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Date: Wed, 9 Nov 2016 23:12:16 +0100
From: Jann Horn <jannh@...gle.com>
To: Josef Bacik <jbacik@...com>
Cc: Daniel Borkmann <daniel@...earbox.net>,
Alexei Starovoitov <ast@...nel.org>,
"David S. Miller" <davem@...emloft.net>, security@...nel.org,
netdev@...r.kernel.org
Subject: Re: 484611357c19 introduces arbitrary kernel write bug (root-only)
Can you resend with "git send-email" or so? "git am" says that the
patch is corrupt, likely because of line wrapping.
On Wed, Nov 9, 2016 at 10:21 PM, Josef Bacik <jbacik@...com> wrote:
> On 11/08/2016 07:23 PM, Jann Horn wrote:
>>
>> In 484611357c19 (not in any stable kernel yet), functionality is
>> introduced that allows root (and afaics nobody else, since nobody else
>> is allowed to perform pointer arithmetic) to basically write to (and
>> read from) arbitrary kernel memory. There are multiple bugs in the
>> validation logic:
>>
>> - A bitwise AND of values in the ranges [a,b] and [c,d] is assumed to
>> always result in a value
>> >= a&b. However, for the combination of ranges [1,1] and [1,2],
>> this calculates a minimum of 1
>> while actually, 1&2 is zero. This is the bug that my crasher
>> (below) triggers.
>> - a%b is assumed to always be smaller than b-1. However, for b==0,
>> this will calculate an upper
>> limit of -1 while the values will actually always be zero.
>> - I'm not sure about this, but I think that, when only one end of the
>> range is bounded, the logic will
>> incorrectly also treat the other end as a bounded, and because of
>> the usage of bound
>> placeholders that are smaller than the actual maximum values, this
>> could be used to perform
>> out-of-bounds accesses.
>>
>> The fun part here is that, as soon as the validation is just
>> off-by-one, arithmetic transformations can be used to turn that into
>> out-of-bounds accesses at arbitrary offsets. The crasher turns the
>> off-by-one into a memory write at offset 0x10000000.
>>
>
> Can you give this a whirl? It addresses your testcase and the other issues
> you've brought up. Thanks
>
> From e47a1de98af2c1bcebd4224f546e3be1fd340b6a Mon Sep 17 00:00:00 2001
> From: Josef Bacik <jbacik@...com>
> Date: Wed, 9 Nov 2016 16:09:52 -0500
> Subject: [PATCH] bpf: fix range arithmetic for bpf map access
>
> I made some invalid assumptions with BPF_AND and BPF_MOD that could result
> in
> invalid accesses to bpf map entries. Fix this up by doing a few things
>
> 1) Kill BPF_MOD support. This doesn't actually get used by the compiler in
> real
> life and just adds extra complexity.
>
> 2) Fix the logic for BPF_AND. If the min value is negative then that is the
> new
> minimum, otherwise it is unconditionally 0.
>
> 3) Don't do operations on the ranges if they are set to the limits, as they
> are
> by definition undefined, and allowing arithmetic operations on those values
> could make them appear valid when they really aren't.
>
> This fixes the testcase provided by Jann as well as a few other theoretical
> problems.
>
> Reported-by: Jann Horn <jannh@...gle.com>
> Signed-off-by: Josef Bacik <jbacik@...com>
> ---
> include/linux/bpf_verifier.h | 3 +-
> kernel/bpf/verifier.c | 65
> ++++++++++++++++++++++++++++----------------
> 2 files changed, 44 insertions(+), 24 deletions(-)
>
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index ac5b393..15ceb7f 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -22,7 +22,8 @@ struct bpf_reg_state {
> * Used to determine if any memory access using this register will
> * result in a bad access.
