Re: [PATCH bpf] bpf: Fix null pointer dereference in resolve_prog_type() for BPF_PROG_TYPE_EXT

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Jun 20, 2024 at 04:54:51PM +0800, Tengda Wu wrote:
> 
> 
> On 2024/6/20 14:46, Leon Hwang wrote:
> > 
> > 
> > On 20/6/24 14:07, Tengda Wu wrote:
> >> When loading a EXT program without specifying `attr->attach_prog_fd`,
> >> the `prog->aux->dst_prog` will be null. At this time, calling
> >> resolve_prog_type() anywhere will result in a null pointer dereference.
> > 
> > Interesting, same NULL pointer dereference causes another issue[0].
> > 
> > As for my case, when resolve_prog_type(), it has to use
> > prog->aux->saved_dst_prog_type instead of prog->aux->dst_prog->type for
> > EXT program, in order to avoid NULL pointer dereference.
> > 
> > [0] https://lore.kernel.org/bpf/20240602122421.50892-2-hffilwlqm@gmail.com/
> > 
> > Thanks,
> > Leon
> >This looks good, but unfortunately, there is still a problem with using 
> `prog->aux->saved_dst_prog_type` to resolve prog type, because its value still 
> comes from `prog->aux->dst_prog`in check_attach_btf_id(). 
> 
> Additionally, resolve_prog_type() not always be used after check_attach_btf_id().
> The following example stack trace proves the existence of this situation. It 
> shows that NULL pointer dereference occurs in add_subprog_and_kfunc(), which
> check_attach_btf_id() has not yet reached. 
> 
> So it may be more effective to check and avoid dst_prog empty when EXT program loads.

also please note it's breaking test_libbpf_probe_prog_types test

  test_libbpf_probe_prog_types:FAIL:BPF_PROG_TYPE_EXT unexpected BPF_PROG_TYPE_EXT: actual 0 != expected 1

because the attach_prog_fd wasn't needed to load EXT program before,
but I guess the following attach would fail.. so it's likely ok

jirka

> 
> >>
> >> Example stack trace:
> >>
> >> [    8.107863] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000004
> >> [    8.108262] Mem abort info:
> >> [    8.108384]   ESR = 0x0000000096000004
> >> [    8.108547]   EC = 0x25: DABT (current EL), IL = 32 bits
> >> [    8.108722]   SET = 0, FnV = 0
> >> [    8.108827]   EA = 0, S1PTW = 0
> >> [    8.108939]   FSC = 0x04: level 0 translation fault
> >> [    8.109102] Data abort info:
> >> [    8.109203]   ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
> >> [    8.109399]   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
> >> [    8.109614]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
> >> [    8.109836] user pgtable: 4k pages, 48-bit VAs, pgdp=0000000101354000
> >> [    8.110011] [0000000000000004] pgd=0000000000000000, p4d=0000000000000000
> >> [    8.112624] Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
> >> [    8.112783] Modules linked in:
> >> [    8.113120] CPU: 0 PID: 99 Comm: may_access_dire Not tainted 6.10.0-rc3-next-20240613-dirty #1
> >> [    8.113230] Hardware name: linux,dummy-virt (DT)
> >> [    8.113390] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> >> [    8.113429] pc : may_access_direct_pkt_data+0x24/0xa0
> >> [    8.113746] lr : add_subprog_and_kfunc+0x634/0x8e8
> >> [    8.113798] sp : ffff80008283b9f0
> >> [    8.113813] x29: ffff80008283b9f0 x28: ffff800082795048 x27: 0000000000000001
> >> [    8.113881] x26: ffff0000c0bb2600 x25: 0000000000000000 x24: 0000000000000000
> >> [    8.113897] x23: ffff0000c1134000 x22: 000000000001864f x21: ffff0000c1138000
> >> [    8.113912] x20: 0000000000000001 x19: ffff0000c12b8000 x18: ffffffffffffffff
> >> [    8.113929] x17: 0000000000000000 x16: 0000000000000000 x15: 0720072007200720
> >> [    8.113944] x14: 0720072007200720 x13: 0720072007200720 x12: 0720072007200720
> >> [    8.113958] x11: 0720072007200720 x10: 0000000000f9fca4 x9 : ffff80008021f4e4
> >> [    8.113991] x8 : 0101010101010101 x7 : 746f72705f6d656d x6 : 000000001e0e0f5f
> >> [    8.114006] x5 : 000000000001864f x4 : ffff0000c12b8000 x3 : 000000000000001c
> >> [    8.114020] x2 : 0000000000000002 x1 : 0000000000000000 x0 : 0000000000000000
> >> [    8.114126] Call trace:
> >> [    8.114159]  may_access_direct_pkt_data+0x24/0xa0
> >> [    8.114202]  bpf_check+0x3bc/0x28c0
> >> [    8.114214]  bpf_prog_load+0x658/0xa58
> >> [    8.114227]  __sys_bpf+0xc50/0x2250
> >> [    8.114240]  __arm64_sys_bpf+0x28/0x40
> >> [    8.114254]  invoke_syscall.constprop.0+0x54/0xf0
> >> [    8.114273]  do_el0_svc+0x4c/0xd8
> >> [    8.114289]  el0_svc+0x3c/0x140
> >> [    8.114305]  el0t_64_sync_handler+0x134/0x150
> >> [    8.114331]  el0t_64_sync+0x168/0x170
> >> [    8.114477] Code: 7100707f 54000081 f9401c00 f9403800 (b9400403)
> >> [    8.118672] ---[ end trace 0000000000000000 ]---
> >>
> >> Fix this by adding dst_prog non-empty check in BPF_PROG_TYPE_EXT case
> >> when calling bpf_prog_load().
> >>
> >> Fixes: 4a9c7bbe2ed4 ("bpf: Resolve to prog->aux->dst_prog->type only for BPF_PROG_TYPE_EXT")
> >> Signed-off-by: Tengda Wu <wutengda@huaweicloud.com>
> >> Cc: stable@vger.kernel.org # v5.18+
> >> ---
> >>  kernel/bpf/syscall.c | 5 ++++-
> >>  1 file changed, 4 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> >> index f45ed6adc092..4490f8ccf006 100644
> >> --- a/kernel/bpf/syscall.c
> >> +++ b/kernel/bpf/syscall.c
> >> @@ -2632,9 +2632,12 @@ bpf_prog_load_check_attach(enum bpf_prog_type prog_type,
> >>  			return 0;
> >>  		return -EINVAL;
> >>  	case BPF_PROG_TYPE_SYSCALL:
> >> -	case BPF_PROG_TYPE_EXT:
> >>  		if (expected_attach_type)
> >>  			return -EINVAL;
> >> +		return 0;
> >> +	case BPF_PROG_TYPE_EXT:
> >> +		if (expected_attach_type || !dst_prog)
> >> +			return -EINVAL;
> >>  		fallthrough;
> >>  	default:
> >>  		return 0;
>