Re: [syzbot] [hardening?] [mm?] BUG: bad usercopy in vfs_readlink

[Jan Kara <jack@suse.cz>]

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On Tue 04-02-25 01:46:28, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    69b8923f5003 Merge tag 'for-linus-6.14-ofs4' of git://git...
> git tree:       upstream
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=1258aeb0580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=57ab43c279fa614d
> dashboard link: https://syzkaller.appspot.com/bug?extid=48a99e426f29859818c0
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15825724580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1658aeb0580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/ea84ac864e92/disk-69b8923f.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/6a465997b4e0/vmlinux-69b8923f.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/d72b67b2bd15/bzImage-69b8923f.xz
> mounted in repro: https://storage.googleapis.com/syzbot-assets/7c2919610764/mount_0.gz
> 
> The issue was bisected to:
> 
> commit bae80473f7b0b25772619e7692019b1549d4a82c
> Author: Mateusz Guzik <mjguzik@gmail.com>
> Date:   Wed Nov 20 11:20:35 2024 +0000
> 
>     ext4: use inode_set_cached_link()
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1248c3df980000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=1148c3df980000
> console output: https://syzkaller.appspot.com/x/log.txt?x=1648c3df980000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+48a99e426f29859818c0@syzkaller.appspotmail.com
> Fixes: bae80473f7b0 ("ext4: use inode_set_cached_link()")

Please check attached patch:

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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From df00b84402fb67d94a9eb6b86633092983cb388c Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@suse.cz>
Date: Wed, 5 Feb 2025 19:02:35 +0100
Subject: [PATCH] ext4: Verify fast symlink length

Verify fast symlink length stored in inode->i_size matches the string
stored in the inode to avoid surprises from corrupted filesystems.

Reported-by: syzbot+48a99e426f29859818c0@syzkaller.appspotmail.com
Fixes: bae80473f7b0 ("ext4: use inode_set_cached_link()")
Suggested-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/ext4/inode.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 7c54ae5fcbd4..fbda5a67f7f9 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5007,8 +5007,16 @@ struct inode *__ext4_iget(struct super_block *sb, unsigned long ino,
 			inode->i_op = &ext4_encrypted_symlink_inode_operations;
 		} else if (ext4_inode_is_fast_symlink(inode)) {
 			inode->i_op = &ext4_fast_symlink_inode_operations;
-			nd_terminate_link(ei->i_data, inode->i_size,
-				sizeof(ei->i_data) - 1);
+			if (inode->i_size == 0 ||
+			    inode->i_size >= EXT4_N_BLOCKS * 4 ||
+			    strnlen((char *)ei->i_data, inode->i_size + 1) !=
+								inode->i_size) {
+				ext4_error_inode(inode, function, line, 0,
+					"invalid fast symlink length %llu",
+					 (unsigned long long)inode->i_size);
+				ret = -EFSCORRUPTED;
+				goto bad_inode;
+			}
 			inode_set_cached_link(inode, (char *)ei->i_data,
 					      inode->i_size);
 		} else {
-- 
2.43.0