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    Subject[PATCH 5.4 289/475] ext4: dont BUG if someone dirty pages without asking ext4 first
    Date
    From: Theodore Ts'o <tytso@mit.edu>

    [ 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>
    ---
    fs/ext4/inode.c | 25 +++++++++++++++++++++++++
    1 file changed, 25 insertions(+)

    diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
    index dcbd8ac8d471..0d62f05f8925 100644
    --- a/fs/ext4/inode.c
    +++ b/fs/ext4/inode.c
    @@ -2161,6 +2161,15 @@ 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
    @@ -2710,6 +2719,22 @@ 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;
    --
    2.34.1


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