netdev - Re: [PATCH bpf] bpf: fix inner map masking to prevent oob under speculation

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Date:   Thu, 17 Jan 2019 18:32:11 +0000
From:   Martin Lau <kafai@...com>
To:     Daniel Borkmann <daniel@...earbox.net>
CC:     "ast@...nel.org" <ast@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf] bpf: fix inner map masking to prevent oob under
 speculation

On Thu, Jan 17, 2019 at 04:34:45PM +0100, Daniel Borkmann wrote:
> During review I noticed that inner meta map setup for map in
> map is buggy in that it does not propagate all needed data
> from the reference map which the verifier is later accessing.
> 
> In particular one such case is index masking to prevent out of
> bounds access under speculative execution due to missing the
> map's unpriv_array/index_mask field propagation. Fix this such
> that the verifier is generating the correct code for inlined
> lookups in case of unpriviledged use.
> 
> Before patch (test_verifier's 'map in map access' dump):
> 
>   # bpftool prog dump xla id 3
>      0: (62) *(u32 *)(r10 -4) = 0
>      1: (bf) r2 = r10
>      2: (07) r2 += -4
>      3: (18) r1 = map[id:4]
>      5: (07) r1 += 272                |
>      6: (61) r0 = *(u32 *)(r2 +0)     |
>      7: (35) if r0 >= 0x1 goto pc+6   | Inlined map in map lookup
>      8: (54) (u32) r0 &= (u32) 0      | with index masking for
>      9: (67) r0 <<= 3                 | map->unpriv_array.
>     10: (0f) r0 += r1                 |
>     11: (79) r0 = *(u64 *)(r0 +0)     |
>     12: (15) if r0 == 0x0 goto pc+1   |
>     13: (05) goto pc+1                |
>     14: (b7) r0 = 0                   |
>     15: (15) if r0 == 0x0 goto pc+11
>     16: (62) *(u32 *)(r10 -4) = 0
>     17: (bf) r2 = r10
>     18: (07) r2 += -4
>     19: (bf) r1 = r0
>     20: (07) r1 += 272                |
>     21: (61) r0 = *(u32 *)(r2 +0)     | Index masking missing (!)
>     22: (35) if r0 >= 0x1 goto pc+3   | for inner map despite
>     23: (67) r0 <<= 3                 | map->unpriv_array set.
>     24: (0f) r0 += r1                 |
>     25: (05) goto pc+1                |
>     26: (b7) r0 = 0                   |
>     27: (b7) r0 = 0
>     28: (95) exit
> 
> After patch:
> 
>   # bpftool prog dump xla id 1
>      0: (62) *(u32 *)(r10 -4) = 0
>      1: (bf) r2 = r10
>      2: (07) r2 += -4
>      3: (18) r1 = map[id:2]
>      5: (07) r1 += 272                |
>      6: (61) r0 = *(u32 *)(r2 +0)     |
>      7: (35) if r0 >= 0x1 goto pc+6   | Same inlined map in map lookup
>      8: (54) (u32) r0 &= (u32) 0      | with index masking due to
>      9: (67) r0 <<= 3                 | map->unpriv_array.
>     10: (0f) r0 += r1                 |
>     11: (79) r0 = *(u64 *)(r0 +0)     |
>     12: (15) if r0 == 0x0 goto pc+1   |
>     13: (05) goto pc+1                |
>     14: (b7) r0 = 0                   |
>     15: (15) if r0 == 0x0 goto pc+12
>     16: (62) *(u32 *)(r10 -4) = 0
>     17: (bf) r2 = r10
>     18: (07) r2 += -4
>     19: (bf) r1 = r0
>     20: (07) r1 += 272                |
>     21: (61) r0 = *(u32 *)(r2 +0)     |
>     22: (35) if r0 >= 0x1 goto pc+4   | Now fixed inlined inner map
>     23: (54) (u32) r0 &= (u32) 0      | lookup with proper index masking
>     24: (67) r0 <<= 3                 | for map->unpriv_array.
>     25: (0f) r0 += r1                 |
>     26: (05) goto pc+1                |
>     27: (b7) r0 = 0                   |
>     28: (b7) r0 = 0
>     29: (95) exit
> 
> Fixes: b2157399cc98 ("bpf: prevent out-of-bounds speculation")
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
The fix looks great.  Thanks for the fix!
In the future if there is another exception other than
array_map, a inner_map->ops->map_meta_alloc() can be introduced?

Acked-by: Martin KaFai Lau <kafai@...com>