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Message-Id: <20210430141911.396953187@linuxfoundation.org>
Date: Fri, 30 Apr 2021 16:20:21 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
bpf@...r.kernel.org, Piotr Krysiuk <piotras@...il.com>,
Benedict Schlueter <benedict.schlueter@....de>,
Daniel Borkmann <daniel@...earbox.net>,
John Fastabend <john.fastabend@...il.com>,
Alexei Starovoitov <ast@...nel.org>,
Frank van der Linden <fllinden@...zon.com>
Subject: [PATCH 5.4 7/8] bpf: Tighten speculative pointer arithmetic mask
From: Daniel Borkmann <daniel@...earbox.net>
commit 7fedb63a8307dda0ec3b8969a3b233a1dd7ea8e0 upstream.
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@...il.com>
Reported-by: Benedict Schlueter <benedict.schlueter@....de>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Reviewed-by: John Fastabend <john.fastabend@...il.com>
Acked-by: Alexei Starovoitov <ast@...nel.org>
Tested-by: Benedict Schlueter <benedict.schlueter@....de>
[fllinden@...zon.com: backported to 5.4]
Signed-off-by: Frank van der Linden <fllinden@...zon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
kernel/bpf/verifier.c | 73 ++++++++++++++++++++++++++++++--------------------
1 file changed, 44 insertions(+), 29 deletions(-)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4278,7 +4278,7 @@ static int retrieve_ptr_limit(const stru
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))
@@ -4287,26 +4287,18 @@ static int retrieve_ptr_limit(const stru
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;
@@ -4361,10 +4353,12 @@ static int sanitize_ptr_alu(struct bpf_v
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);
@@ -4383,18 +4377,33 @@ static int sanitize_ptr_alu(struct bpf_v
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
@@ -4506,6 +4515,7 @@ static int adjust_ptr_min_max_vals(struc
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;
@@ -4564,12 +4574,15 @@ static int adjust_ptr_min_max_vals(struc
!check_reg_sane_offset(env, ptr_reg, ptr_reg->type))
return -EINVAL;
- 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
*/
@@ -4620,10 +4633,6 @@ static int adjust_ptr_min_max_vals(struc
}
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",
@@ -4706,6 +4715,12 @@ static int adjust_ptr_min_max_vals(struc
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;
}
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