Commit 1aff297f authored Dec 07, 2025 by Qu Wenruo Committed by David Sterba Jan 06, 2026

btrfs: avoid access-beyond-folio for bs > ps encoded writes



[POTENTIAL BUG]
If the system page size is 4K and fs block size is 8K, and max_inline
mount option is set to 6K, we can inline a 6K sized data extent.

Then a encoded write submitted a compressed extent which is at file
offset 0, and the compressed length is 6K, which is allowed to be inlined.

Now a read beyond page boundary is triggered inside write_extent_buffer()
from insert_inline_extent().

[CAUSE]
Currently the function __cow_file_range_inline() can only accept a
single folio.

For regular compressed write path, we always allocate the compressed
folios using the minimal order matching the block size, thus the
@compressed_folio should always cover a full fs block thus it is fine.

But for encoded writes, they allocate page size folios, this means we
can hit a case where the compressed data is smaller than block size but
still larger than page size, in that case __cow_file_range_inline() will
be called with @compressed_size larger than a page.

In that case we will trigger a read beyond the folio inside
insert_inline_extent().

Thankfully this is not that common, as the default max_inline is only
2048 bytes, smaller than PAGE_SIZE, and bs > ps support is still
experimental.

[FIX]
We need to either allow insert_inline_extent() to accept a page array to
properly support such case, or reject such inline extent.

The latter is a much simpler solution, and considering bs > ps will stay
as a corner case and non-default max_inline will be even rarer, I don't
think we really need to fulfill such niche.

So just reject any inline extent that's larger than PAGE_SIZE, and add
an extra ASSERT() to insert_inline_extent() to catch such beyond-boundary
access.

Fixes: ec207990 ("btrfs: enable encoded read/write/send for bs > ps cases")
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

parent c1c050f9

fs/btrfs/inode.c

+18 −4

Original line number	Diff line number	Diff line
		@@ -480,13 +480,15 @@ static int insert_inline_extent(struct btrfs_trans_handle *trans,
		ASSERT(size <= sectorsize);

		/*
		* The compressed size also needs to be no larger than a sector.
		* That's also why we only need one page as the parameter.
		* The compressed size also needs to be no larger than a page.
		* That's also why we only need one folio as the parameter.
		*/
		if (compressed_folio)
		if (compressed_folio) {
		ASSERT(compressed_size <= sectorsize);
		else
		ASSERT(compressed_size <= PAGE_SIZE);
		} else {
		ASSERT(compressed_size == 0);
		}

		if (compressed_size && compressed_folio)
		cur_size = compressed_size;
		@@ -573,6 +575,18 @@ static bool can_cow_file_range_inline(struct btrfs_inode *inode,
		if (offset != 0)
		return false;

		/*
		* Even for bs > ps cases, cow_file_range_inline() can only accept a
		* single folio.
		*
		* This can be problematic and cause access beyond page boundary if a
		* page sized folio is passed into that function.
		* And encoded write is doing exactly that.
		* So here limits the inlined extent size to PAGE_SIZE.
		*/
		if (size > PAGE_SIZE \|\| compressed_size > PAGE_SIZE)
		return false;

		/* Inline extents are limited to sectorsize. */
		if (size > fs_info->sectorsize)
		return false;