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bpf: make the verifier tracks the "not equal" for regs

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We can derive some new information for BPF_JNE in regs_refine_cond_op().
Take following code for example:

  /* The type of "a" is u32 */
  if (a > 0 && a < 100) {
    /* the range of the register for a is [0, 99], not [1, 99],
     * and will cause the following error:
     *
     *   invalid zero-sized read
     *
     * as a can be 0.
     */
    bpf_skb_store_bytes(skb, xx, xx, a, 0);
  }

In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the
TRUE branch, the dst_reg will be marked as known to 0. However, in the
fallthrough(FALSE) branch, the dst_reg will not be handled, which makes
the [min, max] for a is [0, 99], not [1, 99].

For BPF_JNE, we can reduce the range of the dst reg if the src reg is a
const and is exactly the edge of the dst reg.

Signed-off-by: Menglong Dong <menglong8.dong@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Link: https://lore.kernel.org/r/20231219134800.1550388-2-menglong8.dong@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Menglong Dong 2023-12-19 21:47:57 +08:00 committed by Alexei Starovoitov
parent 1728df7fc1
commit d028f87517
1 changed files with 37 additions and 1 deletions
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38
kernel/bpf/verifier.c
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@ -14336,7 +14336,43 @@ again:
}
break;
case BPF_JNE:
/* we don't derive any new information for inequality yet */
if (!is_reg_const(reg2, is_jmp32))
swap(reg1, reg2);
if (!is_reg_const(reg2, is_jmp32))
break;
/* try to recompute the bound of reg1 if reg2 is a const and
* is exactly the edge of reg1.
*/
val = reg_const_value(reg2, is_jmp32);
if (is_jmp32) {
/* u32_min_value is not equal to 0xffffffff at this point,
* because otherwise u32_max_value is 0xffffffff as well,
* in such a case both reg1 and reg2 would be constants,
* jump would be predicted and reg_set_min_max() won't
* be called.
*
* Same reasoning works for all {u,s}{min,max}{32,64} cases
* below.
*/
if (reg1->u32_min_value == (u32)val)
reg1->u32_min_value++;
if (reg1->u32_max_value == (u32)val)
reg1->u32_max_value--;
if (reg1->s32_min_value == (s32)val)
reg1->s32_min_value++;
if (reg1->s32_max_value == (s32)val)
reg1->s32_max_value--;
} else {
if (reg1->umin_value == (u64)val)
reg1->umin_value++;
if (reg1->umax_value == (u64)val)
reg1->umax_value--;
if (reg1->smin_value == (s64)val)
reg1->smin_value++;
if (reg1->smax_value == (s64)val)
reg1->smax_value--;
}
break;
case BPF_JSET:
if (!is_reg_const(reg2, is_jmp32))