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bpf: Tighten speculative pointer arithmetic mask

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This work tightens the offset mask we use for unprivileged pointer arithmetic
in order to mitigate a corner case reported by Piotr and Benedict where in
the speculative domain it is possible to advance, for example, the map value
pointer by up to value_size-1 out-of-bounds in order to leak kernel memory
via side-channel to user space.

Before this change, the computed ptr_limit for retrieve_ptr_limit() helper
represents largest valid distance when moving pointer to the right or left
which is then fed as aux->alu_limit to generate masking instructions against
the offset register. After the change, the derived aux->alu_limit represents
the largest potential value of the offset register which we mask against which
is just a narrower subset of the former limit.

For minimal complexity, we call sanitize_ptr_alu() from 2 observation points
in adjust_ptr_min_max_vals(), that is, before and after the simulated alu
operation. In the first step, we retieve the alu_state and alu_limit before
the operation as well as we branch-off a verifier path and push it to the
verification stack as we did before which checks the dst_reg under truncation,
in other words, when the speculative domain would attempt to move the pointer
out-of-bounds.

In the second step, we retrieve the new alu_limit and calculate the absolute
distance between both. Moreover, we commit the alu_state and final alu_limit
via update_alu_sanitation_state() to the env's instruction aux data, and bail
out from there if there is a mismatch due to coming from different verification
paths with different states.

Reported-by: Piotr Krysiuk <piotras@gmail.com>
Reported-by: Benedict Schlueter <benedict.schlueter@rub.de>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Benedict Schlueter <benedict.schlueter@rub.de>
This commit is contained in:
Daniel Borkmann 2021-03-24 10:38:26 +01:00
parent f528819334
commit 7fedb63a83
1 changed files with 44 additions and 29 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

