[PATCH bpf-next v2 0/2] bpf: Validate byte type for helper stack reads

[PATCH bpf-next v2 0/2] bpf: Validate byte type for helper stack reads

[Yiyang Chen]

The verifier checks helper memory arguments byte by byte, but its scalar
spill fast path only checks the slot-wide spill marker. For a CAP_BPF load,
a narrow scalar spill can therefore make an untouched sibling byte appear
initialized when the sibling is passed to a helper.

Patch 1 makes the fast path require STACK_SPILL for the byte being checked.
Patch 2 adds verifier_spill_fill coverage for a CAP_BPF map update helper
reading an invalid sibling byte.

Changes in v2:
- Rewrite the patch 1 commit message to explain the uninitialized-stack-read
  invariant and distinguish it from the pointer-leak check, with references
  to the related producer-side fix and direct-read selftest.
- Add the regression Fixes tag to patch 2 and use the neighboring inline-asm
  line-continuation style.

v1: https://lore.kernel.org/bpf/cover.1783920884.git.chenyy23@mails.tsinghua.edu.cn/

Yiyang Chen (2):
  bpf: Check byte type when validating helper stack reads
  selftests/bpf: Cover helper reads of invalid spill siblings

 kernel/bpf/verifier.c                         |  3 +-
 .../selftests/bpf/progs/verifier_spill_fill.c | 34 +++++++++++++++++++
 2 files changed, 36 insertions(+), 1 deletion(-)


base-commit: 1d91ea01185656ac3ee63c5f9f6f8bde3c746b3d
-- 
2.34.1

[PATCH bpf-next v2 1/2] bpf: Check byte type when validating helper stack reads

[Yiyang Chen]

check_stack_range_initialized() accepts a STACK_INVALID byte only when
allow_uninit_stack is set. It then has a separate path for spilled
registers: scalar spills are accepted directly, while pointer spills
require allow_ptr_leaks.

bpf_is_spilled_reg() tests slot_type[BPF_REG_SIZE - 1], the canonical top
byte of the slot. After a sub-register scalar spill, such as a one-byte
store to fp-8, save_register_state() marks only the covered byte
STACK_SPILL. mark_stack_slot_misc() leaves the untouched sibling bytes
STACK_INVALID. When the helper range reaches fp-7, the STACK_INVALID case
correctly declines it if allow_uninit_stack is false, but the following
scalar-spill path accepts it based on fp-8's slot-wide spill marker.

For a CAP_BPF load without CAP_PERFMON, allow_uninit_stack is false, so an
untouched sibling byte such as fp-7 should reach -EACCES. Instead, passing
fp-7 as the value to bpf_map_update_elem() after a one-byte spill to fp-8
is accepted, and the helper copies a byte the program never wrote. A direct
read of fp-7 rejects; only the helper-argument path is affected.

Require the current byte to be STACK_SPILL before taking the
spilled-register path. Full-width spills still mark every byte STACK_SPILL.
A permitted STACK_INVALID read is still handled by the preceding
allow_uninit_stack case. This mirrors the producer-side fix in
mark_stack_slot_misc(), which preserves STACK_INVALID siblings, and applies
the same invariant to helper arguments. The existing direct-read selftest
does not cover this path.

Fixes: 354e8f1970f8 ("bpf: Support <8-byte scalar spill and refill")
Signed-off-by: Yiyang Chen <chenyy23@mails.tsinghua.edu.cn>
---
 kernel/bpf/verifier.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a78cdabf85607..7c61ade27d2a6 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6735,7 +6735,8 @@ static int check_stack_range_initialized(
 			goto mark;
 		}
 
-		if (bpf_is_spilled_reg(&state->stack[spi]) &&
+		if (*stype == STACK_SPILL &&
+		    bpf_is_spilled_reg(&state->stack[spi]) &&
 		    (state->stack[spi].spilled_ptr.type == SCALAR_VALUE ||
 		     env->allow_ptr_leaks)) {
 			if (clobber) {
-- 
2.34.1

[PATCH bpf-next v2 2/2] selftests/bpf: Cover helper reads of invalid spill siblings

[Yiyang Chen]

Add a verifier_spill_fill case that passes an untouched byte next to a
one-byte scalar spill to bpf_map_update_elem(). The full-capability load
retains the existing uninitialized-stack behavior, while the CAP_BPF-only
load must reject the STACK_INVALID helper read.

Fixes: 354e8f1970f8 ("bpf: Support <8-byte scalar spill and refill")
Signed-off-by: Yiyang Chen <chenyy23@mails.tsinghua.edu.cn>
---
 .../selftests/bpf/progs/verifier_spill_fill.c | 34 +++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
index 8b166c42c4e0e..2e62a7ddcbce2 100644
--- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
+++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
@@ -11,6 +11,13 @@ struct {
 	__uint(max_entries, 4096);
 } map_ringbuf SEC(".maps");
 
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, __u32);
+	__type(value, __u8);
+} map_partial_spill SEC(".maps");
+
 SEC("socket")
 __description("check valid spill/fill")
 __success __failure_unpriv __msg_unpriv("R0 leaks addr")
@@ -1307,6 +1314,33 @@ __naked void stack_noperfmon_spill_32bit_onto_64bit_slot(void)
 	: __clobber_all);
 }
 
+SEC("socket")
+__description("helper read must reject an invalid sibling of a narrow spill")
+__success
+__caps_unpriv(CAP_BPF)
+__failure_unpriv __msg_unpriv("invalid read from stack R3 off -7+0 size 1")
+__naked void helper_read_invalid_partial_spill_sibling(void)
+{
+	asm volatile ("					\
+	r5 = 1;						\
+	*(u8 *)(r10 - 8) = r5;				\
+	r2 = 0;						\
+	*(u32 *)(r10 - 16) = r2;			\
+	r1 = %[map_partial_spill] ll;			\
+	r2 = r10;					\
+	r2 += -16;					\
+	r3 = r10;					\
+	r3 += -7;					\
+	r4 = 0;						\
+	call %[bpf_map_update_elem];			\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm(bpf_map_update_elem),
+	  __imm_addr(map_partial_spill)
+	: __clobber_all);
+}
+
 /*
  * stacksafe(): check if 32-bit scalar spill in old state is considered
  * equivalent to STACK_MISC in cur state.
-- 
2.34.1