linux-kernel - [PATCH v2] vfat: don't read garbage after last dirent

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Message-Id: <20181216231510.26854-1-mcroce@redhat.com>
Date:   Mon, 17 Dec 2018 00:15:10 +0100
From:   Matteo Croce <mcroce@...hat.com>
To:     OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:     Timothy Redaelli <tredaelli@...hat.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: [PATCH v2] vfat: don't read garbage after last dirent

The FAT32 File System Specification[1] states that:

    If DIR_Name[0] == 0x00, then the directory entry is free, and there
    are no allocated directory entries after this one.

    The special 0 value, indicates to FAT file system driver code that
    the rest of the entries in this directory do not need to be examined
    because they are all free.

This is not enforced by Linux, and is possible to read garbage if not
all dirents after the last one are filled with zeroes.

[1] http://download.microsoft.com/download/1/6/1/161ba512-40e2-4cc9-843a-923143f3456c/fatgen103.doc

Reported-by: Timothy Redaelli <tredaelli@...hat.com>
Signed-off-by: Matteo Croce <mcroce@...hat.com>
---
v2:
* add the check also in lookup and dir empty check
* fix two tipos in the commit message  

 fs/fat/dir.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/fat/dir.c b/fs/fat/dir.c
index c8366cb8eccd..955edf5df286 100644
--- a/fs/fat/dir.c
+++ b/fs/fat/dir.c
@@ -588,7 +588,7 @@ static int __fat_readdir(struct inode *inode, struct file *file,
 
 	bh = NULL;
 get_new:
-	if (fat_get_entry(inode, &cpos, &bh, &de) == -1)
+	if (fat_get_entry(inode, &cpos, &bh, &de) == -1 || !de->name[0])
 		goto end_of_dir;
 parse_record:
 	nr_slots = 0;
@@ -916,7 +916,8 @@ int fat_dir_empty(struct inode *dir)
 
 	bh = NULL;
 	cpos = 0;
-	while (fat_get_short_entry(dir, &cpos, &bh, &de) >= 0) {
+	while (fat_get_short_entry(dir, &cpos, &bh, &de) >= 0 &&
+	       de->name[0]) {
 		if (strncmp(de->name, MSDOS_DOT   , MSDOS_NAME) &&
 		    strncmp(de->name, MSDOS_DOTDOT, MSDOS_NAME)) {
 			result = -ENOTEMPTY;
@@ -961,7 +962,7 @@ int fat_scan(struct inode *dir, const unsigned char *name,
 	sinfo->slot_off = 0;
 	sinfo->bh = NULL;
 	while (fat_get_short_entry(dir, &sinfo->slot_off, &sinfo->bh,
-				   &sinfo->de) >= 0) {
+				   &sinfo->de) >= 0 && sinfo->de->name[0]) {
 		if (!strncmp(sinfo->de->name, name, MSDOS_NAME)) {
 			sinfo->slot_off -= sizeof(*sinfo->de);
 			sinfo->nr_slots = 1;
-- 
2.19.2