bpf: Reject variable offset alu on PTR_TO_FLOW_KEYS

Commit Message

Hao Sun Jan. 9, 2024, 3:36 p.m. UTC

For PTR_TO_FLOW_KEYS, check_flow_keys_access() only uses fixed off
for validation. However, variable offset ptr alu is not prohibited
for this ptr kind. So the variable offset is not checked.

The following prog is accepted:
func#0 @0
0: R1=ctx() R10=fp0
0: (bf) r6 = r1                       ; R1=ctx() R6_w=ctx()
1: (79) r7 = *(u64 *)(r6 +144)        ; R6_w=ctx() R7_w=flow_keys()
2: (b7) r8 = 1024                     ; R8_w=1024
3: (37) r8 /= 1                       ; R8_w=scalar()
4: (57) r8 &= 1024                    ; R8_w=scalar(smin=smin32=0,
smax=umax=smax32=umax32=1024,var_off=(0x0; 0x400))
5: (0f) r7 += r8
mark_precise: frame0: last_idx 5 first_idx 0 subseq_idx -1
mark_precise: frame0: regs=r8 stack= before 4: (57) r8 &= 1024
mark_precise: frame0: regs=r8 stack= before 3: (37) r8 /= 1
mark_precise: frame0: regs=r8 stack= before 2: (b7) r8 = 1024
6: R7_w=flow_keys(smin=smin32=0,smax=umax=smax32=umax32=1024,var_off
=(0x0; 0x400)) R8_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=1024,
var_off=(0x0; 0x400))
6: (79) r0 = *(u64 *)(r7 +0)          ; R0_w=scalar()
7: (95) exit

This prog loads flow_keys to r7, and adds the variable offset r8
to r7, and finally causes out-of-bounds access:

BUG: unable to handle page fault for address: ffffc90014c80038
...
Call Trace:
 <TASK>
 bpf_dispatcher_nop_func include/linux/bpf.h:1231 [inline]
 __bpf_prog_run include/linux/filter.h:651 [inline]
 bpf_prog_run include/linux/filter.h:658 [inline]
 bpf_prog_run_pin_on_cpu include/linux/filter.h:675 [inline]
 bpf_flow_dissect+0x15f/0x350 net/core/flow_dissector.c:991
 bpf_prog_test_run_flow_dissector+0x39d/0x620 net/bpf/test_run.c:1359
 bpf_prog_test_run kernel/bpf/syscall.c:4107 [inline]
 __sys_bpf+0xf8f/0x4560 kernel/bpf/syscall.c:5475
 __do_sys_bpf kernel/bpf/syscall.c:5561 [inline]
 __se_sys_bpf kernel/bpf/syscall.c:5559 [inline]
 __x64_sys_bpf+0x73/0xb0 kernel/bpf/syscall.c:5559
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0x3f/0x110 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

Fix this by rejecting ptr alu with variable offset on flow_keys.
Applying the patch makes the program rejected with "R7 pointer
arithmetic on flow_keys prohibited"

Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
Signed-off-by: Hao Sun <[e-mail]>
---
 kernel/bpf/verifier.c | 4 ++++
 1 file changed, 4 insertions(+)

Comments

Eduard Zingerman Jan. 9, 2024, 4:21 p.m. UTC | #1

On Tue, 2024-01-09 at 16:36 +0100, Hao Sun wrote:
> For PTR_TO_FLOW_KEYS, check_flow_keys_access() only uses fixed off
> for validation. However, variable offset ptr alu is not prohibited
> for this ptr kind. So the variable offset is not checked.
> 
[...]
> 
> Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
> Signed-off-by: Hao Sun <[e-mail]>
> ---
>  kernel/bpf/verifier.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index adbf330d364b..65f598694d55 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -12826,6 +12826,10 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
>  	}
>  
>  	switch (base_type(ptr_reg->type)) {
> +	case PTR_TO_FLOW_KEYS:
> +		if (known)
> +			break;
> +		fallthrough;
>  	case CONST_PTR_TO_MAP:
>  		/* smin_val represents the known value */
>  		if (known && smin_val == 0 && opcode == BPF_ADD)

This change makes sense, could you please add a testcase?

