[syzbot] [afs?] general protection fault in afs_atcell_get_link

[syzbot] [afs?] general protection fault in afs_atcell_get_link

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    b35233e7bfa0 Merge tag 'for-6.14/dm-fixes-2' of git://git...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1232704c580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=317038cbd53153e8
dashboard link: https://syzkaller.appspot.com/bug?extid=76a6f18e3af82e84f264
compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17d3fc78580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=169fb874580000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/7feb34a89c2a/non_bootable_disk-b35233e7.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/1b21b01e0ec9/vmlinux-b35233e7.xz
kernel image: https://storage.googleapis.com/syzbot-assets/d08ea80ce857/bzImage-b35233e7.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+76a6f18e3af82e84f264@syzkaller.appspotmail.com

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000056: 0000 [#1] PREEMPT SMP KASAN NOPTI
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
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f2317244161
Code: 75 57 89 f0 25 00 00 41 00 3d 00 00 41 00 74 49 80 3d 0a 8f 07 00 00 74 6d 89 da 48 89 ee bf 9c ff ff ff b8 01 01 00 00 0f 05 <48> 3d 00 f0 ff ff 0f 87 93 00 00 00 48 8b 54 24 28 64 48 2b 14 25
RSP: 002b:00007ffd89f6cc50 EFLAGS: 00000202 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f2317244161
RDX: 0000000000000000 RSI: 00007ffd89f6cd80 RDI: 00000000ffffff9c
RBP: 00007ffd89f6cd80 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000202 R12: 00007ffd89f6de70
R13: 00005555939a8700 R14: 000055559399f338 R15: 0000000000000001
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
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
----------------
Code disassembly (best guess):
   0:	89 c3                	mov    %eax,%ebx
   2:	89 c6                	mov    %eax,%esi
   4:	e8 53 af 3b fe       	call   0xfe3baf5c
   9:	85 db                	test   %ebx,%ebx
   b:	75 64                	jne    0x71
   d:	e8 4a b4 3b fe       	call   0xfe3bb45c
  12:	48 8d bd b0 02 00 00 	lea    0x2b0(%rbp),%rdi
  19:	48 b8 00 00 00 00 00 	movabs $0xdffffc0000000000,%rax
  20:	fc ff df
  23:	48 89 fa             	mov    %rdi,%rdx
  26:	48 c1 ea 03          	shr    $0x3,%rdx
* 2a:	80 3c 02 00          	cmpb   $0x0,(%rdx,%rax,1) <-- trapping instruction
  2e:	0f 85 1f 01 00 00    	jne    0x153
  34:	4c 89 f6             	mov    %r14,%rsi
  37:	bf 03 00 00 00       	mov    $0x3,%edi
  3c:	4c                   	rex.WR
  3d:	8b                   	.byte 0x8b
  3e:	a5                   	movsl  %ds:(%rsi),%es:(%rdi)
  3f:	b0                   	.byte 0xb0


---
This report is generated by a bot. It may contain errors.
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syzbot engineers can be reached at syzkaller@googlegroups.com.

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If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
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Re: [syzbot] Re: [syzbot] [afs?] general protection fault in afs_atcell_get_link

[syzbot]

For archival purposes, forwarding an incoming command email to
linux-kernel@vger.kernel.org.

***

Subject: Re: [syzbot] [afs?] general protection fault in afs_atcell_get_link
Author: enjuk@amazon.com

> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    b35233e7bfa0 Merge tag 'for-6.14/dm-fixes-2' of git://git...
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1232704c580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=317038cbd53153e8
> dashboard link: https://syzkaller.appspot.com/bug?extid=76a6f18e3af82e84f264
> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17d3fc78580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=169fb874580000

#syz test

diff --git a/fs/afs/dynroot.c b/fs/afs/dynroot.c
index 9732a1e17db3..3ea5e388ee16 100644
--- a/fs/afs/dynroot.c
+++ b/fs/afs/dynroot.c
@@ -213,6 +213,9 @@ static const char *afs_atcell_get_link(struct dentry *dentry, struct inode *inod
 	if (!dentry) {
 		/* We're in RCU-pathwalk. */
 		cell = rcu_dereference(net->ws_cell);
+		if (!cell)
+			return ERR_PTR(-ENOENT);
+
 		if (dotted)
 			name = cell->name - 1;
 		else

Re: [syzbot] [afs?] general protection fault in afs_atcell_get_link

[Edward Adam Davis]

#syz test

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));

Re: [syzbot] [afs?] general protection fault in afs_atcell_get_link

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+76a6f18e3af82e84f264@syzkaller.appspotmail.com
Tested-by: syzbot+76a6f18e3af82e84f264@syzkaller.appspotmail.com

Tested on:

commit:         76b6905c Merge tag 'mm-hotfixes-stable-2025-03-17-20-0..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1188f278580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2e330e9768b5b8ff
dashboard link: https://syzkaller.appspot.com/bug?extid=76a6f18e3af82e84f264
compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
patch:          https://syzkaller.appspot.com/x/patch.diff?x=173145e4580000

Note: testing is done by a robot and is best-effort only.

[PATCH] afs: Fix a null-ptr-deref in afs_atcell_get_link

[Edward Adam Davis]

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@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=76a6f18e3af82e84f264
Signed-off-by: Edward Adam Davis <eadavis@qq.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