Re: [PATCH 2/3] Smack: handle zero-length security labels without panic

[Konstantin Khlebnikov]

On Sat, Aug 9, 2014 at 12:25 AM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> Quoting Konstantin Khlebnikov (k.khlebnikov@xxxxxxxxxxx):
>> Zero-length security labels are invalid but kernel should handle them.
>>
>> This patch fixes kernel panic after setting zero-length security labels:
>> # attr -S -s "SMACK64" -V "" file
>>
>> And after writing zero-length string into smackfs files syslog and onlycp:
>> # python -c 'import os; os.write(1, "")' > /smack/syslog
>>
>> The problem is caused by brain-damaged logic in function smk_parse_smack()
>> which takes pointer to buffer and its length but if length below or equal zero
>> it thinks that the buffer is zero-terminated. Unfortunately callers of this
>> function are widely used and proper fix requires serious refactoring.
>>
>> Signed-off-by: Konstantin Khlebnikov <k.khlebnikov@xxxxxxxxxxx>
>
> Acked-by: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>
>
>>
>> ---
>>
>> Example:
>>
>> [   28.063935] BUG: unable to handle kernel NULL pointer dereference at           (null)
>> [   28.064623] IP: [<ffffffff812ea780>] strlen+0x0/0x30
>> [   28.064623] PGD 29a77067 PUD 2972d067 PMD 0
>> [   28.064623] Oops: 0000 [#1] SMP
>> [   28.064623] Modules linked in:
>> [   28.064623] CPU: 0 PID: 824 Comm: attr Not tainted 3.16.0+ #6
>> [   28.064623] task: ffff880029bd88c0 ti: ffff8800297d0000 task.ti: ffff8800297d0000
>> [   28.064623] RIP: 0010:[<ffffffff812ea780>]  [<ffffffff812ea780>] strlen+0x0/0x30
>> [   28.064623] RSP: 0018:ffff8800297d3ca8  EFLAGS: 00010246
>> [   28.064623] RAX: 0000000000000001 RBX: ffff880029c096c0 RCX: 0000000000000001
>> [   28.064623] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
>> [   28.064623] RBP: ffff8800297d3cd0 R08: ffffffff81e3d940 R09: ffff88002a3e8588
>> [   28.064623] R10: 8080808080808080 R11: 0000000000000000 R12: 0000000000000000
>> [   28.064623] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>> [   28.064623] FS:  00007effe391a700(0000) GS:ffff88002a400000(0000) knlGS:0000000000000000
>> [   28.064623] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [   28.064623] CR2: 0000000000000000 CR3: 00000000297a1000 CR4: 00000000001406b0
>> [   28.064623] Stack:
>> [   28.064623]  ffffffff812a6fa5 ffff880029c096c0 0000000000000001 0000000000000000
>> [   28.064623]  0000000000000000 ffff8800297d3cf8 ffffffff812a70ae ffff880029c096c0
>> [   28.064623]  0000000000000001 0000000000000000 ffff8800297d3d88 ffffffff812a64dc
>> [   28.064623] Call Trace:
>> [   28.064623]  [<ffffffff812a6fa5>] ? smk_parse_smack+0xe5/0x110
>> [   28.064623]  [<ffffffff812a70ae>] smk_import_entry+0xe/0x130
>> [   28.064623]  [<ffffffff812a64dc>] smack_inode_setxattr+0x11c/0x280
>> [   28.064623]  [<ffffffff8116a767>] ? lookup_fast+0x177/0x2e0
>> [   28.064623]  [<ffffffff81177001>] ? dput+0x21/0x1a0
>> [   28.064623]  [<ffffffff812a0eeb>] security_inode_setxattr+0x1b/0x30
>> [   28.064623]  [<ffffffff8118488f>] vfs_setxattr+0x6f/0xb0
>> [   28.064623]  [<ffffffff81184a76>] setxattr+0x1a6/0x1f0
>> [   28.064623]  [<ffffffff8116af6d>] ? final_putname+0x1d/0x40
>> [   28.064623]  [<ffffffff8116b1c4>] ? putname+0x24/0x40
>> [   28.064623]  [<ffffffff8117144a>] ? user_path_at_empty+0x5a/0xa0
>> [   28.064623]  [<ffffffff81162b84>] ? __sb_start_write+0x44/0xe0
>> [   28.064623]  [<ffffffff8113cd21>] ? do_brk+0x241/0x320
>> [   28.064623]  [<ffffffff81184d97>] SyS_lsetxattr+0x87/0xe0
>> [   28.064623]  [<ffffffff81806f69>] system_call_fastpath+0x16/0x1b
>> [   28.064623] Code: 89 f8 48 89 e5 f6 82 40 3e a6 81 20 74 15 0f 1f 44 00 00 48 83 c0 01 0f b6 10 f6 82 40 3e a6 81 20 75 f0 5d c3 66 0f 1f 44 00 00 <80> 3f 00 55 48 89 e5 74 15 48 89 f8 0f 1f 40 00 48 83 c0 01 80
>> [   28.064623] RIP  [<ffffffff812ea780>] strlen+0x0/0x30
>> [   28.064623]  RSP <ffff8800297d3ca8>
>> [   28.064623] CR2: 0000000000000000
>> [   28.093561] ---[ end trace de1055429a98a5be ]---
>> ---
>>  security/smack/smack_lsm.c |    2 +-
>>  security/smack/smackfs.c   |    4 ++--
>>  2 files changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
>> index b11ab23..afa5ad0 100644
>> --- a/security/smack/smack_lsm.c
>> +++ b/security/smack/smack_lsm.c
>> @@ -923,7 +923,7 @@ static int smack_inode_setxattr(struct dentry *dentry, const char *name,
>>               rc = -EPERM;
>>
>>       if (rc == 0 && check_import) {
>> -             skp = smk_import_entry(value, size);
>> +             skp = size ? smk_import_entry(value, size) : NULL;
>>               if (skp == NULL || (check_star &&
>>                   (skp == &smack_known_star || skp == &smack_known_web)))
>>                       rc = -EINVAL;
>> diff --git a/security/smack/smackfs.c b/security/smack/smackfs.c
>> index 3c720ff..56a1439 100644
>> --- a/security/smack/smackfs.c
>> +++ b/security/smack/smackfs.c
>> @@ -1677,7 +1677,7 @@ static ssize_t smk_write_onlycap(struct file *file, const char __user *buf,
>>       if (smack_onlycap != NULL && smack_onlycap != skp)
>>               return -EPERM;
>>
>> -     data = kzalloc(count, GFP_KERNEL);
>> +     data = kzalloc(count + 1, GFP_KERNEL);
>>       if (data == NULL)
>>               return -ENOMEM;
>>
>> @@ -2228,7 +2228,7 @@ static ssize_t smk_write_syslog(struct file *file, const char __user *buf,
>>       if (!smack_privileged(CAP_MAC_ADMIN))
>>               return -EPERM;
>>
>
> How about putting a comment here explaining that this way if len is 0 you
> make sure to have '\0' to avoid parsing problems?

There are plenty of these comments. That design is uncommon and
extremely dangerous,
it cannot be neutralized just by a comment.

Probably magic length '0' might be replaced with magic '-1' which is much safer.

>
>> -     data = kzalloc(count, GFP_KERNEL);
>> +     data = kzalloc(count + 1, GFP_KERNEL);
>>       if (data == NULL)
>>               return -ENOMEM;
>>
> --
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