Re: [PATCH 0/2] apparmor unaligned memory fixes

Helge Deller
2025-11-26T11:03:21Z

On 11/26/25 11:44, david laight wrote:
> On Wed, 26 Nov 2025 01:11:45 -0800
> John Johansen <john.johansen@canonical.com> wrote:
>
>> On 11/25/25 13:13, Helge Deller wrote:
>>> On 11/25/25 20:20, John Johansen wrote:
>>>> On 11/25/25 07:11, Helge Deller wrote:
>>>>> * John Johansen <john.johansen@canonical.com>:
>>>>>> On 11/18/25 04:49, Helge Deller wrote:
>>>>>>> Hi Adrian,
>>>>>>>
>>>>>>> On 11/18/25 12:43, John Paul Adrian Glaubitz wrote:
>>>>>>>> On Tue, 2025-11-18 at 12:09 +0100, Helge Deller wrote:
>>>>>>>>> My patch fixed two call sites, but I suspect you see another call site which
>>>>>>>>> hasn't been fixed yet.
>>>>>>>>>
>>>>>>>>> Can you try attached patch? It might indicate the caller of the function and
>>>>>>>>> maybe prints the struct name/address which isn't aligned.
>>>>>>>>>
>>>>>>>>> Helge
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> diff --git a/security/apparmor/match.c b/security/apparmor/match.c
>>>>>>>>> index c5a91600842a..b477430c07eb 100644
>>>>>>>>> --- a/security/apparmor/match.c
>>>>>>>>> +++ b/security/apparmor/match.c
>>>>>>>>> @@ -313,6 +313,9 @@ struct aa_dfa *aa_dfa_unpack(void *blob, size_t size, int flags)
>>>>>>>>>        if (size < sizeof(struct table_set_header))
>>>>>>>>>            goto fail;
>>>>>>>>> +    if (WARN_ON(((unsigned long)data) & (BITS_PER_LONG/8 - 1)))
>>>>>>>>> +        pr_warn("dfa blob stream %pS not aligned.\n", data);
>>>>>>>>> +
>>>>>>>>>        if (ntohl(*(__be32 *) data) != YYTH_MAGIC)
>>>>>>>>>            goto fail;
>>>>>>>>
>>>>>>>> Here is the relevant output with the patch applied:
>>>>>>>>
>>>>>>>> [   73.840639] ------------[ cut here ]------------
>>>>>>>> [   73.901376] WARNING: CPU: 0 PID: 341 at security/apparmor/match.c:316 aa_dfa_unpack+0x6cc/0x720
>>>>>>>> [   74.015867] Modules linked in: binfmt_misc evdev flash sg drm drm_panel_orientation_quirks backlight i2c_core configfs nfnetlink autofs4 ext4 crc16 mbcache jbd2 hid_generic usbhid sr_mod hid cdrom
>>>>>>>> sd_mod ata_generic ohci_pci ehci_pci ehci_hcd ohci_hcd pata_ali libata sym53c8xx scsi_transport_spi tg3 scsi_mod usbcore libphy scsi_common mdio_bus usb_common
>>>>>>>> [   74.428977] CPU: 0 UID: 0 PID: 341 Comm: apparmor_parser Not tainted 6.18.0-rc6+ #9 NONE
>>>>>>>> [   74.536543] Call Trace:
>>>>>>>> [   74.568561] [<0000000000434c24>] dump_stack+0x8/0x18
>>>>>>>> [   74.633757] [<0000000000476438>] __warn+0xd8/0x100
>>>>>>>> [   74.696664] [<00000000004296d4>] warn_slowpath_fmt+0x34/0x74
>>>>>>>> [   74.771006] [<00000000008db28c>] aa_dfa_unpack+0x6cc/0x720
>>>>>>>> [   74.843062] [<00000000008e643c>] unpack_pdb+0xbc/0x7e0
>>>>>>>> [   74.910545] [<00000000008e7740>] unpack_profile+0xbe0/0x1300
>>>>>>>> [   74.984888] [<00000000008e82e0>] aa_unpack+0xe0/0x6a0
>>>>>>>> [   75.051226] [<00000000008e3ec4>] aa_replace_profiles+0x64/0x1160
>>>>>>>> [   75.130144] [<00000000008d4d90>] policy_update+0xf0/0x280
>>>>>>>> [   75.201057] [<00000000008d4fc8>] profile_replace+0xa8/0x100
>>>>>>>> [   75.274258] [<0000000000766bd0>] vfs_write+0x90/0x420
>>>>>>>> [   75.340594] [<00000000007670cc>] ksys_write+0x4c/0xe0
>>>>>>>> [   75.406932] [<0000000000767174>] sys_write+0x14/0x40
>>>>>>>> [   75.472126] [<0000000000406174>] linux_sparc_syscall+0x34/0x44
