bpf: Fix NULL pointer dereference when registering bpf trampoline

A panic was reported on arm64:

[   44.517109] audit: type=1334 audit(1658859870.268:59): prog-id=19 op=LOAD
[   44.622031] Unable to handle kernel NULL pointer dereference at
virtual address 0000000000000010
[   44.624321] Mem abort info:
[   44.625049]   ESR = 0x0000000096000004
[   44.625935]   EC = 0x25: DABT (current EL), IL = 32 bits
[   44.627182]   SET = 0, FnV = 0
[   44.627930]   EA = 0, S1PTW = 0
[   44.628684]   FSC = 0x04: level 0 translation fault
[   44.629788] Data abort info:
[   44.630474]   ISV = 0, ISS = 0x00000004
[   44.631362]   CM = 0, WnR = 0
[   44.632041] user pgtable: 4k pages, 48-bit VAs, pgdp=0000000100ab5000
[   44.633494] [0000000000000010] pgd=0000000000000000, p4d=0000000000000000
[   44.635202] Internal error: Oops: 96000004 [#1] SMP
[   44.636452] Modules linked in: xfs crct10dif_ce ghash_ce virtio_blk
virtio_console virtio_mmio qemu_fw_cfg
[   44.638713] CPU: 2 PID: 1 Comm: systemd Not tainted 5.19.0-rc7 #1
[   44.640164] Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
[   44.641799] pstate: 00400005 (nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[   44.643404] pc : ftrace_set_filter_ip+0x24/0xa0
[   44.644659] lr : bpf_trampoline_update.constprop.0+0x428/0x4a0
[   44.646118] sp : ffff80000803b9f0
[   44.646950] x29: ffff80000803b9f0 x28: ffff0b5d80364400 x27: ffff80000803bb48
[   44.648721] x26: ffff8000085ad000 x25: ffff0b5d809d2400 x24: 0000000000000000
[   44.650493] x23: 00000000ffffffed x22: ffff0b5dd7ea0900 x21: 0000000000000000
[   44.652279] x20: 0000000000000000 x19: 0000000000000000 x18: ffffffffffffffff
[   44.654067] x17: 0000000000000000 x16: 0000000000000000 x15: ffffffffffffffff
[   44.655787] x14: ffff0b5d809d2498 x13: ffff0b5d809d2432 x12: 0000000005f5e100
[   44.657535] x11: abcc77118461cefd x10: 000000000000005f x9 : ffffa7219cb5b190
[   44.659254] x8 : ffffa7219c8e0000 x7 : 0000000000000000 x6 : ffffa7219db075e0
[   44.661066] x5 : ffffa7219d3130e0 x4 : ffffa7219cab9da0 x3 : 0000000000000000
[   44.662837] x2 : 0000000000000000 x1 : ffffa7219cb7a5c0 x0 : 0000000000000000
[   44.664675] Call trace:
[   44.665274]  ftrace_set_filter_ip+0x24/0xa0
[   44.666327]  bpf_trampoline_update.constprop.0+0x428/0x4a0
[   44.667696]  __bpf_trampoline_link_prog+0xcc/0x1c0
[   44.668834]  bpf_trampoline_link_prog+0x40/0x64
[   44.669919]  bpf_tracing_prog_attach+0x120/0x490
[   44.671011]  link_create+0xe0/0x2b0
[   44.671869]  __sys_bpf+0x484/0xd30
[   44.672706]  __arm64_sys_bpf+0x30/0x40
[   44.673678]  invoke_syscall+0x78/0x100
[   44.674623]  el0_svc_common.constprop.0+0x4c/0xf4
[   44.675783]  do_el0_svc+0x38/0x4c
[   44.676624]  el0_svc+0x34/0x100
[   44.677429]  el0t_64_sync_handler+0x11c/0x150
[   44.678532]  el0t_64_sync+0x190/0x194
[   44.679439] Code: 2a0203f4 f90013f5 2a0303f5 f9001fe1 (f9400800)
[   44.680959] ---[ end trace 0000000000000000 ]---
[   44.682111] Kernel panic - not syncing: Oops: Fatal exception
[   44.683488] SMP: stopping secondary CPUs
[   44.684551] Kernel Offset: 0x2721948e0000 from 0xffff800008000000
[   44.686095] PHYS_OFFSET: 0xfffff4a380000000
[   44.687144] CPU features: 0x010,00022811,19001080
[   44.688308] Memory Limit: none
[   44.689082] ---[ end Kernel panic - not syncing: Oops: Fatal exception ]---

It's caused by a NULL tr->fops passed to ftrace_set_filter_ip(). tr->fops
is initialized to NULL and is assigned to an allocated memory address if
CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS is enabled. Since there is no
direct call on arm64 yet, the config can't be enabled.

To fix it, call ftrace_set_filter_ip() only if tr->fops is not NULL.

Fixes: 00963a2e ("bpf: Support bpf_trampoline on functions with IPMODIFY (e.g. livepatch)")
Reported-by: Bruno Goncalves <bgoncalv@redhat.com>
Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Bruno Goncalves <bgoncalv@redhat.com>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20220728114048.3540461-1-xukuohai@huaweicloud.com

kernel/bpf/trampoline.c

@@ -248,8 +248,11 @@ static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
 	int ret;
 
 	faddr = ftrace_location((unsigned long)ip);
-	if (faddr)
+	if (faddr) {
+		if (!tr->fops)
+			return -ENOTSUPP;
 		tr->func.ftrace_managed = true;
+	}
 
 	if (bpf_trampoline_module_get(tr))
 		return -ENOENT;