> From: Theodore Ts'o <tytso@xxxxxxx> > > [ 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> > --- > fs/ext4/inode.c | 25 +++++++++++++++++++++++++ > 1 file changed, 25 insertions(+) > > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c > index 7959aae4857e..96cf0f57ca95 100644 > --- a/fs/ext4/inode.c > +++ b/fs/ext4/inode.c > @@ -2148,6 +2148,15 @@ 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 > @@ -2697,6 +2706,22 @@ 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; > -- > 2.34.1 > > > I see the command but can't find the corresponding bug. The email is sent to syzbot+HASH@xxxxxxxxxxxxxxxxxxxxxxxxx address but the HASH does not correspond to any known bug. Please double check the address.