>>>>>>>> [   75.548802] ---[ end trace 0000000000000000 ]---
>>>>>>>> [   75.609503] dfa blob stream 0xfff0000008926b96 not aligned.
>>>>>>>> [   75.682695] Kernel unaligned access at TPC[8db2a8] aa_dfa_unpack+0x6e8/0x720
>>>>>>>
>>>>>>> The non-8-byte-aligned address (0xfff0000008926b96) is coming from userspace
>>>>>>> (via the write syscall).
>>>>>>> Some apparmor userspace tool writes into the apparmor ".replace" virtual file with
>>>>>>> a source address which is not correctly aligned.
>>>>>>
>>>>>> the userpace buffer passed to write(2) has to be aligned? Its certainly nice if it
>>>>>> is but the userspace tooling hasn't been treating it as aligned. With that said,
>>>>>> the dfa should be padded to be aligned. So this tripping in the dfa is a bug,
>>>>>> and there really should be some validation to catch it.
>>>>>>
>>>>>>> You should be able to debug/find the problematic code with strace from userspace.
>>>>>>> Maybe someone with apparmor knowledge here on the list has an idea?
>>>>>>>
>>>>>> This is likely an unaligned 2nd profile, being split out and loaded separately
>>>>>> from the rest of the container. Basically the loader for some reason (there
>>>>>> are a few different possible reasons) is poking into the container format and
>>>>>> pulling out the profile at some offset, this gets loaded to the kernel but
>>>>>> it would seem that its causing an issue with the dfa alignment within the container,
>>>>>> which should be aligned to the original container.
>>>>>
>>>>>
>>>>> Regarding this:
>>>>>
>>>>>> Kernel side, we are going to need to add some extra verification checks, it should
>>>>>> be catching this, as unaligned as part of the unpack. Userspace side, we will have
>>>>>> to verify my guess and fix the loader.
>>>>>
>>>>> I wonder if loading those tables are really time critical?
>>>>
>>>> no, most policy is loaded once on boot and then at package upgrades. There are some
>>>> bits that may be loaded at application startup like, snap, libvirt, lxd, basically
>>>> container managers might do some thing custom per container.
>>>>
>>>> Its the run time we want to minimize, the cost of.
>>>>
>>>> Policy already can be unaligned (the container format rework to fix this is low
>>>> priority), and is treated as untrusted. It goes through an unpack, and translation to
>>>> machine native, with as many bounds checks, necessary transforms etc done at unpack
>>>> time as possible, so that the run time costs can be minimized.
>>>>
>>>>> If not, maybe just making the kernel aware that the tables might be unaligned
>>>>> can help, e.g. with the following (untested) patch.
>>>>> Adrian, maybe you want to test?
>>>>>
>>>>
>>>>> ------------------------
>>>>>
>>>>> [PATCH] Allow apparmor to handle unaligned dfa tables
>>>>>
>>>>> The dfa tables can originate from kernel or userspace and 8-byte alignment
>>>>> isn't always guaranteed and as such may trigger unaligned memory accesses
>>>>> on various architectures.
>>>>> Work around it by using the get_unaligned_xx() helpers.
>>>>>
>>>>> Signed-off-by: Helge Deller <deller@gmx.de>
>>>>>
>>>> lgtm,
>>>>
>>>> Acked-by: John Johansen <john.johansen@canonical.com>
>>>>
>>>> I'll pull this into my tree regardless of whether it fixes the issue
>>>> for Adrian, as it definitely fixes an issue.
>>>>
>>>> We can added additional patches on top s needed.
