linux-kernel - [PATCH] ext4: fix kernel segfault after iterator overflow

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Date: Thu, 27 Jun 2024 10:56:01 +0200
From: Jan Henrik Weinstock <jan@....re>
To: tytso@....edu,
	adilger.kernel@...ger.ca
Cc: linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	lukas@....re,
	simon@....re,
	Jan Henrik Weinstock <jan@....re>
Subject: [PATCH] ext4: fix kernel segfault after iterator overflow

When search_buf gets placed at the end of the virtual address space
        de = (struct ext4_dir_entry_2 *) ((char *) de + de_len);
might overflow to zero and a subsequent loop iteration will crash.

Observed on a simulated riscv32 system using 2GB of memory and a rootfs
on MMC.

Signed-off-by: Jan Henrik Weinstock <jan@....re>
---
 fs/ext4/namei.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index a630b27a4..030a11412 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -1537,7 +1537,8 @@ int ext4_search_dir(struct buffer_head *bh, char *search_buf, int buf_size,
 
 	de = (struct ext4_dir_entry_2 *)search_buf;
 	dlimit = search_buf + buf_size;
-	while ((char *) de < dlimit - EXT4_BASE_DIR_LEN) {
+	while ((char *) de < dlimit - EXT4_BASE_DIR_LEN &&
+	       (char *) de >= search_buf) {
 		/* this code is executed quadratically often */
 		/* do minimal checking `by hand' */
 		if (de->name + de->name_len <= dlimit &&
-- 
2.45.2