[PATCH AUTOSEL 4.4 27/35] btrfs: subpage: check if there are compressed extents inside one page

[PATCH AUTOSEL 4.4 27/35] btrfs: subpage: check if there are compressed extents inside one page

[Sasha Levin]

From: Qu Wenruo <wqu@suse.com>

[ Upstream commit 3670e6451bc9c39ab3a46f1da19360219e4319f3 ]

[BUG]
When testing experimental subpage compressed write support, it hits a
NULL pointer dereference inside read path:

 Unable to handle kernel NULL pointer dereference at virtual address 0000000000000018
 pc : __pi_memcmp+0x28/0x1ec
 lr : check_data_csum+0xd0/0x274 [btrfs]
 Call trace:
  __pi_memcmp+0x28/0x1ec
  btrfs_verify_data_csum+0xf4/0x244 [btrfs]
  end_bio_extent_readpage+0x1d0/0x6b0 [btrfs]
  bio_endio+0x15c/0x1dc
  end_workqueue_fn+0x44/0x64 [btrfs]
  btrfs_work_helper+0x74/0x250 [btrfs]
  process_one_work+0x1d4/0x47c
  worker_thread+0x180/0x400
  kthread+0x11c/0x120
  ret_from_fork+0x10/0x30
 Code: 54000261 d100044c d343fd8c f8408403 (f8408424)
 ---[ end trace 9e2c59f33ea40866 ]---

[CAUSE]
When reading two compressed extents inside the same page, like the
following layout, we trigger above crash:

	0	32K	64K
	|-------|\\\\\\\|
	     |	     \- Compressed extent (A)
	     \--------- Compressed extent (B)

For compressed read, we don't need to populate its io_bio->csum, as we
rely on compressed_bio->csum to verify the compressed data, and then
copy the decompressed to inode pages.

Normally btrfs_verify_data_csum() skip such page by checking and
clearing its PageChecked flag

But since that flag is still for the full page, when endio for inode
page range [0, 32K) gets executed, it clears PageChecked flag for the
full page.

Then when endio for inode page range [32K, 64K) gets executed, since the
page no longer has PageChecked flag, it just continues checking, even
though io_bio->csum is NULL.

[FIX]
Thankfully there are only two users of PageChecked bit:

- Cow fixup
  Since subpage has its own way to trace page dirty (dirty_bitmap) and
  ordered bit (ordered_bitmap), it should never trigger cow fixup.

- Compressed read
  We can distinguish such read by just checking io_bio->csum.

So just check io_bio->csum before doing the verification to avoid such
NULL pointer dereference.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/btrfs/inode.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index da4ad006739d..8a2dc96b62b4 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -3146,6 +3146,20 @@ static int btrfs_readpage_end_io_hook(struct btrfs_io_bio *io_bio,
 		return 0;
 	}
 
+	/*
+	 * For subpage case, above PageChecked is not safe as it's not subpage
+	 * compatible.
+	 * But for now only cow fixup and compressed read utilize PageChecked
+	 * flag, while in this context we can easily use io_bio->csum to
+	 * determine if we really need to do csum verification.
+	 *
+	 * So for now, just exit if io_bio->csum is NULL, as it means it's
+	 * compressed read, and its compressed data csum has already been
+	 * verified.
+	 */
+	if (io_bio->csum == NULL)
+		return 0;
+
 	if (BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM)
 		return 0;
 
-- 
2.30.2

[PATCH AUTOSEL 4.4 28/35] btrfs: subpage: fix race between prepare_pages() and btrfs_releasepage()

[Sasha Levin]

From: Qu Wenruo <wqu@suse.com>

[ Upstream commit e0467866198f7f536806f39e5d0d91ae8018de08 ]

[BUG]
When running generic/095, there is a high chance to crash with subpage
data RW support:

 assertion failed: PagePrivate(page) && page->private
 ------------[ cut here ]------------
 kernel BUG at fs/btrfs/ctree.h:3403!
 Internal error: Oops - BUG: 0 [#1] SMP
 CPU: 1 PID: 3567 Comm: fio Tainted: 5.12.0-rc7-custom+ #17
 Hardware name: Khadas VIM3 (DT)
 Call trace:
  assertfail.constprop.0+0x28/0x2c [btrfs]
  btrfs_subpage_assert+0x80/0xa0 [btrfs]
  btrfs_subpage_set_uptodate+0x34/0xec [btrfs]
  btrfs_page_clamp_set_uptodate+0x74/0xa4 [btrfs]
  btrfs_dirty_pages+0x160/0x270 [btrfs]
  btrfs_buffered_write+0x444/0x630 [btrfs]
  btrfs_direct_write+0x1cc/0x2d0 [btrfs]
  btrfs_file_write_iter+0xc0/0x160 [btrfs]
  new_sync_write+0xe8/0x180
  vfs_write+0x1b4/0x210
  ksys_pwrite64+0x7c/0xc0
  __arm64_sys_pwrite64+0x24/0x30
  el0_svc_common.constprop.0+0x70/0x140
  do_el0_svc+0x28/0x90
  el0_svc+0x2c/0x54
  el0_sync_handler+0x1a8/0x1ac
  el0_sync+0x170/0x180
 Code: f0000160 913be042 913c4000 955444bc (d4210000)
 ---[ end trace 3fdd39f4cccedd68 ]---

[CAUSE]
Although prepare_pages() calls find_or_create_page(), which returns the
page locked, but in later prepare_uptodate_page() calls, we may call
btrfs_readpage() which will unlock the page before it returns.

This leaves a window where btrfs_releasepage() can sneak in and release
the page, clearing page->private and causing above ASSERT().

[FIX]
In prepare_uptodate_page(), we should not only check page->mapping, but
also PagePrivate() to ensure we are still holding the correct page which
has proper fs context setup.

Reported-by: Ritesh Harjani <riteshh@linux.ibm.com>
Tested-by: Ritesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/btrfs/file.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 2426dc56426f..c917a04118cc 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1307,7 +1307,18 @@ static int prepare_uptodate_page(struct inode *inode,
 			unlock_page(page);
 			return -EIO;
 		}
-		if (page->mapping != inode->i_mapping) {
+
+		/*
+		 * Since btrfs_readpage() will unlock the page before it
+		 * returns, there is a window where btrfs_releasepage() can
+		 * be called to release the page.
+		 * Here we check both inode mapping and PagePrivate() to
+		 * make sure the page was not released.
+		 *
+		 * The private flag check is essential for subpage as we need
+		 * to store extra bitmap using page->private.
+		 */
+		if (page->mapping != inode->i_mapping || !PagePrivate(page)) {
 			unlock_page(page);
 			return -EAGAIN;
 		}
-- 
2.30.2

Re: [PATCH AUTOSEL 4.4 27/35] btrfs: subpage: check if there are compressed extents inside one page

[David Sterba]

On Thu, Sep 09, 2021 at 08:01:08AM -0400, Sasha Levin wrote:
> From: Qu Wenruo <wqu@suse.com>
> 
> [ Upstream commit 3670e6451bc9c39ab3a46f1da19360219e4319f3 ]

Please drop this patch from stable queue, thanks.

Re: [PATCH AUTOSEL 4.4 28/35] btrfs: subpage: fix race between prepare_pages() and btrfs_releasepage()

[David Sterba]

On Thu, Sep 09, 2021 at 08:01:09AM -0400, Sasha Levin wrote:
> From: Qu Wenruo <wqu@suse.com>
> 
> [ Upstream commit e0467866198f7f536806f39e5d0d91ae8018de08 ]

Please drop this patch from stable queue, thanks.