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bpf: Fix mask direction swap upon off reg sign change

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Masking direction as indicated via mask_to_left is considered to be
calculated once and then used to derive pointer limits. Thus, this
needs to be placed into bpf_sanitize_info instead so we can pass it
to sanitize_ptr_alu() call after the pointer move. Piotr noticed a
corner case where the off reg causes masking direction change which
then results in an incorrect final aux->alu_limit.

Fixes: 7fedb63a83 ("bpf: Tighten speculative pointer arithmetic mask")
Reported-by: Piotr Krysiuk <piotras@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Piotr Krysiuk <piotras@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Daniel Borkmann 2021-05-21 10:19:22 +00:00
parent 3d0220f686
commit bb01a1bba5
1 changed files with 12 additions and 10 deletions
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22
kernel/bpf/verifier.c
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@ -6409,18 +6409,10 @@ enum {
};
static int retrieve_ptr_limit(const struct bpf_reg_state *ptr_reg,
const struct bpf_reg_state *off_reg,
u32 *alu_limit, u8 opcode)
u32 *alu_limit, bool mask_to_left)
{
bool off_is_neg = off_reg->smin_value < 0;
bool mask_to_left = (opcode == BPF_ADD && off_is_neg) ||
(opcode == BPF_SUB && !off_is_neg);
u32 max = 0, ptr_limit = 0;
if (!tnum_is_const(off_reg->var_off) &&
(off_reg->smin_value < 0) != (off_reg->smax_value < 0))
return REASON_BOUNDS;
switch (ptr_reg->type) {
case PTR_TO_STACK:
/* Offset 0 is out-of-bounds, but acceptable start for the
@ -6488,6 +6480,7 @@ static bool sanitize_needed(u8 opcode)
struct bpf_sanitize_info {
struct bpf_insn_aux_data aux;
bool mask_to_left;
};
static int sanitize_ptr_alu(struct bpf_verifier_env *env,
@ -6519,7 +6512,16 @@ static int sanitize_ptr_alu(struct bpf_verifier_env *env,
if (vstate->speculative)
goto do_sim;
err = retrieve_ptr_limit(ptr_reg, off_reg, &alu_limit, opcode);
if (!commit_window) {
if (!tnum_is_const(off_reg->var_off) &&
(off_reg->smin_value < 0) != (off_reg->smax_value < 0))
return REASON_BOUNDS;
info->mask_to_left = (opcode == BPF_ADD && off_is_neg) ||
(opcode == BPF_SUB && !off_is_neg);
}
err = retrieve_ptr_limit(ptr_reg, &alu_limit, info->mask_to_left);
if (err < 0)
return err;