Re: [PATCH bpf-next v4 00/20] Add return value range check for BPF LSM

Paul Moore
2024-07-19T02:13:27Z

On Fri, Jul 12, 2024 at 5:44 PM Paul Moore <paul@paul-moore.com> wrote:
> On Thu, Jul 11, 2024 at 7:13 AM Xu Kuohai <xukuohai@huaweicloud.com> wrote:
> > From: Xu Kuohai <xukuohai@huawei.com>
> >
> > LSM BPF prog returning a positive number attached to the hook
> > file_alloc_security makes kernel panic.
> >
> > Here is a panic log:
> >
> > [ 441.235774] BUG: kernel NULL pointer dereference, address: 00000000000009
> > [ 441.236748] #PF: supervisor write access in kernel mode
> > [ 441.237429] #PF: error_code(0x0002) - not-present page
> > [ 441.238119] PGD 800000000b02f067 P4D 800000000b02f067 PUD b031067 PMD 0
> > [ 441.238990] Oops: 0002 [#1] PREEMPT SMP PTI
> > [ 441.239546] CPU: 0 PID: 347 Comm: loader Not tainted 6.8.0-rc6-gafe0cbf23373 #22
> > [ 441.240496] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.15.0-0-g2dd4b4
> > [ 441.241933] RIP: 0010:alloc_file+0x4b/0x190
> > [ 441.242485] Code: 8b 04 25 c0 3c 1f 00 48 8b b0 30 0c 00 00 e8 9c fe ff ff 48 3d 00 f0 ff fb
> > [ 441.244820] RSP: 0018:ffffc90000c67c40 EFLAGS: 00010203
> > [ 441.245484] RAX: ffff888006a891a0 RBX: ffffffff8223bd00 RCX: 0000000035b08000
> > [ 441.246391] RDX: ffff88800b95f7b0 RSI: 00000000001fc110 RDI: f089cd0b8088ffff
> > [ 441.247294] RBP: ffffc90000c67c58 R08: 0000000000000001 R09: 0000000000000001
> > [ 441.248209] R10: 0000000000000001 R11: 0000000000000001 R12: 0000000000000001
> > [ 441.249108] R13: ffffc90000c67c78 R14: ffffffff8223bd00 R15: fffffffffffffff4
> > [ 441.250007] FS: 00000000005f3300(0000) GS:ffff88803ec00000(0000) knlGS:0000000000000000
> > [ 441.251053] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [ 441.251788] CR2: 00000000000001a9 CR3: 000000000bdc4003 CR4: 0000000000170ef0
> > [ 441.252688] Call Trace:
> > [ 441.253011] <TASK>
> > [ 441.253296] ? __die+0x24/0x70
> > [ 441.253702] ? page_fault_oops+0x15b/0x480
> > [ 441.254236] ? fixup_exception+0x26/0x330
> > [ 441.254750] ? exc_page_fault+0x6d/0x1c0
> > [ 441.255257] ? asm_exc_page_fault+0x26/0x30
> > [ 441.255792] ? alloc_file+0x4b/0x190
> > [ 441.256257] alloc_file_pseudo+0x9f/0xf0
> > [ 441.256760] __anon_inode_getfile+0x87/0x190
> > [ 441.257311] ? lock_release+0x14e/0x3f0
> > [ 441.257808] bpf_link_prime+0xe8/0x1d0
> > [ 441.258315] bpf_tracing_prog_attach+0x311/0x570
> > [ 441.258916] ? __pfx_bpf_lsm_file_alloc_security+0x10/0x10
> > [ 441.259605] __sys_bpf+0x1bb7/0x2dc0
> > [ 441.260070] __x64_sys_bpf+0x20/0x30
> > [ 441.260533] do_syscall_64+0x72/0x140
> > [ 441.261004] entry_SYSCALL_64_after_hwframe+0x6e/0x76
> > [ 441.261643] RIP: 0033:0x4b0349
> > [ 441.262045] Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 88
> > [ 441.264355] RSP: 002b:00007fff74daee38 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
> > [ 441.265293] RAX: ffffffffffffffda RBX: 00007fff74daef30 RCX: 00000000004b0349
> > [ 441.266187] RDX: 0000000000000040 RSI: 00007fff74daee50 RDI: 000000000000001c
> > [ 441.267114] RBP: 000000000000001b R08: 00000000005ef820 R09: 0000000000000000
> > [ 441.268018] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000004
> > [ 441.268907] R13: 0000000000000004 R14: 00000000005ef018 R15: 00000000004004e8
> >
> > This is because the filesystem uses IS_ERR to check if the return value
> > is an error code. If it is not, the filesystem takes the return value
> > as a file pointer. Since the positive number returned by the BPF prog
> > is not a real file pointer, this misinterpretation causes a panic.
> >
> > Since other LSM modules always return either a negative error code
> > or a valid pointer, this specific issue only exists in BPF LSM. The
> > proposed solution is to reject LSM BPF progs returning unexpected
> > values in the verifier. This patch set adds return value check to
> > ensure only BPF progs returning expected values are accepted.
> >
> > Since each LSM hook has different excepted return values, we need to
> > know the expected return values for each individual hook to do the
> > check. Earlier versions of the patch set used LSM hook annotations
> > to specify the return value range for each hook. Based on Paul's
> > suggestion, current version gets rid of such annotations and instead
> > converts hook return values to a common pattern: return 0 on success
> > and negative error code on failure.
> >
> > Basically, LSM hooks are divided into two types: hooks that return a
> > negative error code and zero or other values, and hooks that do not
> > return a negative error code. This patch set converts all hooks of the
> > first type and part of the second type to return 0 on success and a
> > negative error code on failure (see patches 1-10). For certain hooks,
> > like ismaclabel and inode_xattr_skipcap, the hook name already imply
> > that returning 0 or 1 is the best choice, so they are not converted.
> > There are four unconverted hooks. Except for ismaclabel, which is not
> > used by BPF LSM, the other three are specified with a BTF ID list to
> > only return 0 or 1.
>
> Thank you for following up on your initial work with this patchset, Xu
> Kuohai. It doesn't look like I'm going to be able to finish my review
> by the end of the day today, so expect that a bit later, but so far I
> think most of the changes look good and provide a nice improvement :)

You should have my feedback now, let me know if you have any questions.

One additional comment I might make is that you may either want to
wait until after v6.11-rc1 is released and I've had a chance to rebase
the lsm/{dev,next} branches and merge the patchsets which are
currently queued; there are a few patches queued up which will have an
impact on this work. While it's an unstable branch, you can take a
peek at those queues patches in the lsm/dev-staging branch.

https://github.com/LinuxSecurityModule/kernel/blob/main/README.md

--
paul-moore.com