Re: fs: NULL deref in atime_needs_update

[MickaÃl SalaÃn]

Hi,

Actually I found the same bug (without fuzzing) and I can reproduce it in a deterministic way (e.g. by creating a LSM that return 1 for the security_file_open hook). At least, from v4.2.8 I can easily trigger traces like this :

BUG: unable to handle kernel NULL pointer dereference at 0000000000000050
IP: [<ffffffff81170871>] atime_needs_update+0x11/0xc0
PGD 127b17067 PUD 12ab2e067 PMD 0 
Oops: 0000 [#45] SMP 
[...]
RIP: 0010:[<ffffffff81170871>]  [<ffffffff81170871>] atime_needs_update+0x11/0xc0
RSP: 0018:ffff880127853c18  EFLAGS: 00010246
RAX: ffff88012ad0c080 RBX: ffff88012ad0c1d8 RCX: ffff88012ad0c080
RDX: 0000000000000000 RSI: ffff88012ad0c1d8 RDI: ffff880127853d98
RBP: ffff880127853c28 R08: ffff8800cc0a2540 R09: ffff8800cfbfc320
R10: ffff8800cc0a2540 R11: 0000000000000001 R12: ffff8800cb5d6300
R13: 0000000000000000 R14: ffff88012ad0c080 R15: ffff880127853e7c
FS:  00007f1054aae700(0000) GS:ffff88012fc40000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000050 CR3: 0000000127977000 CR4: 00000000000406e0
Stack:
ffff88012ad0c1d8 ffff8800cb5d6300 ffff880127853c60 ffffffff8117094e
ffff8800c9ade3c0 0000000000000000 00000000a670294f ffff880127853d70
ffff880127853d98 ffff880127853c98 ffffffff8116071c ffff8800cb4ada80
Call Trace:
[<ffffffff8117094e>] ? touch_atime+0x2e/0xd0
[<ffffffff8116071c>] ? trailing_symlink+0xec/0x280
[<ffffffff81163a78>] ? path_openat+0x468/0x1240
[<ffffffff8111856d>] ? pagevec_lru_move_fn+0xed/0x110
[<ffffffff81117ff0>] ? __activate_page+0x130/0x130
[<ffffffff8116593c>] ? do_filp_open+0x8c/0x100
[<ffffffff81164dec>] ? filename_lookup+0xec/0x180
[<ffffffff8115bc24>] ? do_open_execat+0x74/0x170
[<ffffffff8115d437>] ? do_execveat_common.isra.42+0x1a7/0x6a0
[<ffffffff8115db90>] ? SyS_execve+0x30/0x40
[<ffffffff8156ad65>] ? stub_execve+0x5/0x5
[<ffffffff8156aadb>] ? entry_SYSCALL_64_fastpath+0x16/0x6a
Code: 89 c7 e8 63 eb ff ff 48 89 d8 5b c3 0f 1f 40 00 66 2e 0f 1f 84 00 00 00 00 00 55 48 89 e5 41 54 53 f6 46 0c 02 75 72 48 8b 56 28 <48> 8b 42 50 a9 01 04 00 00 75 63 f6 c4 08 75 65 4c 8b 27 41 8b 
RIP  [<ffffffff81170871>] atime_needs_update+0x11/0xc0
RSP <ffff880127853c18>
CR2: 0000000000000050
---[ end trace 97dc4f4bb0214bd8 ]---


