[PATCH v2] erofs: fix inconsistent per-file compression format

[Gao Xiang]

EROFS can select compression algorithms on a per-file basis, and each
per-file compression algorithm needs to be marked in the on-disk
superblock for initialization.

However, syzkaller can generate inconsistent crafted images that use
an unsupported algorithm for specific inodes; thus, an unexpected
"BUG: kernel NULL pointer dereference" can be raised.

Fix this by checking against `sbi->available_compr_algs` for each
compressed inode.  Incorrect !erofs_sb_has_compr_cfgs preset bitmap
is now fixed together since it was harmless previously.

Reported-by: <bugreport@xxxxxxxxxxxxxx>
Fixes: 14373711dd54 ("erofs: add on-disk compression configurations")
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>
---
change since v1:
 - fix left-shift overflow (undefined behavior).

 fs/erofs/decompressor.c | 2 +-
 fs/erofs/zmap.c         | 9 ++++++---
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/fs/erofs/decompressor.c b/fs/erofs/decompressor.c
index 021be5feb1bc..af98e88908ee 100644
--- a/fs/erofs/decompressor.c
+++ b/fs/erofs/decompressor.c
@@ -398,7 +398,7 @@ int z_erofs_parse_cfgs(struct super_block *sb, struct erofs_super_block *dsb)
 	int size, ret = 0;
 
 	if (!erofs_sb_has_compr_cfgs(sbi)) {
-		sbi->available_compr_algs = Z_EROFS_COMPRESSION_LZ4;
+		sbi->available_compr_algs = 1 << Z_EROFS_COMPRESSION_LZ4;
 		return z_erofs_load_lz4_config(sb, dsb, NULL, 0);
 	}
 
diff --git a/fs/erofs/zmap.c b/fs/erofs/zmap.c
index 7b55111fd533..6f10bc8bbb1c 100644
--- a/fs/erofs/zmap.c
+++ b/fs/erofs/zmap.c
@@ -578,6 +578,7 @@ static int z_erofs_fill_inode_lazy(struct inode *inode)
 {
 	struct erofs_inode *const vi = EROFS_I(inode);
 	struct super_block *const sb = inode->i_sb;
+	struct erofs_sb_info *sbi = EROFS_SB(sb);
 	int err, headnr;
 	erofs_off_t pos;
 	struct erofs_buf buf = __EROFS_BUF_INITIALIZER;
@@ -624,10 +625,12 @@ static int z_erofs_fill_inode_lazy(struct inode *inode)
 
 	headnr = 0;
 	if (vi->z_algorithmtype[0] >= Z_EROFS_COMPRESSION_MAX ||
-	    vi->z_algorithmtype[++headnr] >= Z_EROFS_COMPRESSION_MAX) {
-		erofs_err(sb, "unknown HEAD%u format %u for nid %llu, please upgrade kernel",
+	    !(sbi->available_compr_algs & (1 << vi->z_algorithmtype[0])) ||
+	    vi->z_algorithmtype[++headnr] >= Z_EROFS_COMPRESSION_MAX ||
+	    !(sbi->available_compr_algs & (1 << vi->z_algorithmtype[headnr]))) {
+		erofs_err(sb, "inconsistent HEAD%u algorithm format %u for nid %llu",
 			  headnr + 1, vi->z_algorithmtype[headnr], vi->nid);
-		err = -EOPNOTSUPP;
+		err = -EFSCORRUPTED;
 		goto out_put_metabuf;
 	}
 
-- 
2.39.3