73
kernel/bpf/verifier.c
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@ -5871,7 +5871,7 @@ static int retrieve_ptr_limit(const struct bpf_reg_state *ptr_reg,
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 off, max = 0, ptr_limit = 0;
u32 max = 0, ptr_limit = 0;
if (!tnum_is_const(off_reg->var_off) &&
(off_reg->smin_value < 0) != (off_reg->smax_value < 0))
@ -5880,26 +5880,18 @@ static int retrieve_ptr_limit(const struct bpf_reg_state *ptr_reg,
switch (ptr_reg->type) {
case PTR_TO_STACK:
/* Offset 0 is out-of-bounds, but acceptable start for the
* left direction, see BPF_REG_FP.
* left direction, see BPF_REG_FP. Also, unknown scalar
* offset where we would need to deal with min/max bounds is
* currently prohibited for unprivileged.
*/
max = MAX_BPF_STACK + mask_to_left;
/* Indirect variable offset stack access is prohibited in
* unprivileged mode so it's not handled here.
*/
off = ptr_reg->off + ptr_reg->var_off.value;
if (mask_to_left)
ptr_limit = MAX_BPF_STACK + off;
else
ptr_limit = -off - 1;
ptr_limit = -(ptr_reg->var_off.value + ptr_reg->off);
break;
case PTR_TO_MAP_VALUE:
max = ptr_reg->map_ptr->value_size;
if (mask_to_left) {
ptr_limit = ptr_reg->umax_value + ptr_reg->off;
} else {
off = ptr_reg->smin_value + ptr_reg->off;
ptr_limit = ptr_reg->map_ptr->value_size - off - 1;
}
ptr_limit = (mask_to_left ?
ptr_reg->smin_value :
ptr_reg->umax_value) + ptr_reg->off;
break;
default:
return REASON_TYPE;
@ -5954,10 +5946,12 @@ static int sanitize_ptr_alu(struct bpf_verifier_env *env,
struct bpf_insn *insn,
const struct bpf_reg_state *ptr_reg,
const struct bpf_reg_state *off_reg,
struct bpf_reg_state *dst_reg)
struct bpf_reg_state *dst_reg,
struct bpf_insn_aux_data *tmp_aux,
const bool commit_window)
{
struct bpf_insn_aux_data *aux = commit_window ? cur_aux(env) : tmp_aux;
struct bpf_verifier_state *vstate = env->cur_state;
struct bpf_insn_aux_data *aux = cur_aux(env);
bool off_is_neg = off_reg->smin_value < 0;
bool ptr_is_dst_reg = ptr_reg == dst_reg;
u8 opcode = BPF_OP(insn->code);
@ -5976,18 +5970,33 @@ static int sanitize_ptr_alu(struct bpf_verifier_env *env,
if (vstate->speculative)
goto do_sim;
alu_state = off_is_neg ? BPF_ALU_NEG_VALUE : 0;
alu_state |= ptr_is_dst_reg ?
BPF_ALU_SANITIZE_SRC : BPF_ALU_SANITIZE_DST;
err = retrieve_ptr_limit(ptr_reg, off_reg, &alu_limit, opcode);
if (err < 0)
return err;
if (commit_window) {
/* In commit phase we narrow the masking window based on
* the observed pointer move after the simulated operation.
*/
alu_state = tmp_aux->alu_state;
alu_limit = abs(tmp_aux->alu_limit - alu_limit);
} else {
alu_state = off_is_neg ? BPF_ALU_NEG_VALUE : 0;
alu_state |= ptr_is_dst_reg ?
BPF_ALU_SANITIZE_SRC : BPF_ALU_SANITIZE_DST;
}
err = update_alu_sanitation_state(aux, alu_state, alu_limit);
if (err < 0)
return err;
do_sim:
/* If we're in commit phase, we're done here given we already
* pushed the truncated dst_reg into the speculative verification
* stack.
*/
if (commit_window)
return 0;
/* Simulate and find potential out-of-bounds access under
* speculative execution from truncation as a result of
* masking when off was not within expected range. If off
@ -6130,6 +6139,7 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
smin_ptr = ptr_reg->smin_value, smax_ptr = ptr_reg->smax_value;
u64 umin_val = off_reg->umin_value, umax_val = off_reg->umax_value,
umin_ptr = ptr_reg->umin_value, umax_ptr = ptr_reg->umax_value;
struct bpf_insn_aux_data tmp_aux = {};
u8 opcode = BPF_OP(insn->code);
u32 dst = insn->dst_reg;
int ret;
@ -6196,12 +6206,15 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
/* pointer types do not carry 32-bit bounds at the moment. */
__mark_reg32_unbounded(dst_reg);
switch (opcode) {
case BPF_ADD:
ret = sanitize_ptr_alu(env, insn, ptr_reg, off_reg, dst_reg);
if (sanitize_needed(opcode)) {
ret = sanitize_ptr_alu(env, insn, ptr_reg, off_reg, dst_reg,
&tmp_aux, false);
if (ret < 0)
return sanitize_err(env, insn, ret, off_reg, dst_reg);
}
switch (opcode) {
case BPF_ADD:
/* We can take a fixed offset as long as it doesn't overflow
* the s32 'off' field
*/
@ -6252,10 +6265,6 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
}
break;
case BPF_SUB:
ret = sanitize_ptr_alu(env, insn, ptr_reg, off_reg, dst_reg);
if (ret < 0)
return sanitize_err(env, insn, ret, off_reg, dst_reg);
if (dst_reg == off_reg) {
/* scalar -= pointer. Creates an unknown scalar */
verbose(env, "R%d tried to subtract pointer from scalar\n",
@ -6338,6 +6347,12 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
if (sanitize_check_bounds(env, insn, dst_reg) < 0)
return -EACCES;
if (sanitize_needed(opcode)) {
ret = sanitize_ptr_alu(env, insn, dst_reg, off_reg, dst_reg,
&tmp_aux, true);
if (ret < 0)
return sanitize_err(env, insn, ret, off_reg, dst_reg);
}
return 0;
}