [1] https://lore.kernel.org/linux-mm/20180103100430.GE4911@quack2.suse.cz
		*/
		if (!page_has_buffers(page)) {
		ext4_warning_inode(mpd->inode, "page %lu does not have buffers attached", page->index);
		ClearPageDirty(page);
		unlock_page(page);
		continue;
		}

		if (mpd->map.m_len == 0)
		mpd->first_page = page->index;
		mpd->next_page = page->index + 1;