erofs: fix blksize < PAGE_SIZE for file-backed mounts

JIRA: https://issues.redhat.com/browse/RHEL-82421
Upstream status: Linus

commit bae0854
Author: Hongzhen Luo <hongzhen@linux.alibaba.com>
Date:   Tue Oct 15 18:38:36 2024 +0800

    erofs: fix blksize < PAGE_SIZE for file-backed mounts

    Adjust sb->s_blocksize{,_bits} directly for file-backed
    mounts when the fs block size is smaller than PAGE_SIZE.

    Previously, EROFS used sb_set_blocksize(), which caused
    a panic if bdev-backed mounts is not used.

    Fixes: fb17675 ("erofs: add file-backed mount support")
    Signed-off-by: Hongzhen Luo <hongzhen@linux.alibaba.com>
    Link: https://lore.kernel.org/r/20241015103836.3757438-1-hongzhen@linux.alibaba.com
    Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

CVE: CVE-2024-56750
Signed-off-by: Ian Kent <ikent@redhat.com>

Author: Ian Kent

fs/erofs/super.c

@@ -631,7 +631,11 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 			errorfc(fc, "unsupported blksize for fscache mode");
 			return -EINVAL;
 		}
-		if (!sb_set_blocksize(sb, 1 << sbi->blkszbits)) {
+
+		if (erofs_is_fileio_mode(sbi)) {
+			sb->s_blocksize = 1 << sbi->blkszbits;
+			sb->s_blocksize_bits = sbi->blkszbits;
+		} else if (!sb_set_blocksize(sb, 1 << sbi->blkszbits)) {
 			errorfc(fc, "failed to set erofs blksize");
 			return -EINVAL;
 		}