[PATCH v2 3/7] block: take i_rwsem for the splice read path

[Tal Zussman <tz2294@columbia.edu>]

def_blk_fops wires ->splice_read directly to filemap_splice_read(),
which allocates folios based on mapping_min_folio_order() without any
lock against set_blocksize(). A splice from a block device can race
set_blocksize() raising the minimum folio order and insert a folio that
is too small for the mapping. blkdev_read_iter() wraps filemap_read()
in inode_lock_shared() for this reason, but the splice path was missed.

Splicing from a block device while toggling the block size between 512
bytes and 64K with BLKBSZSET hits this within seconds on a
CONFIG_DEBUG_VM kernel:

  page dumped because: VM_BUG_ON_FOLIO(folio_order(folio) < mapping_min_folio_order(mapping))
  kernel BUG at mm/filemap.c:858!
  Oops: invalid opcode: 0000 [#1] SMP NOPTI
  RIP: 0010:__filemap_add_folio+0x51c/0x570
  Call Trace:
   filemap_add_folio+0x64/0x140
   page_cache_ra_order+0x1dd/0x3d0
   filemap_get_pages+0x153/0x760
   filemap_splice_read+0x13f/0x300
   splice_file_to_pipe+0xc0/0xd0
   do_splice+0x6a8/0x890
   __do_splice+0xb0/0x210
   __x64_sys_splice+0x80/0x100
   do_syscall_64+0x10e/0x520
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

Take inode_lock_shared() around filemap_splice_read(), like the read
path does.

Fixes: 3c20917120ce ("block/bdev: enable large folio support for large logical block sizes")
Assisted-by: Claude:claude-fable-5
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 block/fops.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/block/fops.c b/block/fops.c
index d5f569333f46..a51814821100 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -855,6 +855,22 @@ static ssize_t blkdev_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	return ret;
 }
 
+/*
+ * Take i_rwsem to avoid racing with set_blocksize changing i_blkbits/folio
+ * order and punching out the pagecache.
+ */
+static ssize_t blkdev_splice_read(struct file *in, loff_t *ppos,
+		struct pipe_inode_info *pipe, size_t len, unsigned int flags)
+{
+	struct inode *bd_inode = bdev_file_inode(in);
+	ssize_t ret;
+
+	inode_lock_shared(bd_inode);
+	ret = filemap_splice_read(in, ppos, pipe, len, flags);
+	inode_unlock_shared(bd_inode);
+	return ret;
+}
+
 #define	BLKDEV_FALLOC_FL_SUPPORTED					\
 		(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE |		\
 		 FALLOC_FL_ZERO_RANGE | FALLOC_FL_WRITE_ZEROES)
@@ -956,7 +972,7 @@ const struct file_operations def_blk_fops = {
 #ifdef CONFIG_COMPAT
 	.compat_ioctl	= compat_blkdev_ioctl,
 #endif
-	.splice_read	= filemap_splice_read,
+	.splice_read	= blkdev_splice_read,
 	.splice_write	= iter_file_splice_write,
 	.fallocate	= blkdev_fallocate,
 	.uring_cmd	= blkdev_uring_cmd,

-- 
2.39.5