Patch "ext4: don't BUG if someone dirty pages without asking ext4 first" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

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

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     ext4-don-t-bug-if-someone-dirty-pages-without-asking.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 00ba817730141657d34605d239c25aa1ec426542
Author: Theodore Ts'o <tytso@xxxxxxx>
Date:   Thu Mar 3 09:38:47 2022 -0500

    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@xxxxxxxxxxxxxx
    [2] https://lore.kernel.org/r/Yg0m6IjcNmfaSokM@xxxxxxxxxx
    
    Reported-by: syzbot+d59332e2db681cf18f0318a06e994ebbb529a8db@xxxxxxxxxxxxxxxxxxxxxxxxx
    Reported-by: Lee Jones <lee.jones@xxxxxxxxxx>
    Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
    Link: https://lore.kernel.org/r/YiDS9wVfq4mM2jGK@xxxxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 22a5140546fb..fff52292c01e 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1992,6 +1992,15 @@ 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
@@ -2595,6 +2604,22 @@ 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@xxxxxxxxxxxxxx
+			 */
+			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;