Bug #1748

bpf_validate() uses BPF_RVAL() when it should use BPF_SRC()

Added by guy almost 4 years ago. Updated almost 4 years ago.

Status:ClosedStart date:
Priority:NormalDue date:
Assignee:swildner% Done:

0%

Category:-
Target version:-

Description

In bpf_validate, when it checks whether the divisor in a BPF_DIV instruction is a constant 0, it does

case BPF_DIV:
/*
* Check for constant division by 0.
*/
if (BPF_RVAL(p->code) == BPF_K && p->k == 0)
return 0;
break;

BPF_RVAL() is the macro to get the return value of a RET instruction; it extracts the 0x18 bits. The BPF_DIV opcode is 0x30, which has the 0x10 bit set; a BPF_DIV instruction with a constant 0 as the divisor would be BPF_DIV|BPF_K, which is 0x30; BPF_RVAL(p->code) would be 0x10, which isn't equal to BPF_K, which is 0x00.

The macro to get the source argument of an arithmetic instruction is BPF_SRC(), which extracts only the 0x08 bit; BPF_SRC(p->code) would be 0x00, which is equal to BPF_K, so it should be doing

case BPF_DIV:
/*
* Check for constant division by 0.
*/
if (BPF_SRC(p->code) == BPF_K && p->k == 0)
return 0;
break;

History

#1 Updated by swildner almost 4 years ago

OpenBSD has changed this to BPF_SRC() based on a report by Guy. NetBSD still
uses BPF_RVAL (couldn't find a PR either), while FreeBSD uses this:

/*
* Check for constant division by 0.
*/
if (p->code == (BPF_ALU|BPF_DIV|BPF_K) && p->k == 0)
return (0);

#2 Updated by swildner almost 4 years ago

Thanks, committed! -> 3361ad4a4e7a29e045f839fbd155dbb34030f7d7

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