>>>
>>> My patch does not modify the UNPACK_ARRAY() macro, which we
>>> possibly should adjust as well.
>>
>> Indeed. See the patch below. I am not surprised testing hasn't triggered this
>> case, but a malicious userspace could certainly construct a policy that would
>> trigger it. Yes it would have to be root, but I still would like to prevent
>> root from being able to trigger this.
>>
>>> Adrian's testing seems to trigger only a few unaligned accesses,
>>> so maybe it's not a issue currently.
>>>
>> I don't think the userspace compiler is generating one that is bad, but it
>> possible to construct one and get it to the point where it can trip in
>> UNPACK_ARRAY
>>
>> commit 2c87528c1e7be3976b61ac797c6c8700364c4c63
>> Author: John Johansen <john.johansen@canonical.com>
>> Date: Tue Nov 25 13:59:32 2025 -0800
>>
>> apparmor: fix unaligned memory access of UNPACK_ARRAY
>>
>> The UNPACK_ARRAY macro has the potential to have unaligned memory
>> access when the unpacking an unaligned profile, which is caused by
>> userspace splitting up a profile container before sending it to the
>> kernel.
>>
>> While this is corner case, policy loaded from userspace should be
>> treated as untrusted so ensure that userspace can not trigger an
>> unaligned access.
>>
>> Signed-off-by: John Johansen <john.johansen@canonical.com>
>>
>> diff --git a/security/apparmor/include/match.h b/security/apparmor/include/match.h
>> index 1fbe82f5021b1..203f7c07529f5 100644
>> --- a/security/apparmor/include/match.h
>> +++ b/security/apparmor/include/match.h
>> @@ -104,7 +104,7 @@ struct aa_dfa {
>> struct table_header *tables[YYTD_ID_TSIZE];
>> };
>>
>> -#define byte_to_byte(X) (X)
>> +#define byte_to_byte(X) *(X)
>
> Even though is is only used once that ought to be (*(X))
>
>>
>> #define UNPACK_ARRAY(TABLE, BLOB, LEN, TTYPE, BTYPE, NTOHX) \
>> do { \
>> @@ -112,7 +112,7 @@ struct aa_dfa {
>> TTYPE *__t = (TTYPE *) TABLE; \
>> BTYPE *__b = (BTYPE *) BLOB; \
>> for (__i = 0; __i < LEN; __i++) { \
>> - __t[__i] = NTOHX(__b[__i]); \
>> + __t[__i] = NTOHX(&__b[__i]); \
>> } \
>> } while (0)
>>
>> diff --git a/security/apparmor/match.c b/security/apparmor/match.c
>> index 26e82ba879d44..3dcc342337aca 100644
>> --- a/security/apparmor/match.c
>> +++ b/security/apparmor/match.c
>> @@ -71,10 +71,10 @@ static struct table_header *unpack_table(char *blob, size_t bsize)
>> u8, u8, byte_to_byte);
>
> Is that that just memcpy() ?

No, it's memcpy() only on big-endian machines.
On little-endian machines it converts from big-endian
16/32-bit ints to little-endian 16/32-bit ints.

But I see some potential for optimization here:
a) on big-endian machines just use memcpy()
b) on little-endian machines use memcpy() to copy from possibly-unaligned
memory to then known-to-be-aligned destination. Then use a loop with
be32_to_cpu() instead of get_unaligned_xx() as it's faster.

Thoughts?

Helge
> David
>
>> else if (th.td_flags == YYTD_DATA16)
>> UNPACK_ARRAY(table->td_data, blob, th.td_lolen,
>> - u16, __be16, be16_to_cpu);
>> + u16, __be16, get_unaligned_be16);
>> else if (th.td_flags == YYTD_DATA32)
>> UNPACK_ARRAY(table->td_data, blob, th.td_lolen,
>> - u32, __be32, be32_to_cpu);
>> + u32, __be32, get_unaligned_be32);
>> else
>> goto fail;
>> /* if table was vmalloced make sure the page tables are synced
>>
>>
>>
>