[PATCH bpf-next v3 13/13] bpf: poison dead stack slots

Eduard Zingerman
2026-04-10T09:29:42Z

From: Alexei Starovoitov <ast@kernel.org>

As a sanity check poison stack slots that stack liveness determined
to be dead, so that any read from such slots will cause program rejection.
If stack liveness logic is incorrect the poison can cause
valid program to be rejected, but it also will prevent unsafe program
to be accepted.

Allow global subprogs "read" poisoned stack slots.
The static stack liveness determined that subprog doesn't read certain
stack slots, but sizeof(arg_type) based global subprog validation
isn't accurate enough to know which slots will actually be read by
the callee, so it needs to check full sizeof(arg_type) at the caller.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
include/linux/bpf_verifier.h | 1 +
kernel/bpf/log.c | 5 +-
kernel/bpf/verifier.c | 89 ++++++++++++++++------
.../selftests/bpf/progs/verifier_spill_fill.c | 2 +
4 files changed, 70 insertions(+), 27 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 6e7f53f6a1d4..6d4488012fef 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -220,6 +220,7 @@ enum bpf_stack_slot_type {
STACK_DYNPTR,
STACK_ITER,
STACK_IRQ_FLAG,
+ STACK_POISON,
};

#define BPF_REG_SIZE 8 /* size of eBPF register in bytes */
diff --git a/kernel/bpf/log.c b/kernel/bpf/log.c
index f0902ecb7df6..d5779a3426d9 100644
--- a/kernel/bpf/log.c
+++ b/kernel/bpf/log.c
@@ -542,7 +542,8 @@ static char slot_type_char[] = {
[STACK_ZERO] = '0',
[STACK_DYNPTR] = 'd',
[STACK_ITER] = 'i',
- [STACK_IRQ_FLAG] = 'f'
+ [STACK_IRQ_FLAG] = 'f',
+ [STACK_POISON] = 'p',
};

#define UNUM_MAX_DECIMAL U16_MAX
@@ -779,7 +780,7 @@ void print_verifier_state(struct bpf_verifier_env *env, const struct bpf_verifie

for (j = 0; j < BPF_REG_SIZE; j++) {
slot_type = state->stack[i].slot_type[j];
- if (slot_type != STACK_INVALID)
+ if (slot_type != STACK_INVALID && slot_type != STACK_POISON)
valid = true;
types_buf[j] = slot_type_char[slot_type];
}
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b279c4c93b09..2fe7d602f70c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1327,6 +1327,7 @@ static bool is_stack_slot_special(const struct bpf_stack_state *stack)
case STACK_IRQ_FLAG:
return true;
case STACK_INVALID:
+ case STACK_POISON:
case STACK_MISC:
case STACK_ZERO:
return false;
@@ -1356,9 +1357,11 @@ static bool is_spilled_scalar_after(const struct bpf_stack_state *stack, int im)
stack->spilled_ptr.type == SCALAR_VALUE;
}

-/* Mark stack slot as STACK_MISC, unless it is already STACK_INVALID, in which
- * case they are equivalent, or it's STACK_ZERO, in which case we preserve
- * more precise STACK_ZERO.
+/*
+ * Mark stack slot as STACK_MISC, unless it is already:
+ * - STACK_INVALID, in which case they are equivalent.
+ * - STACK_ZERO, in which case we preserve more precise STACK_ZERO.
+ * - STACK_POISON, which truly forbids access to the slot.
* Regardless of allow_ptr_leaks setting (i.e., privileged or unprivileged
* mode), we won't promote STACK_INVALID to STACK_MISC. In privileged case it is
* unnecessary as both are considered equivalent when loading data and pruning,
@@ -1369,14 +1372,14 @@ static void mark_stack_slot_misc(struct bpf_verifier_env *env, u8 *stype)
{
if (*stype == STACK_ZERO)
return;
- if (*stype == STACK_INVALID)
+ if (*stype == STACK_INVALID || *stype == STACK_POISON)
return;
*stype = STACK_MISC;
}

static void scrub_spilled_slot(u8 *stype)
{
- if (*stype != STACK_INVALID)
+ if (*stype != STACK_INVALID && *stype != STACK_POISON)
*stype = STACK_MISC;
}

@@ -5563,8 +5566,10 @@ static int check_stack_write_var_off(struct bpf_verifier_env *env,
* For privileged programs, we will accept such reads to slots
* that may or may not be written because, if we're reject
* them, the error would be too confusing.
+ * Conservatively, treat STACK_POISON in a similar way.
*/
- if (*stype == STACK_INVALID && !env->allow_uninit_stack) {
+ if ((*stype == STACK_INVALID || *stype == STACK_POISON) &&
+ !env->allow_uninit_stack) {
verbose(env, "uninit stack in range of var-offset write prohibited for !root; insn %d, off: %d",
insn_idx, i);
return -EINVAL;
@@ -5700,8 +5705,13 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
}
if (type == STACK_INVALID && env->allow_uninit_stack)
continue;
- verbose(env, "invalid read from stack off %d+%d size %d\n",
- off, i, size);
+ if (type == STACK_POISON) {
+ verbose(env, "reading from stack off %d+%d size %d, slot poisoned by dead code elimination\n",
+ off, i, size);
+ } else {
+ verbose(env, "invalid read from stack off %d+%d size %d\n",
+ off, i, size);
+ }
return -EACCES;
}