Regards,
 MickaÃl


On 05/02/2016 22:11, Dmitry Vyukov wrote:
> Hello,
> 
> I've hit the following GPF while running syzkaller fuzzer:
> 
> general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN
> Modules linked in:
> CPU: 1 PID: 5178 Comm: syz-executor Not tainted 4.5.0-rc2+ #65
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> task: ffff880064768000 ti: ffff8800622c0000 task.ti: ffff8800622c0000
> RIP: 0010:[<ffffffff8181aa5d>]  [<ffffffff8181aa5d>]
> atime_needs_update+0x2d/0x460
> RSP: 0018:ffff8800622c7a30  EFLAGS: 00010203
> RAX: dffffc0000000000 RBX: 0000000000000000 RCX: dffffc0000000000
> RDX: 0000000000000001 RSI: 0000000000000000 RDI: 000000000000000c
> RBP: ffff8800622c7a58 R08: 0000000000000001 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000001 R12: ffff8800622c7c08
> R13: ffff8800622c7c08 R14: ffff8800301ca322 R15: ffff8800622c7bb0
> FS:  00007fd1c9f8b700(0000) GS:ffff88003ed00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> CR2: 0000000020f31000 CR3: 0000000062274000 CR4: 00000000000006e0
> Stack:
>  ffff8800622c7bf4 0000000000000000 ffff8800622c7c08 ffff8800301ca322
>  ffff8800622c7bb0 ffff8800622c7b38 ffffffff817ecd91 ffff880030bf5200
>  ffff8800622c7bb8 1ffff1000c458f56 ffff8800622c7c00 ffff8800622c7be0
> Call Trace:
>  [<     inline     >] get_link fs/namei.c:1006
>  [<ffffffff817ecd91>] link_path_walk+0xaf1/0x1030 fs/namei.c:1968
>  [<ffffffff817ed311>] path_parentat+0x41/0x150 fs/namei.c:2176
>  [<ffffffff817f4c5c>] filename_parentat+0x17c/0x3c0 fs/namei.c:2198
>  [<     inline     >] user_path_parent fs/namei.c:2412
>  [<     inline     >] SYSC_renameat2 fs/namei.c:4411
>  [<     inline     >] SyS_renameat2 fs/namei.c:4375
>  [<     inline     >] SYSC_renameat fs/namei.c:4521
>  [<ffffffff817f9a72>] SyS_renameat+0x192/0x820 fs/namei.c:4518
>  [<ffffffff8669e0b6>] entry_SYSCALL_64_fastpath+0x16/0x7a
> arch/x86/entry/entry_64.S:185
> Code: 89 e5 41 57 41 56 41 55 41 54 49 89 fc 53 48 89 f3 e8 08 25 d5
> ff 48 8d 7b 0c 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <0f>
> b6 14 02 48 89 f8 83 e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85
> RIP  [<ffffffff8181aa5d>] atime_needs_update+0x2d/0x460 fs/inode.c:1611
>  RSP <ffff8800622c7a30>
> ---[ end trace 1a4c9bda4680ce46 ]---
> 
> On commit df48ab3c2f5ffca88b7803ffbadd074bd5a0a2ef.
> 
> Objdump shows that inode is NULL in atime_needs_update.
> 
> Unfortunately reproduction of this crash is very hard. The program
> executes something along the lines of:
> 
> mmap(0x20000000, 15945728, PROT_READ|PROT_WRITE,
> MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x20000000
> mkdir("./bus", 0662515705056234013740)  = 0
> openat(AT_FDCWD, "./bus", O_RDONLY|O_EXCL) = 3
> symlinkat("../bus", 3, "./bus")         = 0
> renameat(3, "./bus", 3, "./bus/file0")  = 0
> mmap(0x20f35000, 4096, PROT_READ|PROT_WRITE,
> MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x20f35000
> mount("./bus", "./bus", 0x20f2aee4,
> MS_RDONLY|MS_NODEV|MS_RELATIME|MS_NODIRATIME|MS_BIND|MS_MOVE|MS_REC|MS_UNBINDABLE|MS_SLAVE|MS_SHARED|0xc000380,
> 0x20093f5f) = 0
> open("./bus/file0", O_RDWR|O_EXCL)      = -1 EISDIR (Is a directory)
> exit_group(0)                           = ?
> 
> But in multiple threads so that some calls can be doubled and/or
> overlapped. And all this happens on a tmpfs mount.
> 
> But I was able to reproduce it 8 or so times, so I am sure that it is real.
> 
> For future reference, I was running these programs:
> https://gist.githubusercontent.com/dvyukov/124c457d308fa724d88a/raw/fec2d86e125a7fd2fa2916791d65d7daead7cbbb/gistfile1.txt
> Following these instructions:
> https://github.com/google/syzkaller/wiki/How-to-execute-syzkaller-programs
>

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