> */
> - u64 min_value, max_value;
> + s64 min_value;
> + u64 max_value;
> u32 id;
> union {
> /* valid when type == CONST_IMM | PTR_TO_STACK |
> UNKNOWN_VALUE */
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 9002575..840533a 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -234,8 +234,8 @@ static void print_verifier_state(struct
> bpf_verifier_state *state)
> reg->map_ptr->value_size,
> reg->id);
> if (reg->min_value != BPF_REGISTER_MIN_RANGE)
> - verbose(",min_value=%llu",
> - (unsigned long long)reg->min_value);
> + verbose(",min_value=%lld",
> + (long long)reg->min_value);
> if (reg->max_value != BPF_REGISTER_MAX_RANGE)
> verbose(",max_value=%llu",
> (unsigned long long)reg->max_value);
> @@ -1490,7 +1490,7 @@ static void check_reg_overflow(struct bpf_reg_state
> *reg)
> {
> if (reg->max_value > BPF_REGISTER_MAX_RANGE)
> reg->max_value = BPF_REGISTER_MAX_RANGE;
> - if ((s64)reg->min_value < BPF_REGISTER_MIN_RANGE)
> + if (reg->min_value < BPF_REGISTER_MIN_RANGE)
> reg->min_value = BPF_REGISTER_MIN_RANGE;
> }
>
> @@ -1498,7 +1498,8 @@ static void adjust_reg_min_max_vals(struct
> bpf_verifier_env *env,
> struct bpf_insn *insn)
> {
> struct bpf_reg_state *regs = env->cur_state.regs, *dst_reg;
> - u64 min_val = BPF_REGISTER_MIN_RANGE, max_val =
> BPF_REGISTER_MAX_RANGE;
> + s64 min_val = BPF_REGISTER_MIN_RANGE;
> + u64 max_val = BPF_REGISTER_MAX_RANGE;
> bool min_set = false, max_set = false;
> u8 opcode = BPF_OP(insn->code);
>
> @@ -1534,22 +1535,45 @@ static void adjust_reg_min_max_vals(struct
> bpf_verifier_env *env,
> return;
> }
>
> + /* If one of our values was at the end of our ranges then we can't
> just
> + * do our normal operations to the register, we need to set the
> values
> + * to the min/max since they are undefined.
> + */
> + if (min_val == BPF_REGISTER_MIN_RANGE)
> + dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
> + if (max_val == BPF_REGISTER_MAX_RANGE)
> + dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
> +
> switch (opcode) {
> case BPF_ADD:
> - dst_reg->min_value += min_val;
> - dst_reg->max_value += max_val;
> + if (dst_reg->min_value != BPF_REGISTER_MIN_RANGE)
> + dst_reg->min_value += min_val;
> + if (dst_reg->max_value != BPF_REGISTER_MAX_RANGE)
> + dst_reg->max_value += max_val;
> break;
> case BPF_SUB:
> - dst_reg->min_value -= min_val;
> - dst_reg->max_value -= max_val;
> + if (dst_reg->min_value != BPF_REGISTER_MIN_RANGE)
> + dst_reg->min_value -= min_val;
> + if (dst_reg->max_value != BPF_REGISTER_MAX_RANGE)
> + dst_reg->max_value -= max_val;
> break;
> case BPF_MUL:
> - dst_reg->min_value *= min_val;
> - dst_reg->max_value *= max_val;
> + if (dst_reg->min_value != BPF_REGISTER_MIN_RANGE)
> + dst_reg->min_value *= min_val;
> + if (dst_reg->max_value != BPF_REGISTER_MAX_RANGE)
> + dst_reg->max_value *= max_val;
> break;
> case BPF_AND:
> - /* & is special since it could end up with 0 bits set. */
> - dst_reg->min_value &= min_val;
> + /* & is special since it's could be any value within our
> range,
> + * including 0. But if the thing we're AND'ing against is
> + * negative and we're negative then that's the minimum
> value,
> + * otherwise the minimum will always be 0.
> + */
> + if (min_val < 0 && dst_reg->min_value < 0)
> + dst_reg->min_value = min_t(s64, dst_reg->min_value,
> + min_val);
> + else
> + dst_reg->min_value = 0;
> dst_reg->max_value = max_val;
> break;
> case BPF_LSH:
> @@ -1559,24 +1583,19 @@ static void adjust_reg_min_max_vals(struct
> bpf_verifier_env *env,
> */
> if (min_val > ilog2(BPF_REGISTER_MAX_RANGE))
> dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
> - else
> + else if (dst_reg->min_value != BPF_REGISTER_MIN_RANGE)
> dst_reg->min_value <<= min_val;
>
> if (max_val > ilog2(BPF_REGISTER_MAX_RANGE))
> dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
> - else
> + else if (dst_reg->max_value != BPF_REGISTER_MAX_RANGE)
> dst_reg->max_value <<= max_val;
> break;
> case BPF_RSH:
> - dst_reg->min_value >>= min_val;
> - dst_reg->max_value >>= max_val;
> - break;
> - case BPF_MOD:
> - /* % is special since it is an unsigned modulus, so the
> floor
> - * will always be 0.
> - */
> - dst_reg->min_value = 0;
> - dst_reg->max_value = max_val - 1;
> + if (dst_reg->min_value != BPF_REGISTER_MIN_RANGE)
> + dst_reg->min_value >>= min_val;
> + if (dst_reg->max_value != BPF_REGISTER_MAX_RANGE)
> + dst_reg->max_value >>= max_val;
> break;
> default:
> reset_reg_range_values(regs, insn->dst_reg);
> --
> 2.5.5
>
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