[PATCH] btrfs: fix a out-of-boundary access for copy_compressed_data_to_page()

[Qu Wenruo <wqu@suse.com>]

[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 calltrace 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 compressed pages), but also an input parameter, for
the maximum amount of pages we can utilize.

In commit d4088803f511 ("btrfs: subpage: make lzo_compress_pages()
compatible"), the refactor 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 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.

Reported-by: Omar Sandoval <osandov@fb.com>
Fixes: d4088803f511 ("btrfs: subpage: make lzo_compress_pages() compatible")
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/lzo.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/lzo.c b/fs/btrfs/lzo.c
index 00cffc183ec0..f410ceabcdbd 100644
--- a/fs/btrfs/lzo.c
+++ b/fs/btrfs/lzo.c
@@ -125,6 +125,7 @@ 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)
 {
@@ -132,6 +133,9 @@ static int copy_compressed_data_to_page(char *compressed_data,
 	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) {
@@ -195,6 +202,7 @@ 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;
@@ -202,6 +210,7 @@ int lzo_compress_pages(struct list_head *ws, struct address_space *mapping,
 	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;
 
-- 
2.33.1