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Message-Id: <20241028202645.412589-1-danielyangkang@gmail.com>
Date: Mon, 28 Oct 2024 13:26:44 -0700
From: Daniel Yang <danielyangkang@...il.com>
To: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
linux-kernel@...r.kernel.org (open list)
Cc: Daniel Yang <danielyangkang@...il.com>,
syzbot+3999cae1c2d59c2cc8b9@...kaller.appspotmail.com
Subject: [PATCH] Fix BUG: KCSAN: data-race in fat16_ent_get / fat16_ent_put
Issue is that fat_free() calls fat_get_cluster() and fat_free_clusters()
at the same time. If the same fatent gets modified, it can lead to a
race condition where fat16_ent_put() and fat16_ent_get() will read/write
conflict on fatent->u.ent16_p.
To fix: add critical sections in fat_free() on the offending function
calls so that they can't both be running at the same time. Since the
critical sections are short, a spinlock is used since the overhead is
not that high.
Signed-off-by: Daniel Yang <danielyangkang@...il.com>
Reported-by: syzbot+3999cae1c2d59c2cc8b9@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=3999cae1c2d59c2cc8b9
---
fs/fat/file.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)
diff --git a/fs/fat/file.c b/fs/fat/file.c
index e887e9ab7..d7ae152a9 100644
--- a/fs/fat/file.c
+++ b/fs/fat/file.c
@@ -7,6 +7,7 @@
* regular file handling primitives for fat-based filesystems
*/
+#include "linux/spinlock.h"
#include <linux/capability.h>
#include <linux/module.h>
#include <linux/compat.h>
@@ -306,6 +307,9 @@ static long fat_fallocate(struct file *file, int mode,
return err;
}
+/* Prevent data race in fat_free. */
+static DEFINE_SPINLOCK(cluster_lock);
+
/* Free all clusters after the skip'th cluster. */
static int fat_free(struct inode *inode, int skip)
{
@@ -343,7 +347,10 @@ static int fat_free(struct inode *inode, int skip)
struct fat_entry fatent;
int ret, fclus, dclus;
+ /* Ensure fat_get_cluster isn't called while freeing. */
+ spin_lock(&cluster_lock);
ret = fat_get_cluster(inode, skip - 1, &fclus, &dclus);
+ spin_unlock(&cluster_lock);
if (ret < 0)
return ret;
else if (ret == FAT_ENT_EOF)
@@ -373,7 +380,12 @@ static int fat_free(struct inode *inode, int skip)
inode->i_blocks = skip << (MSDOS_SB(sb)->cluster_bits - 9);
/* Freeing the remained cluster chain */
- return fat_free_clusters(inode, free_start);
+ int ret;
+
+ spin_lock(&cluster_lock);
+ ret = fat_free_clusters(inode, free_start);
+ spin_unlock(&cluster_lock);
+ return ret;
}
void fat_truncate_blocks(struct inode *inode, loff_t offset)
--
2.39.2
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