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Date: Fri, 24 Oct 2014 18:00:43 +0300
From: Dmitry Kasatkin <d.kasatkin@...sung.com>
To: Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc: linux-security-module@...r.kernel.org,
linux-ima-devel@...ts.sourceforge.net,
linux-kernel@...r.kernel.org, jack@...e.cz, jmorris@...ei.org,
dmitry.kasatkin@...il.com
Subject: Re: [PATCH v2 1/2] ima: check xattr value length in
ima_inode_setxattr()
On 24/10/14 17:18, Mimi Zohar wrote:
> On Fri, 2014-10-24 at 10:07 +0300, Dmitry Kasatkin wrote:
>> ima_inode_setxattr() can be called with no value. Function does not
>> check the length so that following command can be used to produce
>> kernel oops: setfattr -n security.ima FOO. This patch fixes it.
>>
>> Changes in v2:
>> * testing validity of xattr type
>> * allow setting hash only in fix or log mode (Mimi)
> I only mentioned "fix" mode, not "log" mode (explanation below).
>
We need it in log mode as well (explanation bellow)
>> [ 261.562522] BUG: unable to handle kernel NULL pointer dereference at (null)
>> [ 261.564109] IP: [<ffffffff812af272>] ima_inode_setxattr+0x3e/0x5a
>> [ 261.564109] PGD 3112f067 PUD 42965067 PMD 0
>> [ 261.564109] Oops: 0000 [#1] SMP
>> [ 261.564109] Modules linked in: bridge stp llc evdev serio_raw i2c_piix4 button fuse
>> [ 261.564109] CPU: 0 PID: 3299 Comm: setxattr Not tainted 3.16.0-kds+ #2924
>> [ 261.564109] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
>> [ 261.564109] task: ffff8800428c2430 ti: ffff880042be0000 task.ti: ffff880042be0000
>> [ 261.564109] RIP: 0010:[<ffffffff812af272>] [<ffffffff812af272>] ima_inode_setxattr+0x3e/0x5a
>> [ 261.564109] RSP: 0018:ffff880042be3d50 EFLAGS: 00010246
>> [ 261.564109] RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000015
>> [ 261.564109] RDX: 0000001500000000 RSI: 0000000000000000 RDI: ffff8800375cc600
>> [ 261.564109] RBP: ffff880042be3d68 R08: 0000000000000000 R09: 00000000004d6256
>> [ 261.564109] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88002149ba00
>> [ 261.564109] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>> [ 261.564109] FS: 00007f6c1e219740(0000) GS:ffff88005da00000(0000) knlGS:0000000000000000
>> [ 261.564109] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [ 261.564109] CR2: 0000000000000000 CR3: 000000003b35a000 CR4: 00000000000006f0
>> [ 261.564109] Stack:
>> [ 261.564109] ffff88002149ba00 ffff880042be3df8 0000000000000000 ffff880042be3d98
>> [ 261.564109] ffffffff812a101b ffff88002149ba00 ffff880042be3df8 0000000000000000
>> [ 261.564109] 0000000000000000 ffff880042be3de0 ffffffff8116d08a ffff880042be3dc8
>> [ 261.564109] Call Trace:
>> [ 261.564109] [<ffffffff812a101b>] security_inode_setxattr+0x48/0x6a
>> [ 261.564109] [<ffffffff8116d08a>] vfs_setxattr+0x6b/0x9f
>> [ 261.564109] [<ffffffff8116d1e0>] setxattr+0x122/0x16c
>> [ 261.564109] [<ffffffff811687e8>] ? mnt_want_write+0x21/0x45
>> [ 261.564109] [<ffffffff8114d011>] ? __sb_start_write+0x10f/0x143
>> [ 261.564109] [<ffffffff811687e8>] ? mnt_want_write+0x21/0x45
>> [ 261.564109] [<ffffffff811687c0>] ? __mnt_want_write+0x48/0x4f
>> [ 261.564109] [<ffffffff8116d3e6>] SyS_setxattr+0x6e/0xb0
>> [ 261.564109] [<ffffffff81529da9>] system_call_fastpath+0x16/0x1b
>> [ 261.564109] Code: 48 89 f7 48 c7 c6 58 36 81 81 53 31 db e8 73 27 04 00 85 c0 75 28 bf 15 00 00 00 e8 8a a5 d9 ff 84 c0 75 05 83 cb ff eb 15 31 f6 <41> 80 7d 00 03 49 8b 7c 24 68 40 0f 94 c6 e8 e1 f9 ff ff 89 d8
>> [ 261.564109] RIP [<ffffffff812af272>] ima_inode_setxattr+0x3e/0x5a
>> [ 261.564109] RSP <ffff880042be3d50>
>> [ 261.564109] CR2: 0000000000000000
>> [ 261.599998] ---[ end trace 39a89a3fc267e652 ]---
>>
>> Reported-by: Jan Kara <jack@...e.cz>
>> Signed-off-by: Dmitry Kasatkin <d.kasatkin@...sung.com>
>> ---
>> security/integrity/ima/ima_appraise.c | 13 +++++++++++--
>> 1 file changed, 11 insertions(+), 2 deletions(-)
>>
>> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
>> index 9226854..e302cbf 100644
>> --- a/security/integrity/ima/ima_appraise.c
>> +++ b/security/integrity/ima/ima_appraise.c
>> @@ -378,8 +378,17 @@ int ima_inode_setxattr(struct dentry *dentry, const char *xattr_name,
>> result = ima_protect_xattr(dentry, xattr_name, xattr_value,
>> xattr_value_len);
>> if (result == 1) {
>> - ima_reset_appraise_flags(dentry->d_inode,
>> - (xvalue->type == EVM_IMA_XATTR_DIGSIG) ? 1 : 0);
>> + bool digsig;
>> +
>> + if (!xattr_value_len ||
>> + (xvalue->type != IMA_XATTR_DIGEST &&
>> + xvalue->type != IMA_XATTR_DIGEST_NG &&
>> + xvalue->type != EVM_IMA_XATTR_DIGSIG))
> "xvalue->type" is an enumerated type. Testing each possible value seems
> kind of a brittle method for vetting the value. I suggest testing the
> existing last value or, better yet, define a last value, so if someone
> adds or changes the order, nothing breaks.
I was considering to define _LAST value, but we have EVM_XATTR_HMAC in
the middle...
In fact I was expecting to get some feedback about it, because in
reality it is just a sanity check.
It does not prevent DoS because it is possible to set correctly
formatted but wrong value and force DoS.
>
>> + return -EINVAL;
>> + digsig = (xvalue->type == EVM_IMA_XATTR_DIGSIG);
>> + if (!digsig && (ima_appraise & IMA_APPRAISE_ENFORCE))
>> + return -EPERM;
> According to the new ima_appraise "log" mode, commit "2faa6ef ima:
> provide 'ima_appraise=log' kernel option", "log" mode permits normal
> execution without "fixing" anything. Normal execution, here, prevents
> writing the extended attribute.
'log' mode is also special mode for system developing and debugging.
It is beneficial to be able to 'label' target object with correct value...
- Dmitry
> Mimi
>
>> + ima_reset_appraise_flags(dentry->d_inode, digsig);
>> result = 0;
>> }
>> return result;
>
>
--
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