@@ -5750,8 +5760,13 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
continue;
if (type == STACK_INVALID && env->allow_uninit_stack)
continue;
- verbose(env, "invalid read from stack off %d+%d size %d\n",
- off, i, size);
+ if (type == STACK_POISON) {
+ verbose(env, "reading from stack off %d+%d size %d, slot poisoned by dead code elimination\n",
+ off, i, size);
+ } else {
+ verbose(env, "invalid read from stack off %d+%d size %d\n",
+ off, i, size);
+ }
return -EACCES;
}
if (dst_regno >= 0)
@@ -8316,16 +8331,22 @@ static int check_stack_range_initialized(
/* Some accesses can write anything into the stack, others are
* read-only.
*/
- bool clobber = false;
+ bool clobber = type == BPF_WRITE;
+ /*
+ * Negative access_size signals global subprog/kfunc arg check where
+ * STACK_POISON slots are acceptable. static stack liveness
+ * might have determined that subprog doesn't read them,
+ * but BTF based global subprog validation isn't accurate enough.
+ */
+ bool allow_poison = access_size < 0 || clobber;
+
+ access_size = abs(access_size);

if (access_size == 0 && !zero_size_allowed) {
verbose(env, "invalid zero-sized read\n");
return -EACCES;
}

- if (type == BPF_WRITE)
- clobber = true;
-
err = check_stack_access_within_bounds(env, regno, off, access_size, type);
if (err)
return err;
@@ -8424,7 +8445,12 @@ static int check_stack_range_initialized(
goto mark;
}

- if (tnum_is_const(reg->var_off)) {
+ if (*stype == STACK_POISON) {
+ if (allow_poison)
+ goto mark;
+ verbose(env, "reading from stack R%d off %d+%d size %d, slot poisoned by dead code elimination\n",
+ regno, min_off, i - min_off, access_size);
+ } else if (tnum_is_const(reg->var_off)) {
verbose(env, "invalid read from stack R%d off %d+%d size %d\n",
regno, min_off, i - min_off, access_size);
} else {
@@ -8607,8 +8633,10 @@ static int check_mem_reg(struct bpf_verifier_env *env, struct bpf_reg_state *reg
mark_ptr_not_null_reg(reg);
}

- err = check_helper_mem_access(env, regno, mem_size, BPF_READ, true, NULL);
- err = err ?: check_helper_mem_access(env, regno, mem_size, BPF_WRITE, true, NULL);
+ int size = base_type(reg->type) == PTR_TO_STACK ? -(int)mem_size : mem_size;
+
+ err = check_helper_mem_access(env, regno, size, BPF_READ, true, NULL);
+ err = err ?: check_helper_mem_access(env, regno, size, BPF_WRITE, true, NULL);

if (may_be_null)
*reg = saved_reg;
@@ -20069,7 +20097,7 @@ static void __clean_func_state(struct bpf_verifier_env *env,
__mark_reg_not_init(env, spill);
}
for (j = start; j < end; j++)
- st->stack[i].slot_type[j] = STACK_INVALID;
+ st->stack[i].slot_type[j] = STACK_POISON;
}
}
}
@@ -20407,7 +20435,8 @@ static bool is_stack_misc_after(struct bpf_verifier_env *env,

for (i = im; i < ARRAY_SIZE(stack->slot_type); ++i) {
if ((stack->slot_type[i] == STACK_MISC) ||
- (stack->slot_type[i] == STACK_INVALID && env->allow_uninit_stack))
+ ((stack->slot_type[i] == STACK_INVALID || stack->slot_type[i] == STACK_POISON) &&
+ env->allow_uninit_stack))
continue;
return false;
}
@@ -20443,13 +20472,22 @@ static bool stacksafe(struct bpf_verifier_env *env, struct bpf_func_state *old,

spi = i / BPF_REG_SIZE;

- if (exact == EXACT &&
- (i >= cur->allocated_stack ||
- old->stack[spi].slot_type[i % BPF_REG_SIZE] !=
- cur->stack[spi].slot_type[i % BPF_REG_SIZE]))
- return false;
+ if (exact == EXACT) {
+ u8 old_type = old->stack[spi].slot_type[i % BPF_REG_SIZE];
+ u8 cur_type = i < cur->allocated_stack ?
+ cur->stack[spi].slot_type[i % BPF_REG_SIZE] : STACK_INVALID;
+
+ /* STACK_INVALID and STACK_POISON are equivalent for pruning */
+ if (old_type == STACK_POISON)
+ old_type = STACK_INVALID;
+ if (cur_type == STACK_POISON)
+ cur_type = STACK_INVALID;
+ if (i >= cur->allocated_stack || old_type != cur_type)
+ return false;
+ }

- if (old->stack[spi].slot_type[i % BPF_REG_SIZE] == STACK_INVALID)
+ if (old->stack[spi].slot_type[i % BPF_REG_SIZE] == STACK_INVALID ||
+ old->stack[spi].slot_type[i % BPF_REG_SIZE] == STACK_POISON)
continue;

if (env->allow_uninit_stack &&
@@ -20547,6 +20585,7 @@ static bool stacksafe(struct bpf_verifier_env *env, struct bpf_func_state *old,
case STACK_MISC:
case STACK_ZERO:
case STACK_INVALID:
+ case STACK_POISON:
continue;
/* Ensure that new unhandled slot types return false by default */
default:
diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
index c6ae64b99cd6..6bc721accbae 100644
--- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
+++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
@@ -780,6 +780,8 @@ __naked void stack_load_preserves_const_precision_subreg(void)
"r1 += r2;"
"*(u8 *)(r1 + 0) = r2;" /* this should be fine */

+ "r2 = *(u64 *)(r10 -8);" /* keep slots alive */
+ "r2 = *(u64 *)(r10 -16);"
"r0 = 0;"
"exit;"
:

--
2.53.0