[PATCH bpf] bpf: verifier: fix NULL pointer dereference in do_misc_fixups()

[Puranjay Mohan]

The addr_space_cast instruction is convered to a normal 32 bit mov by the
verifier if the cast from as(0) to as(1) or if the user has set the flag
BPF_F_NO_USER_CONV in the arena.

If the BPF program doesn't have an associated arena
env->prog->aux->arena is NULL and the verifier currently doesn't check
for it being NULL before accessing map_flags. This can cause a NULL
pointer dereference:

root at rv-tester:~# ./reproducer
 Unable to handle kernel access to user memory without uaccess routines at virtual address 0000000000000030
 Oops [#1]
 Modules linked in: sch_fq_codel drm fuse i2c_core drm_panel_orientation_quirks backlight configfs ip_tables x_tables
 CPU: 2 PID: 265 Comm: reproducer Not tainted 6.8.0 #3
 Hardware name: riscv-virtio,qemu (DT)
 epc : do_misc_fixups+0x43c/0x1168
  ra : bpf_check+0xda8/0x22b6
 epc : ffffffff8017eeaa ra : ffffffff801936d6 sp : ff200000011bb890
  gp : ffffffff82293468 tp : ff60000084fcb840 t0 : ff60000084e38048
  t1 : 0000000000000048 t2 : ff5fffff80000000 s0 : ff200000011bba60
  s1 : ff2000000101d058 a0 : ff6000008b980000 a1 : 0000000000000004
  a2 : 00000000000000e1 a3 : 0000000000000001 a4 : 0000000000010000
  a5 : 0000000000000000 a6 : 0000000000000001 a7 : ff2000000101d000
  s2 : 0000000000000002 s3 : 0000000000000000 s4 : 0000000000000000
  s5 : 0000000000000002 s6 : 0000000000000000 s7 : ff6000008b980aa0
  s8 : 0000000000010005 s9 : 0000000000000004 s10: ff6000008b980000
  s11: 0000000000000000 t3 : 0000000000002000 t4 : 0000ff0000000000
  t5 : 00ff000000000000 t6 : ff20000000000000
 status: 0000000200000120 badaddr: 0000000000000030 cause: 000000000000000d
 [<ffffffff8017eeaa>] do_misc_fixups+0x43c/0x1168
 [<ffffffff801936d6>] bpf_check+0xda8/0x22b6
 [<ffffffff80174b32>] bpf_prog_load+0x486/0x8dc
 [<ffffffff80176566>] __sys_bpf+0xbd8/0x214e
 [<ffffffff80177d14>] __riscv_sys_bpf+0x22/0x2a
 [<ffffffff80d2493a>] do_trap_ecall_u+0x102/0x17c
 [<ffffffff80d3048c>] ret_from_exception+0x0/0x64
 Code: b345 9783 0024 4685 8b63 16d7 3783 008d 7f9c 7fdc (5b9c) 83c9
 ---[ end trace 0000000000000000 ]---
 Kernel panic - not syncing: Fatal exception
 SMP: stopping secondary CPUs

Add a check for NULL pointer before checking map_flags.

Fixes: 6082b6c328b5 ("bpf: Recognize addr_space_cast instruction in the verifier.")
Reported-by: xingwei lee <xrivendell7 at gmail.com>
Reported-by: yue sun <samsun1006219 at gmail.com>
Closes: https://lore.kernel.org/bpf/CABOYnLz09O1+2gGVJuCxd_24a-7UueXzV-Ff+Fr+h5EKFDiYCQ@mail.gmail.com/
Signed-off-by: Puranjay Mohan <puranjay12 at gmail.com>
---
 kernel/bpf/verifier.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index ca6cacf7b42f..78945e7b856d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -19607,7 +19607,8 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 	for (i = 0; i < insn_cnt;) {
 		if (insn->code == (BPF_ALU64 | BPF_MOV | BPF_X) && insn->imm) {
 			if ((insn->off == BPF_ADDR_SPACE_CAST && insn->imm == 1) ||
-			    (((struct bpf_map *)env->prog->aux->arena)->map_flags & BPF_F_NO_USER_CONV)) {
+			    (env->prog->aux->arena &&
+			     (((struct bpf_map *)env->prog->aux->arena)->map_flags & BPF_F_NO_USER_CONV))) {
 				/* convert to 32-bit mov that clears upper 32-bit */
 				insn->code = BPF_ALU | BPF_MOV | BPF_X;
 				/* clear off, so it's a normal 'wX = wY' from JIT pov */
-- 
2.40.1