[PATCH 5.10.y] erofs: fix the out-of-bounds nameoff handling for trailing dirents

[Sasha Levin <sashal@kernel.org>]

From: Gao Xiang <hsiangkao@linux.alibaba.com>

[ Upstream commit d18a3b5d337fa412a38e776e6b4b857a58836575 ]

Currently we already have boundary-checks for nameoffs, but the trailing
dirents are special since the namelens are calculated with strnlen()
with unchecked nameoffs.

If a crafted EROFS has a trailing dirent with nameoff >= maxsize,
maxsize - nameoff can underflow, causing strnlen() to read past the
directory block.

nameoff0 should also be verified to be a multiple of
`sizeof(struct erofs_dirent)` as well [1].

[1] https://sashiko.dev/#/patchset/20260416063511.3173774-1-hsiangkao%40linux.alibaba.com

Fixes: 3aa8ec716e52 ("staging: erofs: add directory operations")
Fixes: 33bac912840f ("staging: erofs: keep corrupted fs from crashing kernel in erofs_readdir()")
Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Reported-by: Junrui Luo <moonafterrain@outlook.com>
Closes: https://lore.kernel.org/r/A0FD7E0F-7558-49B0-8BC8-EB1ECDB2479A@outlook.com
Cc: stable@vger.kernel.org
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
[ replaced upstream `bsz` with `PAGE_SIZE` and `sizeof(*de)` with `sizeof(struct erofs_dirent)` ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/erofs/dir.c | 30 ++++++++++++++++--------------
 1 file changed, 16 insertions(+), 14 deletions(-)

diff --git a/fs/erofs/dir.c b/fs/erofs/dir.c
index 2776bb832127d..79c83218e9560 100644
--- a/fs/erofs/dir.c
+++ b/fs/erofs/dir.c
@@ -38,20 +38,18 @@ static int erofs_fill_dentries(struct inode *dir, struct dir_context *ctx,
 		nameoff = le16_to_cpu(de->nameoff);
 		de_name = (char *)dentry_blk + nameoff;
 
-		/* the last dirent in the block? */
-		if (de + 1 >= end)
-			de_namelen = strnlen(de_name, maxsize - nameoff);
-		else
+		/* non-trailing dirent in the directory block? */
+		if (de + 1 < end)
 			de_namelen = le16_to_cpu(de[1].nameoff) - nameoff;
+		else if (maxsize <= nameoff)
+			goto err_bogus;
+		else
+			de_namelen = strnlen(de_name, maxsize - nameoff);
 
-		/* a corrupted entry is found */
-		if (nameoff + de_namelen > maxsize ||
-		    de_namelen > EROFS_NAME_LEN) {
-			erofs_err(dir->i_sb, "bogus dirent @ nid %llu",
-				  EROFS_I(dir)->nid);
-			DBG_BUGON(1);
-			return -EFSCORRUPTED;
-		}
+		/* a corrupted entry is found (including negative namelen) */
+		if (!in_range32(de_namelen, 1, EROFS_NAME_LEN) ||
+		    nameoff + de_namelen > maxsize)
+			goto err_bogus;
 
 		debug_one_dentry(d_type, de_name, de_namelen);
 		if (!dir_emit(ctx, de_name, de_namelen,
@@ -63,6 +61,10 @@ static int erofs_fill_dentries(struct inode *dir, struct dir_context *ctx,
 	}
 	*ofs = maxsize;
 	return 0;
+err_bogus:
+	erofs_err(dir->i_sb, "bogus dirent @ nid %llu", EROFS_I(dir)->nid);
+	DBG_BUGON(1);
+	return -EFSCORRUPTED;
 }
 
 static int erofs_readdir(struct file *f, struct dir_context *ctx)
@@ -96,8 +98,8 @@ static int erofs_readdir(struct file *f, struct dir_context *ctx)
 
 		nameoff = le16_to_cpu(de->nameoff);
 
-		if (nameoff < sizeof(struct erofs_dirent) ||
-		    nameoff >= PAGE_SIZE) {
+		if (!nameoff || nameoff >= PAGE_SIZE ||
+		    (nameoff % sizeof(struct erofs_dirent))) {
 			erofs_err(dir->i_sb,
 				  "invalid de[0].nameoff %u @ nid %llu",
 				  nameoff, EROFS_I(dir)->nid);
-- 
2.53.0