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

[Puranjay Mohan]

Puranjay Mohan <puranjay12@xxxxxxxxx> writes:

> 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@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@xxxxxxxxx>
> Reported-by: yue sun <samsun1006219@xxxxxxxxx>
> Closes: https://lore.kernel.org/bpf/CABOYnLz09O1+2gGVJuCxd_24a-7UueXzV-Ff+Fr+h5EKFDiYCQ@xxxxxxxxxxxxxx/
> Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx>
> ---
>  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 &&

Kumar made me aware of the fact that env->prog->aux_arena should never
be NULL if the program has an addr_space_cast instruction. This means
that rather than checking for the NULL pointer here and leaving the
addr_space_cast as it is, We should reject programs that contain an
addr_space_cast instruction but don't have an associated arena.

Sent v2 doing the above: https://lore.kernel.org/bpf/20240322153518.11555-1-puranjay12@xxxxxxxxx/