On Thu, Apr 11, 2024 at 08:27:47PM +0800, Xu Kuohai wrote:
[...] 24: (b4) w0 = -1 ; R0_w=0xffffffff ; int BPF_PROG(test_int_hook, struct vm_area_struct *vma, @ lsm.c:89 25: (95) exit At program exit the register R0 has smin=4294967295 smax=4294967295 should have been in [-4095, 0]
It can be seen that instruction "w0 = -1" zero extended -1 to 64-bit register r0, setting both smin and smax values of r0 to 4294967295. This resulted in a false reject when r0 was checked with range [-4095, 0].
Given bpf_retval_range is a 32-bit range, this patch fixes it by changing the compare between r0 and return range from 64-bit operation to 32-bit operation.
Fixes: 8fa4ecd49b81 ("bpf: enforce exact retval range on subprog/callback exit") Signed-off-by: Xu Kuohai xukuohai@huawei.com
kernel/bpf/verifier.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 05c7c5f2bec0..5393d576c76f 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -9879,7 +9879,7 @@ static bool in_rbtree_lock_required_cb(struct bpf_verifier_env *env) static bool retval_range_within(struct bpf_retval_range range, const struct bpf_reg_state *reg) {
- return range.minval <= reg->smin_value && reg->smax_value <= range.maxval;
- return range.minval <= reg->s32_min_value && reg->s32_max_value <= range.maxval;
Logic-wise LGTM
While the status-quo is that the return value is always truncated to 32-bit, looking back there was an attempt to use 64-bit return value for bpf_prog_run[1] (not merged due to issue on 32-bit architectures). Also from the reading of BPF standardization ABI it would be inferred that return value is in 64-bit range:
BPF has 10 general purpose registers and a read-only frame pointer register, all of which are 64-bits wide.
The BPF calling convention is defined as:
* R0: return value from function calls, and exit value for BPF programs ...
So add relevant people into the thread for opinions.
1: https://lore.kernel.org/bpf/20221115193911.u6prvskdzr5jevni@apollo/