[PATCH v2] erofs: limit the level of fs stacking for file-backed mounts

[Gao Xiang <hsiangkao@linux.alibaba.com>]

Otherwise, it could cause potential kernel stack overflow (e.g., EROFS
mounting itself).

Reviewed-by: Sheng Yong <shengyong1@xiaomi.com>
Fixes: fb176750266a ("erofs: add file-backed mount support")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
---
Change since v1:
 - Return -ENOTBLK instead of -EINVAL since userspace tools like
   util-linux will fall back to using loop to mount again.

   Don't use -ELOOP compared to other stacked fses, since -ENOTBLK is
   more suitable: it means the kernel can't handle it anymore.

 fs/erofs/super.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index f3f8d8c066e4..2db534f76464 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -639,6 +639,22 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 
 	sbi->blkszbits = PAGE_SHIFT;
 	if (!sb->s_bdev) {
+		/*
+		 * (File-backed mounts) EROFS claims it's safe to nest other
+		 * fs contexts (including its own) due to self-controlled RO
+		 * accesses/contexts and no side-effect changes that need to
+		 * context save & restore so it can reuse the current thread
+		 * context.  However, it still needs to bump `s_stack_depth` to
+		 * avoid kernel stack overflow from nested filesystems.
+		 */
+		if (erofs_is_fileio_mode(sbi)) {
+			sb->s_stack_depth =
+				file_inode(sbi->dif0.file)->i_sb->s_stack_depth + 1;
+			if (sb->s_stack_depth > FILESYSTEM_MAX_STACK_DEPTH) {
+				erofs_err(sb, "maximum fs stacking depth exceeded");
+				return -ENOTBLK;
+			}
+		}
 		sb->s_blocksize = PAGE_SIZE;
 		sb->s_blocksize_bits = PAGE_SHIFT;
 
-- 
2.43.5