btrfs: fix a out-of-bound access in copy_compressed_data_to_page()

[BUG]
The following script can cause btrfs to crash:

  $ mount -o compress-force=lzo $DEV /mnt
  $ dd if=/dev/urandom of=/mnt/foo bs=4k count=1
  $ sync

The call trace looks like this:

  general protection fault, probably for non-canonical address 0xe04b37fccce3b000: 0000 [#1] PREEMPT SMP NOPTI
  CPU: 5 PID: 164 Comm: kworker/u20:3 Not tainted 5.15.0-rc7-custom+ #4
  Workqueue: btrfs-delalloc btrfs_work_helper [btrfs]
  RIP: 0010:__memcpy+0x12/0x20
  Call Trace:
   lzo_compress_pages+0x236/0x540 [btrfs]
   btrfs_compress_pages+0xaa/0xf0 [btrfs]
   compress_file_range+0x431/0x8e0 [btrfs]
   async_cow_start+0x12/0x30 [btrfs]
   btrfs_work_helper+0xf6/0x3e0 [btrfs]
   process_one_work+0x294/0x5d0
   worker_thread+0x55/0x3c0
   kthread+0x140/0x170
   ret_from_fork+0x22/0x30
  ---[ end trace 63c3c0f131e61982 ]---

[CAUSE]
In lzo_compress_pages(), parameter @out_pages is not only an output
parameter (for the number of compressed pages), but also an input
parameter, as the upper limit of compressed pages we can utilize.

In commit d408880 ("btrfs: subpage: make lzo_compress_pages()
compatible"), the refactoring doesn't take @out_pages as an input, thus
completely ignoring the limit.

And for compress-force case, we could hit incompressible data that
compressed size would go beyond the page limit, and cause the above
crash.

[FIX]
Save @out_pages as @max_nr_page, and pass it to lzo_compress_pages(),
and check if we're beyond the limit before accessing the pages.

Note: this also fixes crash on 32bit architectures that was suspected to
be caused by merge of btrfs patches to 5.16-rc1. Reported in
https://lore.kernel.org/all/[email protected]/ .

Reported-by: Omar Sandoval <[email protected]>
Fixes: d408880 ("btrfs: subpage: make lzo_compress_pages() compatible")
Reviewed-by: Omar Sandoval <[email protected]>
Reviewed-by: Josef Bacik <[email protected]>
Signed-off-by: Qu Wenruo <[email protected]>
Reviewed-by: David Sterba <[email protected]>
[ add note ]
Signed-off-by: David Sterba <[email protected]>

fs/btrfs/lzo.c

@@ -125,13 +125,17 @@ static inline size_t read_compress_length(const char *buf)
 static int copy_compressed_data_to_page(char *compressed_data,
 					size_t compressed_size,
 					struct page **out_pages,
+					unsigned long max_nr_page,
 					u32 *cur_out,
 					const u32 sectorsize)
 {
 	u32 sector_bytes_left;
 	u32 orig_out;
 	struct page *cur_page;
 
+	if ((*cur_out / PAGE_SIZE) >= max_nr_page)
+		return -E2BIG;
+
 	/*
 	 * We never allow a segment header crossing sector boundary, previous
 	 * run should ensure we have enough space left inside the sector.
@@ -158,6 +162,9 @@ static int copy_compressed_data_to_page(char *compressed_data,
 		u32 copy_len = min_t(u32, sectorsize - *cur_out % sectorsize,
 				     orig_out + compressed_size - *cur_out);
 
+		if ((*cur_out / PAGE_SIZE) >= max_nr_page)
+			return -E2BIG;
+
 		cur_page = out_pages[*cur_out / PAGE_SIZE];
 		/* Allocate a new page */
 		if (!cur_page) {
@@ -195,13 +202,15 @@ int lzo_compress_pages(struct list_head *ws, struct address_space *mapping,
 	struct workspace *workspace = list_entry(ws, struct workspace, list);
 	const u32 sectorsize = btrfs_sb(mapping->host->i_sb)->sectorsize;
 	struct page *page_in = NULL;
+	const unsigned long max_nr_page = *out_pages;
 	int ret = 0;
 	/* Points to the file offset of input data */
 	u64 cur_in = start;
 	/* Points to the current output byte */
 	u32 cur_out = 0;
 	u32 len = *total_out;
 
+	ASSERT(max_nr_page > 0);
 	*out_pages = 0;
 	*total_out = 0;
 	*total_in = 0;
@@ -237,7 +246,8 @@ int lzo_compress_pages(struct list_head *ws, struct address_space *mapping,
 		}
 
 		ret = copy_compressed_data_to_page(workspace->cbuf, out_len,
-						   pages, &cur_out, sectorsize);
+						   pages, max_nr_page,
+						   &cur_out, sectorsize);
 		if (ret < 0)
 			goto out;