linux-kernel - [PATCH] afs: Fix a null-ptr-deref in afs_atcell_get

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Message-ID: <tencent_8CA5671E3C533638973237484A0874917609@qq.com>
Date: Tue, 18 Mar 2025 17:26:09 +0800
From: Edward Adam Davis <eadavis@...com>
To: syzbot+76a6f18e3af82e84f264@...kaller.appspotmail.com
Cc: dhowells@...hat.com,
	linux-afs@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	marc.dionne@...istor.com,
	syzkaller-bugs@...glegroups.com
Subject: [PATCH] afs: Fix a null-ptr-deref in afs_atcell_get_link

syzbot report a null-ptr-deref in afs_atcell_get_link. [1]

Before accessing ws_cell, we need to confirm that it is valid.

[1]
KASAN: null-ptr-deref in range [0x00000000000002b0-0x00000000000002b7]
CPU: 2 UID: 0 PID: 5932 Comm: syz-executor998 Not tainted 6.14.0-rc6-syzkaller-00189-gb35233e7bfa0 #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
RIP: 0010:afs_atcell_get_link+0x33e/0x480 fs/afs/dynroot.c:321
Code: 89 c3 89 c6 e8 53 af 3b fe 85 db 75 64 e8 4a b4 3b fe 48 8d bd b0 02 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 1f 01 00 00 4c 89 f6 bf 03 00 00 00 4c 8b a5 b0
RSP: 0018:ffffc90004c3f988 EFLAGS: 00010206
RAX: dffffc0000000000 RBX: 0000000000000001 RCX: ffffffff837e3e8a
RDX: 0000000000000056 RSI: ffffffff837e3e16 RDI: 00000000000002b0
RBP: 0000000000000000 R08: 0000000000000005 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000001 R12: 0000000000000000
R13: ffff8881082b6000 R14: 0000000000000003 R15: ffff8880345a1480
FS:  000055559399f380(0000) GS:ffff88806a800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00005555939b0738 CR3: 000000002712e000 CR4: 0000000000352ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 pick_link fs/namei.c:1914 [inline]
 step_into+0x1982/0x2220 fs/namei.c:1984
 open_last_lookups fs/namei.c:3777 [inline]
 path_openat+0x74c/0x2d80 fs/namei.c:3986
 do_filp_open+0x20c/0x470 fs/namei.c:4016
 do_sys_openat2+0x17a/0x1e0 fs/open.c:1428
 do_sys_open fs/open.c:1443 [inline]
 __do_sys_openat fs/open.c:1459 [inline]
 __se_sys_openat fs/open.c:1454 [inline]
 __x64_sys_openat+0x175/0x210 fs/open.c:1454
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83

Fixes: 823869e1e616 ("afs: Fix afs_atcell_get_link() to handle RCU pathwalk")
Reported-by: syzbot+76a6f18e3af82e84f264@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=76a6f18e3af82e84f264
Signed-off-by: Edward Adam Davis <eadavis@...com>
---
 fs/afs/dynroot.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/afs/dynroot.c b/fs/afs/dynroot.c
index 008698d706ca..7d997f7a8028 100644
--- a/fs/afs/dynroot.c
+++ b/fs/afs/dynroot.c
@@ -314,6 +314,9 @@ static const char *afs_atcell_get_link(struct dentry *dentry, struct inode *inod
 	const char *name;
 	bool dotted = vnode->fid.vnode == 3;
 
+	if (!rcu_access_pointer(net->ws_cell))
+		return ERR_PTR(-ENOENT);
+
 	if (!dentry) {
 		/* We're in RCU-pathwalk. */
 		cell = rcu_dereference(net->ws_cell);
@@ -325,9 +328,6 @@ static const char *afs_atcell_get_link(struct dentry *dentry, struct inode *inod
 		return name;
 	}
 
-	if (!rcu_access_pointer(net->ws_cell))
-		return ERR_PTR(-ENOENT);
-
 	down_read(&net->cells_lock);
 
 	cell = rcu_dereference_protected(net->ws_cell, lockdep_is_held(&net->cells_lock));
-- 
2.43.0