Re: [PATCH bpf 1/2] xsk: fix OOB map writes when deleting elements

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Maciej Fijalkowski <maciej.fijalkowski@intel.com> writes:

> Jordy says:
>
> "
> In the xsk_map_delete_elem function an unsigned integer
> (map->max_entries) is compared with a user-controlled signed integer
> (k). Due to implicit type conversion, a large unsigned value for
> map->max_entries can bypass the intended bounds check:
>
> 	if (k >= map->max_entries)
> 		return -EINVAL;
>
> This allows k to hold a negative value (between -2147483648 and -2),
> which is then used as an array index in m->xsk_map[k], which results
> in an out-of-bounds access.
>
> 	spin_lock_bh(&m->lock);
> 	map_entry = &m->xsk_map[k]; // Out-of-bounds map_entry
> 	old_xs = unrcu_pointer(xchg(map_entry, NULL));  // Oob write
> 	if (old_xs)
> 		xsk_map_sock_delete(old_xs, map_entry);
> 	spin_unlock_bh(&m->lock);
>
> The xchg operation can then be used to cause an out-of-bounds write.
> Moreover, the invalid map_entry passed to xsk_map_sock_delete can lead
> to further memory corruption.
> "
>
> It indeed results in following splat:
>
> [76612.897343] BUG: unable to handle page fault for address: ffffc8fc2e461108
> [76612.904330] #PF: supervisor write access in kernel mode
> [76612.909639] #PF: error_code(0x0002) - not-present page
> [76612.914855] PGD 0 P4D 0
> [76612.917431] Oops: Oops: 0002 [#1] PREEMPT SMP
> [76612.921859] CPU: 11 UID: 0 PID: 10318 Comm: a.out Not tainted 6.12.0-rc1+ #470
> [76612.929189] Hardware name: Intel Corporation S2600WFT/S2600WFT, BIOS SE5C620.86B.02.01.0008.031920191559 03/19/2019
> [76612.939781] RIP: 0010:xsk_map_delete_elem+0x2d/0x60
> [76612.944738] Code: 00 00 41 54 55 53 48 63 2e 3b 6f 24 73 38 4c 8d a7 f8 00 00 00 48 89 fb 4c 89 e7 e8 2d bf 05 00 48 8d b4 eb 00 01 00 00 31 ff <48> 87 3e 48 85 ff 74 05 e8 16 ff ff ff 4c 89 e7 e8 3e bc 05 00 31
> [76612.963774] RSP: 0018:ffffc9002e407df8 EFLAGS: 00010246
> [76612.969079] RAX: 0000000000000000 RBX: ffffc9002e461000 RCX: 0000000000000000
> [76612.976323] RDX: 0000000000000001 RSI: ffffc8fc2e461108 RDI: 0000000000000000
> [76612.983569] RBP: ffffffff80000001 R08: 0000000000000000 R09: 0000000000000007
> [76612.990812] R10: ffffc9002e407e18 R11: ffff888108a38858 R12: ffffc9002e4610f8
> [76612.998060] R13: ffff888108a38858 R14: 00007ffd1ae0ac78 R15: ffffc9002e4610c0
> [76613.005303] FS:  00007f80b6f59740(0000) GS:ffff8897e0ec0000(0000) knlGS:0000000000000000
> [76613.013517] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [76613.019349] CR2: ffffc8fc2e461108 CR3: 000000011e3ef001 CR4: 00000000007726f0
> [76613.026595] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [76613.033841] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [76613.041086] PKRU: 55555554
> [76613.043842] Call Trace:
> [76613.046331]  <TASK>
> [76613.048468]  ? __die+0x20/0x60
> [76613.051581]  ? page_fault_oops+0x15a/0x450
> [76613.055747]  ? search_extable+0x22/0x30
> [76613.059649]  ? search_bpf_extables+0x5f/0x80
> [76613.063988]  ? exc_page_fault+0xa9/0x140
> [76613.067975]  ? asm_exc_page_fault+0x22/0x30
> [76613.072229]  ? xsk_map_delete_elem+0x2d/0x60
> [76613.076573]  ? xsk_map_delete_elem+0x23/0x60
> [76613.080914]  __sys_bpf+0x19b7/0x23c0
> [76613.084555]  __x64_sys_bpf+0x1a/0x20
> [76613.088194]  do_syscall_64+0x37/0xb0
> [76613.091832]  entry_SYSCALL_64_after_hwframe+0x4b/0x53
> [76613.096962] RIP: 0033:0x7f80b6d1e88d
> [76613.100592] Code: 5b 41 5c c3 66 0f 1f 84 00 00 00 00 00 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 73 b5 0f 00 f7 d8 64 89 01 48
> [76613.119631] RSP: 002b:00007ffd1ae0ac68 EFLAGS: 00000206 ORIG_RAX: 0000000000000141
> [76613.131330] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f80b6d1e88d
> [76613.142632] RDX: 0000000000000098 RSI: 00007ffd1ae0ad20 RDI: 0000000000000003
> [76613.153967] RBP: 00007ffd1ae0adc0 R08: 0000000000000000 R09: 0000000000000000
> [76613.166030] R10: 00007f80b6f77040 R11: 0000000000000206 R12: 00007ffd1ae0aed8
> [76613.177130] R13: 000055ddf42ce1e9 R14: 000055ddf42d0d98 R15: 00007f80b6fab040
> [76613.188129]  </TASK>
>
> Fix this by simply changing key type from int to u32.
>
> Fixes: fbfc504a24f5 ("bpf: introduce new bpf AF_XDP map type BPF_MAP_TYPE_XSKMAP")
> Reported-by: Jordy Zomer <jordyzomer@google.com>
> Suggested-by: Jordy Zomer <jordyzomer@google.com>
> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>

Nice find!

Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>