Also, this switch is written to explicitly disallow and implicitly allow
pointer arithmetics, which might be a bit unsafe when new ptr types are added.
Would it make more sense to instead rewrite it to explicitly allow?
E.g. here is what it currently allows / disallows:

| Pointer type        | Arithmetics allowed |
|---------------------+---------------------|
| PTR_TO_CTX          | yes                 |
| CONST_PTR_TO_MAP    | conditionally       |
| PTR_TO_MAP_VALUE    | yes                 |
| PTR_TO_MAP_KEY      | yes                 |
| PTR_TO_STACK        | yes                 |
| PTR_TO_PACKET_META  | yes                 |
| PTR_TO_PACKET       | yes                 |
| PTR_TO_PACKET_END   | no                  |
| PTR_TO_FLOW_KEYS    | yes                 |
| PTR_TO_SOCKET       | no                  |
| PTR_TO_SOCK_COMMON  | no                  |
| PTR_TO_TCP_SOCK     | no                  |
| PTR_TO_TP_BUFFER    | yes                 |
| PTR_TO_XDP_SOCK     | no                  |
| PTR_TO_BTF_ID       | yes                 |
| PTR_TO_MEM          | yes                 |
| PTR_TO_BUF          | yes                 |
| PTR_TO_FUNC         | yes                 |
| CONST_PTR_TO_DYNPTR | yes                 |

Of these PTR_TO_FUNC and CONST_PTR_TO_DYNPTR (?) should not be allowed
as well, probably (not sure if that could be exploited).

Hao Sun Jan. 9, 2024, 4:59 p.m. UTC | #2

On Tue, Jan 9, 2024 at 5:21 PM Eduard Zingerman <[e-mail]> wrote:
>
> On Tue, 2024-01-09 at 16:36 +0100, Hao Sun wrote:
> > For PTR_TO_FLOW_KEYS, check_flow_keys_access() only uses fixed off
> > for validation. However, variable offset ptr alu is not prohibited
> > for this ptr kind. So the variable offset is not checked.
> >
> [...]
> >
> > Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
> > Signed-off-by: Hao Sun <[e-mail]>
> > ---
> >  kernel/bpf/verifier.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index adbf330d364b..65f598694d55 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -12826,6 +12826,10 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
> >       }
> >
> >       switch (base_type(ptr_reg->type)) {
> > +     case PTR_TO_FLOW_KEYS:
> > +             if (known)
> > +                     break;
> > +             fallthrough;
> >       case CONST_PTR_TO_MAP:
> >               /* smin_val represents the known value */
> >               if (known && smin_val == 0 && opcode == BPF_ADD)
>
> This change makes sense, could you please add a testcase?
>

OK, will do it in the next version tomorrow.

> Also, this switch is written to explicitly disallow and implicitly allow
> pointer arithmetics, which might be a bit unsafe when new ptr types are added.
> Would it make more sense to instead rewrite it to explicitly allow?

Yes, this sounds more safe and clear to me, should be done in another patch.

> E.g. here is what it currently allows / disallows:
>
> | Pointer type        | Arithmetics allowed |
> |---------------------+---------------------|
> | PTR_TO_CTX          | yes                 |
> | CONST_PTR_TO_MAP    | conditionally       |
> | PTR_TO_MAP_VALUE    | yes                 |
> | PTR_TO_MAP_KEY      | yes                 |
> | PTR_TO_STACK        | yes                 |
> | PTR_TO_PACKET_META  | yes                 |
> | PTR_TO_PACKET       | yes                 |
> | PTR_TO_PACKET_END   | no                  |
> | PTR_TO_FLOW_KEYS    | yes                 |

This one should be `conditionally`, variable offset disallowed, fixed allowed.

> | PTR_TO_SOCKET       | no                  |
> | PTR_TO_SOCK_COMMON  | no                  |
> | PTR_TO_TCP_SOCK     | no                  |
> | PTR_TO_TP_BUFFER    | yes                 |
> | PTR_TO_XDP_SOCK     | no                  |
> | PTR_TO_BTF_ID       | yes                 |
> | PTR_TO_MEM          | yes                 |
> | PTR_TO_BUF          | yes                 |
> | PTR_TO_FUNC         | yes                 |
> | CONST_PTR_TO_DYNPTR | yes                 |
>
> Of these PTR_TO_FUNC and CONST_PTR_TO_DYNPTR (?) should not be allowed
> as well, probably (not sure if that could be exploited).

I think both should be disallowed.

If alu sanitation is triggered, alu op on func and dynptr would be
rejected by retrieve_ptr_limit();
otherwise, it could be dangerous.

Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index adbf330d364b..65f598694d55 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12826,6 +12826,10 @@  static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 	}
 
 	switch (base_type(ptr_reg->type)) {
+	case PTR_TO_FLOW_KEYS:
+		if (known)
+			break;
+		fallthrough;
 	case CONST_PTR_TO_MAP:
 		/* smin_val represents the known value */
 		if (known && smin_val == 0 && opcode == BPF_ADD)