From: Theodore Ts'o <[email protected]>
[ 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/[email protected]
[2] https://lore.kernel.org/r/[email protected]
Reported-by: syzbot+d59332e2db681cf18f0318a06e994ebbb529a8db@syzkaller.appspotmail.com
Reported-by: Lee Jones <[email protected]>
Signed-off-by: Theodore Ts'o <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sasha Levin <[email protected]>
---
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/[email protected]
+ */
+ 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
> From: Theodore Ts'o <[email protected]>
>
> [ 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/[email protected]
> [2] https://lore.kernel.org/r/[email protected]
>
> Reported-by: syzbot+d59332e2db681cf18f0318a06e994ebbb529a8db@syzkaller.appspotmail.com
> Reported-by: Lee Jones <[email protected]>
> Signed-off-by: Theodore Ts'o <[email protected]>
> Link: https://lore.kernel.org/r/[email protected]
> Signed-off-by: Sasha Levin <[email protected]>
> ---
> 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/[email protected]
> + */
> + 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 [email protected] address
but the HASH does not correspond to any known bug.
Please double check the address.