Re: [PATCH bpf 1/2] bpf: fix a verifier issue when assigning 32bit reg states to 64bit ones

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Thu, May 28, 2020 at 09:50:43AM -0700, Yonghong Song wrote:
> With the latest trunk llvm (llvm 11), I hit a verifier issue for
> test_prog subtest test_verif_scale1.
> 
> The following simplified example illustrate the issue:
>     w9 = 0  /* R9_w=inv0 */
>     r8 = *(u32 *)(r1 + 80)  /* __sk_buff->data_end */
>     r7 = *(u32 *)(r1 + 76)  /* __sk_buff->data */
>     ......
>     w2 = w9 /* R2_w=inv0 */
>     r6 = r7 /* R6_w=pkt(id=0,off=0,r=0,imm=0) */
>     r6 += r2 /* R6_w=inv(id=0) */
>     r3 = r6 /* R3_w=inv(id=0) */
>     r3 += 14 /* R3_w=inv(id=0) */
>     if r3 > r8 goto end
>     r5 = *(u32 *)(r6 + 0) /* R6_w=inv(id=0) */
>        <== error here: R6 invalid mem access 'inv'
>     ...
>   end:
> 
> In real test_verif_scale1 code, "w9 = 0" and "w2 = w9" are in
> different basic blocks.
> 
> In the above, after "r6 += r2", r6 becomes a scalar, which eventually
> caused the memory access error. The correct register state should be
> a pkt pointer.
> 
> The inprecise register state starts at "w2 = w9".
> The 32bit register w9 is 0, in __reg_assign_32_into_64(),
> the 64bit reg->smax_value is assigned to be U32_MAX.
> The 64bit reg->smin_value is 0 and the 64bit register
> itself remains constant based on reg->var_off.
> 
> In adjust_ptr_min_max_vals(), the verifier checks for a known constant,
> smin_val must be equal to smax_val. Since they are not equal,
> the verifier decides r6 is a unknown scalar, which caused later failure.
> 
> The llvm10 does not have this issue as it generates different code:
>     w9 = 0  /* R9_w=inv0 */
>     r8 = *(u32 *)(r1 + 80)  /* __sk_buff->data_end */
>     r7 = *(u32 *)(r1 + 76)  /* __sk_buff->data */
>     ......
>     r6 = r7 /* R6_w=pkt(id=0,off=0,r=0,imm=0) */
>     r6 += r9 /* R6_w=pkt(id=0,off=0,r=0,imm=0) */
>     r3 = r6 /* R3_w=pkt(id=0,off=0,r=0,imm=0) */
>     r3 += 14 /* R3_w=pkt(id=0,off=14,r=0,imm=0) */
>     if r3 > r8 goto end
>     ...
> 
> To fix the issue, if 32bit register is a const 0,
> then just assign max vaue 0 to 64bit register smax_value as well.
> 
> Fixes: 3f50f132d840 ("bpf: Verifier, do explicit ALU32 bounds tracking")
> Cc: John Fastabend <john.fastabend@gmail.com>
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
>  kernel/bpf/verifier.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 8d7ee40e2748..5123ce54695f 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1174,6 +1174,9 @@ static void __reg_assign_32_into_64(struct bpf_reg_state *reg)
>  		reg->smin_value = 0;
>  	if (reg->s32_max_value > 0)
>  		reg->smax_value = reg->s32_max_value;
> +	else if (reg->s32_max_value == 0 && reg->s32_min_value == 0 &&
> +		 tnum_is_const(reg->var_off))
> +		reg->smax_value = 0; /* const 0 */
>  	else
>  		reg->smax_value = U32_MAX;

wouldn't this be a more general fix ?

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 01c7d3634151..83450d5d24ab 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1217,11 +1217,11 @@ static void __reg_assign_32_into_64(struct bpf_reg_state *reg)
         * but must be positive otherwise set to worse case bounds
         * and refine later from tnum.
         */
-       if (reg->s32_min_value > 0)
+       if (reg->s32_min_value >= 0)
                reg->smin_value = reg->s32_min_value;
        else
                reg->smin_value = 0;
-       if (reg->s32_max_value > 0)
+       if (reg->s32_max_value >= 0)
                reg->smax_value = reg->s32_max_value;
        else
                reg->smax_value = U32_MAX;