BPF_STX instruction preserves STACK_ZERO marks for variable offset
writes in situations like below:
*(u64*)(r10 - 8) = 0 ; STACK_ZERO marks for fp[-8]
r0 = random(-7, -1) ; some random number in range of [-7, -1]
r0 += r10 ; r0 is now a variable offset pointer to stack
r1 = 0
*(u8*)(r0) = r1 ; BPF_STX writing zero, STACK_ZERO mark for
; fp[-8] is preserved
This commit updates verifier.c:check_stack_write_var_off() to process
BPF_ST in a similar manner, e.g. the following example:
*(u64*)(r10 - 8) = 0 ; STACK_ZERO marks for fp[-8]
r0 = random(-7, -1) ; some random number in range of [-7, -1]
r0 += r10 ; r0 is now variable offset pointer to stack
*(u8*)(r0) = 0 ; BPF_ST writing zero, STACK_ZERO mark for
; fp[-8] is preserved
Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
---
kernel/bpf/verifier.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c28afae60874..272563a0b770 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3631,6 +3631,7 @@ static int check_stack_write_var_off(struct bpf_verifier_env *env,
int min_off, max_off;
int i, err;
struct bpf_reg_state *ptr_reg = NULL, *value_reg = NULL;
+ struct bpf_insn *insn = &env->prog->insnsi[insn_idx];
bool writing_zero = false;
/* set if the fact that we're writing a zero is used to let any
* stack slots remain STACK_ZERO
@@ -3643,7 +3644,8 @@ static int check_stack_write_var_off(struct bpf_verifier_env *env,
max_off = ptr_reg->smax_value + off + size;
if (value_regno >= 0)
value_reg = &cur->regs[value_regno];
- if (value_reg && register_is_null(value_reg))
+ if ((value_reg && register_is_null(value_reg)) ||
+ (!value_reg && is_bpf_st_mem(insn) && insn->imm == 0))
writing_zero = true;
err = grow_stack_state(state, round_up(-min_off, BPF_REG_SIZE));
--
2.39.1
