erofs: fix crafted invalid cases for encoded extents

Robert recently reported two corrupted images that can cause system
crashes, which are related to the new encoded extents introduced
in Linux 6.15:

- The first one [1] has plen != 0 (e.g. plen == 0x2000000) but
(plen & Z_EROFS_EXTENT_PLEN_MASK) == 0. It is used to represent
special extents such as sparse extents (!EROFS_MAP_MAPPED), but
previously only plen == 0 was handled;

- The second one [2] has pa 0xffffffffffdcffed and plen 0xb4000,
then "cur [0xfffffffffffff000] += bvec.bv_len [0x1000]" in
"} while ((cur += bvec.bv_len) < end);" wraps around, causing an
out-of-bound access of pcl->compressed_bvecs[] in
z_erofs_submit_queue(). EROFS only supports 48-bit physical block
addresses (up to 1EiB for 4k blocks), so add a sanity check to
enforce this.

Fixes: 1d191b4ca51d ("erofs: implement encoded extent metadata")
Reported-by: Robert Morris <[e-mail]>
Closes: https://lore.kernel.org/r/75022.1759355830@localhost [1]
Closes: https://lore.kernel.org/r/80524.1760131149@localhost [2]
Reviewed-by: Hongbo Li <[e-mail]>
Signed-off-by: Gao Xiang <[e-mail]>

fs/erofs/zmap.c

···
 			vi->z_fragmentoff = map->m_plen;
 			if (recsz > offsetof(struct z_erofs_extent, pstart_lo))
 				vi->z_fragmentoff |= map->m_pa << 32;
-		} else if (map->m_plen) {
+		} else if (map->m_plen & Z_EROFS_EXTENT_PLEN_MASK) {
 			map->m_flags |= EROFS_MAP_MAPPED |
 				EROFS_MAP_FULL_MAPPED | EROFS_MAP_ENCODED;
 			fmt = map->m_plen >> Z_EROFS_EXTENT_PLEN_FMT_BIT;
···
 				    struct erofs_map_blocks *map)
 {
 	struct erofs_sb_info *sbi = EROFS_I_SB(inode);
+	u64 pend;
 
 	if (!(map->m_flags & EROFS_MAP_ENCODED))
 		return 0;
···
 	if (unlikely(map->m_plen > Z_EROFS_PCLUSTER_MAX_SIZE ||
 		     map->m_llen > Z_EROFS_PCLUSTER_MAX_DSIZE))
 		return -EOPNOTSUPP;
+	/* Filesystems beyond 48-bit physical block addresses are invalid */
+	if (unlikely(check_add_overflow(map->m_pa, map->m_plen, &pend) ||
+		     (pend >> sbi->blkszbits) >= BIT_ULL(48)))
+		return -EFSCORRUPTED;
 	return 0;
 }