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

Helge Deller
2025-11-26T11:31:42Z

* Helge Deller <deller@gmx.de>:
> 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?

Like this (untested!) patch:

[PATCH] apparmor: Optimize table creation from possibly unaligned memory

Source blob may come from userspace and might be unaligned.
Try to optize the copying process by avoiding unaligned memory accesses.

Signed-off-by: Helge Deller <deller@gmx.de>

diff --git a/security/apparmor/include/match.h b/security/apparmor/include/match.h
index 1fbe82f5021b..225df6495c84 100644
--- a/security/apparmor/include/match.h
+++ b/security/apparmor/include/match.h
@@ -111,9 +111,14 @@ struct aa_dfa {
typeof(LEN) __i; \
TTYPE *__t = (TTYPE *) TABLE; \
BTYPE *__b = (BTYPE *) BLOB; \
- for (__i = 0; __i < LEN; __i++) { \
- __t[__i] = NTOHX(__b[__i]); \
- } \
+ BUILD_BUG_ON(sizeof(TTYPE) != sizeof(BTYPE)); \
+ /* copy to naturally aligned table address */ \
+ memcpy(__t, __b, (LEN) * sizeof(BTYPE)); \
+ /* convert from big-endian if necessary */ \
+ if (!IS_ENABLED(CONFIG_CPU_BIG_ENDIAN)) \
+ for (__i = 0; __i < LEN; __i++, __t++) { \
+ *__t = NTOHX(*__t); \
+ } \
} while (0)

static inline size_t table_size(size_t len, size_t el_size)