linux-ext4 - [PATCH v2] ext4: fix use-after-free in dx

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Date:   Wed,  8 May 2019 14:04:03 +0530
From:   Sahitya Tummala <stummala@...eaurora.org>
To:     Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Sahitya Tummala <stummala@...eaurora.org>
Subject: [PATCH v2] ext4: fix use-after-free in dx_release()

The buffer_head (frames[0].bh) and it's corresping page can be
potentially free'd once brelse() is done inside the for loop
but before the for loop exits in dx_release(). It can be free'd
in another context, when the page cache is flushed via
drop_caches_sysctl_handler(). This results into below data abort
when accessing info->indirect_levels in dx_release().

Unable to handle kernel paging request at virtual address ffffffc17ac3e01e
Call trace:
 dx_release+0x70/0x90
 ext4_htree_fill_tree+0x2d4/0x300
 ext4_readdir+0x244/0x6f8
 iterate_dir+0xbc/0x160
 SyS_getdents64+0x94/0x174

Signed-off-by: Sahitya Tummala <stummala@...eaurora.org>
---
v2:
add a comment in dx_release()

 fs/ext4/namei.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 980166a..5d9ffa8 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -871,12 +871,15 @@ static void dx_release(struct dx_frame *frames)
 {
 	struct dx_root_info *info;
 	int i;
+	unsigned int indirect_levels;
 
 	if (frames[0].bh == NULL)
 		return;
 
 	info = &((struct dx_root *)frames[0].bh->b_data)->info;
-	for (i = 0; i <= info->indirect_levels; i++) {
+	/* save local copy, "info" may be freed after brelse() */
+	indirect_levels = info->indirect_levels;
+	for (i = 0; i <= indirect_levels; i++) {
 		if (frames[i].bh == NULL)
 			break;
 		brelse(frames[i].bh);
-- 
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Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.