[v1] exfat: fix the new buffer was not zeroed before writing

Commit Message

[e-mail] Dec. 16, 2024, 6:52 a.m. UTC

In exfat, not only the newly allocated space will be mapped as
the new buffer, but also the space between ->valid_size and the
file size will be mapped as the new buffer. If the buffer is
mapped as new in ->write_begin(), it will be zeroed. But if the
buffer has been mapped as new before ->write_begin(), ->write_begin()
will not zero them, resulting in access to uninitialized data.

So this commit uses folio_zero_new_buffers() to zero the new buffers
after ->write_begin().

Fixes: 6630ea49103c ("exfat: move extend valid_size into ->page_mkwrite()")
Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=91ae49e1c1a2634d20c0
Tested-by: [e-mail]
Signed-off-by: Yuezhang Mo <[e-mail]>
---
 fs/exfat/file.c | 6 ++++++
 1 file changed, 6 insertions(+)

Patch

From 40a308647d032e442e3eb75ac1ef2264dda185d8 Mon Sep 17 00:00:00 2001
From: Yuezhang Mo <[e-mail]>
Date: Thu, 12 Dec 2024 16:29:23 +0800
Subject: [PATCH v1] exfat: fix the new buffer was not zeroed before writing

In exfat, not only the newly allocated space will be mapped as
the new buffer, but also the space between ->valid_size and the
file size will be mapped as the new buffer. If the buffer is
mapped as new in ->write_begin(), it will be zeroed. But if the
buffer has been mapped as new before ->write_begin(), ->write_begin()
will not zero them, resulting in access to uninitialized data.

So this commit uses folio_zero_new_buffers() to zero the new buffers
after ->write_begin().

Fixes: 6630ea49103c ("exfat: move extend valid_size into ->page_mkwrite()")
Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=91ae49e1c1a2634d20c0
Tested-by: [e-mail]
Signed-off-by: Yuezhang Mo <[e-mail]>
---
 fs/exfat/file.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/exfat/file.c b/fs/exfat/file.c
index fb38769c3e39..05b51e721783 100644
--- a/fs/exfat/file.c
+++ b/fs/exfat/file.c
@@ -545,6 +545,7 @@  static int exfat_extend_valid_size(struct file *file, loff_t new_valid_size)
 	while (pos < new_valid_size) {
 		u32 len;
 		struct folio *folio;
+		unsigned long off;
 
 		len = PAGE_SIZE - (pos & (PAGE_SIZE - 1));
 		if (pos + len > new_valid_size)
@@ -554,6 +555,9 @@  static int exfat_extend_valid_size(struct file *file, loff_t new_valid_size)
 		if (err)
 			goto out;
 
+		off = offset_in_folio(folio, pos);
+		folio_zero_new_buffers(folio, off, off + len);
+
 		err = ops->write_end(file, mapping, pos, len, len, folio, NULL);
 		if (err < 0)
 			goto out;
@@ -563,6 +567,8 @@  static int exfat_extend_valid_size(struct file *file, loff_t new_valid_size)
 		cond_resched();
 	}
 
+	return 0;
+
 out:
 	return err;
 }
-- 
2.43.0